Orwell Astronomical Society (Ipswich)

Transit of Venus, 08 June 2004

1. Introduction

A transit of Venus (ToV) is said to occur when the planet passes across the face of the Sun. On 08 June 2004, the first ToV for 122 years occurred, occasioning considerable interest from astronomers worldwide. Members of OASI observed the TOV from their homes, from Orwell Park Observatory and from other observing locations in the UK and abroad. This article summarises the experiences of the OASI observers.

1.1 Occurrence of Transits of Venus

The orbital periods of the Earth and Venus around the Sun are respectively 365.3 days and 224.7 days. Venus, travelling on the inner orbit, laps the Earth every 583.9 days. The orbit of Venus is inclined to the plane of the ecliptic (the plane of the Earth's orbit) at 3.4°. The points of intersection of the orbital planes are termed the nodes: the ascending node is where Venus crosses from south to north of the ecliptic, and the descending node is where the planet crosses from north to south. In most cases when Venus passes between the Earth and the Sun, the inclination of its orbit means that the planet passes above or below the solar disk. However, if Venus passes through inferior conjunction sufficiently close to a node, a transit will occur. This condition is met in a period of a few days duration in early June (descending node) and in another period of a few days duration in early December (ascending node).

The details of the orbital relationships of the planets mean that TOVs occur in pairs separated by eight years, where each pair is separated from the next by alternating periods of 105.5 and 121.5 years: a pair of June (descending node) transits is followed by a pair of December (ascending node) transits after an interval of 105.5 years and then by another pair of June transits after a further 121.5 years, and so on. This arrangement will not last indefinitely: secular variations in the orbits of Venus and the Earth will eventually alter the orbital relationships and change the above patterns of recurrence.

2. Historical Perspective

Pierre Gassendi was the first astronomer to observe the transit of a planet when he observed the Transit of Mercury (TOM) on 07 November 1631 OS, having been alerted to the event by a prediction by Kepler published in 1629. (OS refers to Old Style, i.e. Julian calendar, used in England prior to 14 September 1752.) In the same publication, Kepler predicted a TOV in December 1631 and stated that the next would occur in 1791. Unfortunately, the December 1631 TOV took place after the Sun had set as seen from Europe, and was not observed.

In October 1639, a young English astronomical genius, Jeremiah Horrocks, after studying the position of Venus in the sky and comparing it with the available predictions by Kepler and others, came to discover a mistake in Kepler's calculations and concluded that there would be a TOV on 24 November 1639 OS, eight years after the 1631 event. Horrocks and a friend, William Crabtree, observed the 1639 TOV, becoming the first ever to witness the phenomenon. Horrocks used the following method to estimate the Earth-Sun distance from his observations:

1. Use the relative orbital periods of the Earth and Venus to relate the angles subtended by the radius of Venus to heliocentric and topocentric observers during a TOV.
2. Assume that the Earth subtends the same angle as Venus to a heliocentric observer.
3. Estimate the angular diameter of Venus during the TOV by comparing it with the angular diameter of the Sun, considered known or estimated by other means.
4. Calculate the Earth-Sun distance directly by definition of the half-angle subtended by the Earth to a heliocentric observer.

Horrocks' estimate of the Earth - Sun distance was 95,700,000 km, approximately 64% of the currently accepted value. This highlights the fact that his method is, unfortunately, inherently incapable of producing accurate results, and is of interest nowadays primarily only because of its historical significance. The inherent inaccuracies of the method, particularly the assumption that the Earth subtends the same angle as Venus to a heliocentric observer, were compounded by the lack of reliable estimates in Horrocks' era of astronomical constants; as a result, Horrocks was forced to take nothing for granted and estimated the apparent diameter of the Sun, which undoubtedly contributed to the final error in his estimate.

In 1676, Edmund Halley journeyed to the tiny island of St Helena, in the South Atlantic, to observe the southern constellations. On 07 November 1677 OS, while in St Helena, he observed a TOM. Although his observation was dogged by clouds, he did manage to observe the ingress and egress of Mercury, and concluded that he was able to time accurately the first and last instants when the whole body of the planet was contained within the solar disk, as he later reported [1]:

I happened to observe, with the utmost care, Mercury passing over the Sun's disk: and contrary to expectation, I very accurately obtained, with a good 24-foot telescope, the very moment in which Mercury, entering the Sun's limb, seemed to touch it internally, as also that of his going off; forming an angle of internal contact. Hence I discovered the precise quantity of time the whole body of Mercury had then appeared within the Sun's disk, and that without an error of one single second of time; for, the thread of solar light, intercepted between the obscure limb of the planet, and the bright limb of the Sun, though exceedingly slender, affected my sight, and in the twinkling of an eye, both the indenture made on the Sun's limb by Mercury entering into it, vanished, and that made by his going off, appeared.

He realised that observations of the first and last instants when the whole body of the planet was contained within the solar disk could be used to calculate a much more accurate estimate of the Earth - Sun distance than Horrocks' method could provide. Knowledge of the mean value of the Earth - Sun distance, known as the Astronomical Unit (AU), could provide the scale of the entire Solar System and enable astronomers to calculate the radii of all planetary orbits through application of Kepler's third law (P² ~ a³, where P is the period of the planet and a is the semi-major axis of its orbit). In Halley's era, improved knowledge of the scale of the Solar System was of huge practical interest as a means to develop improved techniques for celestial navigation.

However, Halley was 21 years old in 1677 and the next TOV was not until 1761, so he would not live to see it. He continued to work on determining the size of the Solar System (amongst other things!) and in 1716, at the age of almost 60, he presented a proposal, known as an Admonition [1], to the Royal Society for observing the next TOV. Halley's Admonition included predictions of the circumstances of the TOV, recommended observing locations worldwide and outlined a technique to use the observations of the TOV to estimate the Earth - Sun distance.

His approach relied on measurements of contact times of the TOV. The contact times are defined as follows:

1^st contact: the limb of Venus first appears to touch the solar limb.

2^nd contact: the disk of Venus first appears totally inside the solar disk.

3^rd contact: the last instant at which the disk of Venus appears totally inside the solar disk.

4^th contact: the last instant at which the limb of Venus appears to touch the solar limb.

Fundamentally, Halley's approach to estimate the Earth - Sun distance relied on the well known parallax effect, whereby observers at different locations on the Earth see bodies in the Solar System at slightly different apparent locations and therefore witness the same event occur at slightly different times. By applying geometrical techniques analogous to the triangulation employed by terrestrial cartographers, it is possible to combine observations from different locations on the Earth to estimate the distance of celestial objects. Halley proposed to combine results from observers spread widely over the globe, and thereby obtain an accurate estimate of the Earth - Sun distance. He proposed the use of observations of 2^nd and 3^rd contact at each of two well-separated observing stations. A variant of Halley's approach, known as De Lisle's method (named after its originator) requires only observations, at two well-separated observing stations, either of 2^nd contact or of 3^rd contact.

So it was that for the 1761 and 1769 TOVs, observatories dispatched astronomers to distant parts of the globe in the hope of obtaining observations from baselines as extended as possible (to maximise the parallax effect). For the TOV on 06 June 1761, many famous astronomers made expeditions as follows:

• Neville Maskelyne (later fifth Astronomer Royal) and Robert Wadington to St Helena,
• Charles Mason and Jeremiah Dixon (later famous for the Mason-Dixon Line) to the Cape of Good Hope,
• Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil to Pondichery in India,
• Alexandre-Gui Pingré to the Isle of Rodrigué in the Indian Ocean,
• Maximillian Hell to Vardø in Norway,
• Jean-Baptiste d'Auteroche to Tobolsk in Siberia.

Many endured epic personal adventures in their travels and managed to undertake observations despite unexpected obstacles and unfortunate circumstances. In total, more than 120 astronomers from eight nations made observations from about 60 stations of the 1761 TOV. However, many of the observations were spoilt by an unexpected phenomenon - the black drop or teardrop effect, which happened as follows. During the ingress stage of the transit, as Venus approached 2^nd contact, rather than the silhouette of the planet separating cleanly from the solar limb, it appeared to stick to the limb and become stretched into a teardrop shape joined to it. This made it difficult to estimate the precise moment of 2^nd contact. The same phenomenon appeared in reverse at 3^rd contact. The effect was unexpected, particularly in view of Halley's comments about the accuracy which he was able to achieve in timing the TOM in 1677. Astronomers did not fully understand the cause of the effect, but attributed it in part to Venus possessing an atmosphere, which Mercury did not.

Many of the observers of the 1761 TOV also observed the 1769 event. They were joined by Lieutenant James Cook, sailing for Tahiti, where he observed the TOV from an observatory built for the occasion at Point Venus. (On Cook's return voyage after the TOV, he discovered New Zealand and charted part of the Australian coast.)

Despite the problems of the teardrop effect, when the French astronomer Joseph de Lalande analysed results from the 1761 and 1769 events in 1771, he obtained an estimate of the AU of 153,000,000 km, within 2.5% of the currently accepted value.

Governments dispatched numerous observing expeditions for the next pair of TOVs on 09 December 1874 and 06 December 1882. J I Plummer, Colonel Tomline's astronomer at Orwell Park Observatory, was among the astronomers dispatched to observe the 06 December 1882 TOV; he led the expedition which observed from Bermuda.

By the late nineteenth century, equipment had improved considerably since the previous pair of TOVs and, in particular, photography was available. However, photography turned out to be disappointing: the photographic plates recorded a poorly defined solar limb and, when astronomers placed them under a microscope, both the solar limb and the limb of Venus became indistinct and could not be used to provide a good quality estimate of contact times. In addition, the teardrop effect again limited accuracy. However, using data from the 1761, 1769, 1874 and 1882 TOVs, Simon Newcomb, a brilliant astronomer at the US Naval Observatory (USNO), calculated a value for the AU of 149,590,000 km, only 0.005% different from the currently accepted value.

The experience of the 1874 and 1882 TOVs convinced astronomers that the practical difficulties of estimating timings from a TOV meant that the method was not suitable to provide an estimate of the AU of ultimate accuracy. Astronomers therefore turned to other methods of estimating the scale of the Solar System, including in the modern era the use of direct radar ranging and spacecraft tracking.

Currently accepted values for the AU are:

• 149,597,870 km (IAU 1976 system of constants),
• 149,597,870.66 km (best modern estimate).

See [2] for a general overview of the historical development of observations of the TOV.

3. Observations At Orwell Park

The majority of members of OASI who observed the 2004 TOV did so using the 26 cm Tomline Refractor at Orwell Park Observatory.

3.1 Preparation

On 07 May 2003, some 13 months before the TOV, seventeen members of OASI observed the TOM from Orwell Park Observatory. The observers used the Tomline Refractor to project an image of the Sun and the silhouette of the planet onto a projection screen which was perched on the observing steps. Although the observations were successful and enjoyable, some areas for potential significant improvement were noted:

• Contrast of the projected image was poor, due largely to sunlight flooding into the dome through the open aperture.
• Perching the projection screen on the observing steps was most unsatisfactory: it was difficult to align the screen accurately perpendicular to the focal axis of the telescope, and the approach was generally very inconvenient.
• The observers had been unable to identify in advance the point of 1^st contact of Mercury on the projected image of the solar limb. This could have caused a delay in identifying 1^st contact. (In practice 1^st contact was obscured by clouds so the problem did not arise!)

In order to improve the contrast of the image for the ToV, on 02 June 2004, James Appleton and Martin Cook rigged a cloth sunshade to screen the aperture of the dome. The shade had a circular hole through which the object glass of the Tomline Refractor protruded. Martin and James attached the sunshade to the inside of the dome, so that it rotated with the latter, and fitted a system of cords to enable it to be raised and lowered to accommodate changes in the altitude angle of the telescope as it tracked the Sun (altitude 13º at 1^st contact and 60º at 4^th contact). See figures 1 - 4.

On the same day, Garry Coleman tackled the lack of a proper projection screen by fitting an adjustable model which he had designed and built in the preceding weeks. He attached the screen to the Tomline Refractor via specially constructed clamps which coupled rigidly to the ironwork supporting the telescope itself. (The clamps had mating faces constructed from soft wood, to avoid causing any damage.) By construction, the screen was aligned perpendicular to the focal axis of the telescope. The screen enabled projection of an image of the solar disk up to 550 mm in diameter. See figures 5 and 6.

Garry, Martin and James left the sunshade and projection screen fitted and ready for use when they left the observatory on 02 June. This saved valuable early-morning preparation time on 08 June!

Finally, prospective observers at Orwell Park positively identified the predicted point of ingress of Venus on the solar disk. This turned out to be very worthwhile as other members of OASI and the BBC, who did not undertake such rigorous preparation, found to their cost - see below!

3.2 Arrival At Orwell Park Observatory

The fact that TOVs are so rare, and that the last occurred 122 years previously, heightened the sense of anticipation leading towards 08 June 2004. This compounded the usual trepidation concerning the weather in the UK but, fortunately, the day dawned gloriously sunny, with blue sky and only a little thin cloud, and these conditions continued throughout the observations. For once, observers at Orwell Park Observatory enjoyed near-perfect weather conditions!

On the morning of the TOV, James Appleton and Monica Lustig, appointed observing directors for the occasion, arrived at Orwell Park Observatory at 5.40am (04:40 UT), by which time several members of OASI were already waiting to gain access. The party ascended the stairs to the observatory dome and began final preparation of observing equipment. The preparations generally went smoothly but, as always in circumstances where a critical task must be undertaken in a short period of time, there were some unexpected difficulties; for example figure 7 shows James and Martin struggling to attach a large piece of matt white card to the projection screen to provide a suitable background for the projected image. Once the white card was firmly attached, the observers experimented with several eyepieces and settled on a large, old, Army-surplus eyepiece. The eyepiece gave a good image size and contained only three or four optical elements, reducing the likelihood of it overheating due to the Sun's rays. In any case it was of rather poor quality so even if overheating did cause damage, there would be no great loss. After finalising the focus of the image, it became apparent that the preparation of the previous week had been worthwhile: the image of the Sun was bright, over half a metre in diameter, stable, displayed excellent contrast and could be easily photographed. The solar disk showed two small sunspots close to its centre.

The observers then set up stills and video cameras to photograph the projected image. (All the cameras were used to photograph the projected image: there was no use of other photographic techniques, such as prime focus or afocal coupling, with the Tomline Refractor.) A digital camera and 35 mm camera, both belonging to Martin, were attached via a bracket to the eyepiece draw-tube of the Tomline Refractor. Many members brought stills cameras for handheld use. There was a noticeable preponderance of digital cameras over traditional SLRs! Garry Coleman, Roy Gooding and David Payne brought video cameras. Figure 8 shows the two stills cameras attached to the Tomline Refractor and the three floor-standing video cameras.

The observers positioned a radio controlled clock, synchronised to the UK Rugby time signal and with a resolution of one second, for easy visibility. This served as the reference time for all their observations. Observers making visual observations in the main referenced their timings directly to the Rugby clock. Observers making video recordings periodically recorded a few seconds of the clock on video tape to act as a reference for subsequent frame-by-frame analysis (see figure 9).

Roy Gooding positioned an audio cassette recorder in the dome to capture comments on observations and general discussions throughout the event. The tape provided a valuable supplement to the written notes of the observers and a check on the validity of estimated timings. To assist in estimating contact times, the observers synchronised the audio tape to the Rugby clock by recording a timing reference on it.

3.3 Ingress

Shortly before 1^st contact there were 13 people in the dome of Orwell Park Observatory. As the predicted time of 1^st contact approached, a hush of expectation descended as everyone peered intently at the projected image, hoping to be first to spot the silhouette of Venus against the solar disk. Two members who arrived at the door to the dome only seconds before the predicted time of 1^st contact had to wait until after 1^st contact for it to be opened so as not to disturb the concentration of the observers inside.

Shortly after 1^st contact, David Payne noticed that there was a coloured rim around the part of the circumference of Venus showing against the background of the solar disk. There was speculation as to whether or not the colour was due to a diffraction effect associated with the atmosphere of the planet, but the observers did not arrive at a definitive conclusion on this point, and their view of the atmosphere of Venus during the egress phase was rather different.

At 05:26:20 UT, Venus appeared to be roughly half-way between 1^st and 2^nd contact. By 05:34 UT, it was possible to observe directly the silhouette of Venus against the solar disc by naked eye using mylar glasses (the eclipse glasses familiar to many as essential equipment for the UK solar eclipse in 1999). At 05:38:00 UT the teardrop effect became readily visible, although there was some suggestion that it had started earlier. By 05:38:40 UT, Martin Cook detected sunlight showing all around the disk of Venus, indicating that 2^nd contact had occurred. Initially this was rather indistinct, but by 05:39:00 UT, other observers in the dome could also discern sunlight completely surrounding the silhouette of Venus. By 05:39:50 UT, the last evidence of the teardrop effect disappeared, and Venus was clearly fully inside the solar disk.

Shortly after 2^nd contact, the general impression in the dome was that Venus presented a comparatively large disc, much larger than Mercury did during the TOM on 07 May 2003. The precise values of the apparent diameters, obtained from the NASA JPL reference ephemeris DE-405, are as follows: Mercury 12.0 arcsec, Venus 57.7 arcsec, almost five times larger. Figure 10 shows the relative sizes of Mercury on 07 May 2003 and Venus on 08 June 2004, from photographs at Orwell Park Observatory of projections of the solar disk. The individual images have been adjusted so that the radius of the solar limb is the same in each, and therefore the planets appear in the correct relative proportions.

Figures 11 and 12 are digital video stills taken by Garry Coleman during the ingress phase of the TOV. Figure 12 is especially interesting: it was taken at 05:38:26 UT, around the time of 2^nd contact. It shows no evidence of the teardrop effect, although the latter was quite apparent to the visual observers at this time.

Figure 13 shows a mosaic of photographs by Martin Cook during the ingress phase of the TOV, from shortly after 1^st contact through to 2^nd contact. They clearly show the development of the teardrop effect. (Note that due to difficulties pointing the camera, Venus is almost out of the frame in two of the images.)

3.4 Mid-Transit

The atmosphere in the observatory changed considerably once the ingress phase of the TOV was complete. By 7.00am (06:00 UT), several observers who had witnessed the ingress phase left to go to work and a steady stream of other members of OASI began to arrive, keen to witness the remainder of the TOV. Special mention must be made at this juncture of Ted Sampson and Les Lamb, who suffered a more stressful journey to the observatory than most. While travelling along the A14 towards Ipswich in Ted's car, a lorry, two vehicles in front of Ted, shed its load - of cushions! The car between the lorry and Ted braked sharply and unfortunately Ted collided with it. The lorry did not stop. When the police arrived on the scene to take details, they donated the cushions to Ted - very useful for the boat! Fortunately no-one was injured in the accident, but Ted's car was the worse for wear.

Orwell Park School was holding examinations all morning and, between exams, many parties of pupils and teachers called in to the observatory dome to see the TOV. A few parents also called in and as usual were impressed with the evident enthusiasm of the assembled observers!

At 8:55am (07:55 UT), James Appleton gave a five minute radio interview by mobile phone for the Mark Murphy Show on Radio Suffolk - see figure 14.

Between ingress and egress, the observers had the opportunity to examine in detail various aspects of the ToV. Figure 15 shows observers in the dome intently studying the projected image. Five aspects of the image were particularly noteworthy:

• Chromatic aberration. The poor quality of the army-surplus eyepiece became apparent on close inspection of the image! Chromatic aberration, first noticed around the rim of Venus by David Payne shortly after 1^st contact (see above), was apparent in the form of a chromatic ring surrounding essentially the entire circumference of the projected solar disk, most pronounced at the N and S poles. The ring was blue from W through N to E and yellow from E through S to W. Within the solar disk, the silhouette of Venus also suffered from chromatic aberration, with a yellow ring arc at its N pole and a blue/indigo ring arc at its S pole (i.e. colour fringe the opposite way round to the aberration surrounding the solar disk). Altering the position of the telescope slightly so as to change the position of the image resulted in a change to the colour rings. In practice, the chromatic aberration was relatively minor, and did not intrude unduly on the observations - see figure 16.
• Radial distortion. A second problem with the eyepiece was radial distortion. Towards the edge of the field, the figure of Venus appeared noticeably elliptical rather than spherical: it measured 15x18 mm in the projected image. During the TOV, Patrick Cook sketched the position of the planet against the Sun's disk every approximately 30 minutes (see section 4.7 below). The difficulties of sketching accurately the silhouette of Venus turned out to be greater than expected, so figure 17 is a re-drawn version of the original hand-drawn sketch which is generally faithful to the position and scale of the original but substitutes a circular Venus for the observed elliptical silhouette. It shows the path of Venus as a curved arc across the solar disk; the true path is a straight line (see e.g. the report by Nigel Evans below). The apparent curvature of the path of Venus is another artefact of radial distortion of the eyepiece.
• Circulating air currents. Close examination of the projected image of the Sun revealed an occasional small ripple circulating around the edge of the solar disk, sometimes clockwise, sometimes anti-clockwise, and sometimes stationary. The consensus of the observers was that this was an effect of air currents inside the Tomline Refractor, generated by solar heating. This explanation appeared to be the only viable one, but the observers were surprised at the apparent regularity of some of the ripples, and had difficulty reconciling this with the expected random motion of air currents.
• Mystery object! At approximately 09:50 UT, some observers in the dome saw a small, round object cross the solar disk in a period of some seconds. Observers using the 250 mm Dobsonian on the balconies at Orwell Park Observatory (see below) also saw the object. There was much speculation as to what it could be. The consensus of observers at the time was that the object was not an aeroplane as no wings were visible. Subsequent investigation revealed that it could not have been the International Space Station (which an observer in the Ukraine witnessed crossing the Sun during the TOV), as its orbit did not carry it over the UK at that time. Probably it was another satellite, but we will never know…
• Detail on solar disk. Although there was little of interest on the solar disk in terms of sunspots, some mottling (faculae) was apparent visually. Unfortunately, photographs of the image did not record it.

3.5 Egress

Some ten minutes before 3^rd contact, there were 18 people in the dome, all eager to witness the egress phase of the TOV. The door was locked to prevent late arrivals from disturbing the observers; however, the very first late arrival turned out to be Andrew Auster, Headmaster of Orwell Park School, so the observers had to make an exception and allow him entry!

Shortly before the predicted time of 3^rd contact, the observers adjusted the position of the Tomline Refractor to centre Venus in the projection, thereby overcoming the problems of chromatic and radial distortion noted above. As a result of centring Venus, the centre of the Sun was offset from the centre of the image and this risked concentrating the Sun's rays onto the tube of the telescope or the wall of the eyepiece: however, the practical effect was to raise the temperature of the eyepiece by only a very small amount, and no damage ensued.

At 11:03 UT, the teardrop effect became visible. As at 2^nd contact, it caused difficulty in accurate estimation of the time of 3^rd contact. Figures 18 and 19 show extreme instances of the teardrop effect around 3^rd contact - they clearly illustrate the difficulty!

At 11:05:43 UT, a white arc (aureole) became visible around the portion of the disc of Venus clear of the solar disk. The observers believed that this was caused by diffraction of sunlight through the atmosphere of the planet. The arc was most pronounced where it was close to the solar disk. It remained visible until 11:17 UT, although towards the end of this time it became less pronounced. Unfortunately, photographs of the projected image do not show the arc, although at its maximum, it was visually quite unmistakeable. Note that the observation of the white arc during egress was different to the observation of a chromatic ring (possibly indicating the presence of the Venusian atmosphere) around the silhouette of the planet during the ingress phase. In the egress phase, the image of the silhouette of Venus was central, so there was almost no chromatic aberration.

A reverential hush descended in the dome shortly before 4^th contact. Everyone was conscious of witnessing the end of one of Nature's great spectacles. Figure 20, taken shortly before 4^th contact, shows observers crowding around the projected image in anticipation.

Figures 21 and 22 are images of the egress phase of the ToV. Figure 21 comprises digital images by James Appleton while figure 22 has been assembled from photographs taken by Martin Cook with an Olympus OM10 camera. Both figures clearly illustrating the development of the teardrop effect.

Immediately after 4^th contact, a tumult of conversations erupted as observers compared their thoughts and observations of the TOV. Ken Goward popped open a bottle of bubbly by way of celebration, and several observers enjoyed a small glass - see figure 23. There was much congratulation over a wonderful set of observations!

3.6 Sketches Of The TOV

Patrick Cook drew a series of sketches recording the position of Venus on the solar disk throughout the TOV. He recorded the position of the planet every few minutes during ingress and egress phases, and approximately every half hour during mid-transit, both as a series of individual sketches and as a compound sketch. Figures 24 - 29 show Patrick sketching and the results of his labours (see also figure 17 above).

3.7 Timings

Observers in the dome at Orwell Park Observatory recorded the following event times. Timings were estimated from video recordings by frame-by-frame analysis, counted from reference frames displaying the Rugby time clock. Visual timings were estimated by direct reference to the Rugby clock, generally supplemented by the proverbial cry of There it is!
 

3.8 Observations From The Balconies At Orwell Park Observatory

In addition to using the Tomline Refractor in the dome at Orwell Park Observatory, members of OASI observed the TOV from the balconies of the Observatory using the following instruments (see figures 30 - 33):

• the OASI 250 mm Dobsonian reflector (stopped down to 180 mm and fitted with a mylar solar filter),
• the OASI Meade ETX125-EC fitted with metalised glass filter,
• Bill Barton's 50 mm refractor fitted with Hα filter.

The chief observers with the above instruments were Paddy O'Sullivan and Gerry Pilling (250 mm Dobsonian) and Bill Barton (Hα). However, in common with the observers using the Tomline Refractor, during the TOV many members of OASI, schoolchildren from Orwell Park School and their parents and teachers called in at the balconies and witnessed the TOV through the telescopes there.

The telescopes on the balconies provided images that were very much smaller than the 0.5 m achieved in the dome by projection, and the images were correspondingly much sharper and crisper. Interestingly, observers using the 250 mm Dobsonian and the Hα instrument reported very little teardrop effect, in contrast to observers using the Tomline Refractor. (There is no record of the degree of teardrop effect witnessed by observers using the Meade ETX.) The Hα telescope revealed negligible solar corona.

The following table lists the contact times estimated by the observers on the balconies, using a second radio-controlled Rugby clock as reference. Note, where multiple times are shown it is because the observers were not always sure precisely when the limb of Venus appeared to touch the solar limb. The term definite means that the event appeared with certainty to have occurred, therefore corresponds to a time likely later than in reality.
 

4. Nigel Evans Observing From Sharm El Sheikh, Egypt

Weather prospects for the TOV were considerably better in the Middle East than in the UK, and I therefore decided to take the Explorers Tours' trip to Sharm El Sheikh in the Sinai, Egypt. My usual astronomical escapes from the UK are to observe total eclipses of the Sun - but it is possible to think of the TOV as a special case of an eclipse with a magnitude of some 3% and a total area of obscuration of the Sun of only some 0.1%!

Some specialised equipment is needed to record the image of a TOV well. I chose to use two sets of equipment: a 1000 mm f/10 telephoto with a 2x teleconverter onto a digital camera (Canon 10D), and an ETX 90 with a Philips ToUcam webcam. Both were driven on a Vixen mount and would periodically record images of Venus in front of the Sun. I would need to be in attendance for the entire transit - some six hours.

In travelling to Sharm El Sheikh the probability of seeing the TOV was improved, but the daytime temperature was considerably higher than at home - somewhere around 35°C. So I would require shade, but this could not be arranged until arrival in Sharm. In addition, the webcam would need a laptop, which itself would need a mains power supply as batteries would not last six hours: so a 15 m extension lead went to Sharm as well. In using a laptop one of the problems was seeing the screen in very bright sunlight. Some means of shielding the screen was needed - hiding under a blanket did improve the view considerably, but it didn't seem viable in temperatures of 35°C or more! The solution was not finally arranged until in Sharm.

I spent the first day in Sharm checking out potential observing sites within the hotels. We stayed in the Hotel Mercure, adjacent to the Ocean Lodge, the main hotel, where the BBC had set up on the roof to relay live images of the TOV. Eventually I found a reasonable site in the Hotel Mercure - a patio not too far from my room with a power outlet. I also acquired a large cardboard box - more of that later. In the evening, Dr John Mason, BAA Public Relations officer, gave a very interesting talk on previous TOVs, mentioning that some expeditions had missed 1^st contact because they were looking at the wrong quadrant of the Sun...

The website http://www.nauticoartiglio.lu.it/almanacco/trans_venus_en.htm (now defunct) listed predicted contact details as follows for the location of the Hotel Mercure (27.87° N, 34.30° E).
 

Local time at Sharm El Sheikh was UT+3hrs and the Sun was at a much more convenient altitude for 1^st contact than in the UK.

Early in the morning of 08 June, about 4.45am, I taped the mount in place and aligned on Polaris - it seemed the easiest way to align to north! At that hour I suffered two experiences that should be missed:

1. the mosquito man spraying an unpleasant white mist around the hotel,
2. the lawns being watered.

Grass and water are rare in Sinai, so it was curious how the lawns of the hotel looked so green. Mystery solved - the lawns are watered with "sewage water" which has the aroma of stagnant drains!

I provided some shade by moving one of the poolside umbrellas - as the Sun moved across the sky I could shift the umbrella to keep me and the secondary equipment in the shade. The primary equipment, the two cameras, had necessarily to be in the Sun although they did have small shades to keep them out of direct sunlight. Now for more of the large cardboard box... The laptop screen could only be read when the screen was in shade. But, place the laptop in a large cardboard box, with suitable holes for access to the keyboard, leads and a porthole to see the screen and Voila! the screen can be seen in comfort. See figure 34.

At around 8.20am local time everyone was out to see 1^st contact. At cries of I can see it! I was looking at the laptop screen but could see no notch in the solar disk. A quick adjustment of the ETX revealed… that I was looking at the wrong quadrant! I wasn't the only one either - the BBC did the same thing - but fortunately my oversight wasn't on national TV! During the period from 1^st contact until beyond 2^nd contact I took a webcam sequence and a still frame every minute. Once 2^nd contact had passed most people faded away to have breakfast or generally keep in the shade. I needed to stay with the cameras so I was out on the patio for the duration; for six hours I (a) shifted the umbrella around so as to keep in the shade (b) every 10 minutes recorded a webcam sequence (the digital camera had an intervalometer attached) (c) drank lots of water!

As time went by, people popped out to see Venus on the Sun, or wandered round to see what equipment other folk were using. Having equipment of my own meant that I could not wander. Activity picked up towards 3^rd contact as everyone scurried out to set up their equipment to watch for the teardrop. A little amusement was occasioned at around 11:40 local time - at that point the Sun reached nearly 86° altitude making it very difficult for altazimuth mounted equipment to follow. At that time the thermometer showed a temperature in the shade of 42°C.

So how do my results look? I have assembled the digital pictures into several mosaics. Figure 35 is a mosaic of images taken with the Canon 10D and 1000 mm lens with the 2x teleconverter, with a 20 minute interval between the images. Figure 36 is similar, but 10 minutes out of step with the former. Figures 37 and 38 are similar mosaics of images taken with the ToUcam/ETX90. Figure 39 is a mosaic of images captured at about one minute intervals during ingress, created by stacking the best frames from the ToUcam/ETX90. Figure 40 is analogous for the egress phase. Note that the field of view of frames taken with the ToUcam is much smaller than those taken with the 10D.

I think that imaging Venus is really beyond the capabilities of the 1000 mm lens - the images of the planet are not pin-sharp. The webcam has the potential to give sharper images, by choosing the sharpest frames and stacking them using a program such as Registax. However my images are degraded because the ETX was slightly out of alignment. Although there were no clouds in the sky, the seeing was actually very poor. This was particularly evident in the webcam images - the silhouette of Venus kept dancing. No doubt being on a brick patio didn't help, but given what I knew about the grass areas in the hotel, a brick patio was ther preferable location!

Note that the ingress and egress sequences show very little evidence of the teardrop effect.

On the evening of 09 June we had an opportunity to go into the desert to see the night sky from 28°N and, being June, this promised some fine views of Scorpius and Sagittarius. I duly set up my mount and did a test frame on Scorpius - to my horror the stars were trailed! A check showed that the mount was powered but the motor was not moving when set to normal speed, but it would move at 16x speed. It could have been worse - it could have broken the day before! Towards the end of the evening I was able to attach the Canon 10D to Nick James's mount and make a series of pictures with an 8 mm lens - see figure 39.

5. Paul Whiting Observing From Sharm El Sheikh, Egypt

I observed the TOV from Sharm El Sheik in Egypt. As events transpired, the whole transit was viewable against perfectly clear, blue skies both at Orwell Park Observatory and in Egypt, so the only "advantages" of travelling to the Sinai Desert were to experience a temperature of 47°C (in the shade) and to enjoy another two hours extra time in bed before 1^st contact!

In Sharm El Sheikh there was a fair representation of OASI members among the contingent of 280 or so British and one American who had taken over the erstwhile Red Sea divers' stronghold, the Ocean Lodge Hotel, and indeed three neighbouring hotels. The Ocean Lodge Hotel was to be our base for the next 10 days.

The first few days in Egypt included a quick trip to Cairo and the pyramids of Giza. Here we met the vendors and cries of: You Inglish? Here's a free gift for your wives! You give me a gift now. The gift requested usually meant the green folding stuff. Backsheesh was a word we came to dread. If anyone did ANYTHING for you they demanded money. Either that or they "accidentally" forgot to give you your change, no matter how large the note you gave them.

And so back to Sharm and the transit. While we had been away the BBC had arrived and installed a satellite dish to beam back to the UK a view from a Hα telescope mounted on the roof of our hotel. The dedicated astronomers had set up their telescopes, cameras and computers the night before the TOV, locating them on the various roofs around the hotel. Dr John Mason gave his usual pre-event talk (well known to Explorers Tours regulars) and mentioned a magnitude -9 Iridium flare, nearly overhead at 4.00am on the morning of the TOV. It was a measure of people's dedication that only three people of the approximately 280 in the Ocean Lodge bothered to get up to see it: me, my partner Diana and John Mason. Call themselves astronomers? Pah!

On 08 June, we mere mortals with basic equipment - in my case a Solarscope - set up after a leisurely breakfast around the pool area. The Solarscope provided an easily viewed 12 cm image, which readily showed the silhouette of Venus and the teardrop effects at 2^nd and 3^rd contacts, although the Sun was too high in the sky to see 3^rd and 4^th contacts without a deal of limbo dancing. Many folk who had brought only eclipse viewers crowded around the Solarscope image, and this proved to be a good way of passing the boring phase in the middle of the transit.

The output of the BBC image was distributed around the hotel on TVs - watching this in a Bedouin tent added a certain something to the experience. The BBC did some filming around us and even interviewed me but nothing was broadcast. Apparently we were not excited enough at 1^st contact.

After the transit there was more time to relax by the pool and a trip to Luxor. Here we experienced the Valley of the Kings, a balloon ride over the Nile, more hieroglyphs, temples and the taxi drivers. The taxi drivers were amazing. If you went out of the hotel and walked for more than 3 m they came after you. We explained that we were only crossing the road to get to the river bank. The reply was, Then where are you going?! And if the first driver gave up, the next in line took over, thinking that we obviously wanted a taxi but didn't like the first guy.

Final thoughts on Egypt? OK the vendors and taxi drivers were a nuisance but the people were very friendly, and once you got used to having armed guards wherever you went, the scenery and pharaonic heritage were stunning. Even the heat was bearable with the right clothes and gallons of water. I'm glad I went, but I do regret missing observing the transit through the Tomline Refractor with its historic links to Airy and the transit in 1882. Figures 42-51 record my trip and observations.

6. Dave And Ann McCracken Observing From Skellingthorpe, Lincs

We observed the TOV and estimated times of contact from Skellingthorpe, Lincs. In mid-transit we moved our equipment temporarily to the Queen Elizabeth School in Gainsborough, and from 8.00 to 9.45am showed the TOV to groups of schoolchildren using both direct viewing and projection. Atmospheric transparency was initially very poor, with clouds and haze and clear periods, but the sky was clearing towards the end of the TOV.

We used two telescopes:

• A Meade ETX70 refractor fitted with full aperture solar filter. 70 mm aperture, 350 mm focal length. Equatorially mounted. Motor driven. Nikon FM camera with photoadapter and x3 Barlow lens used to record images on Fujicolour Superia 200 ASA film.
• A Celestron WA80 (Wide Angle, 80 mm aperture) refractor used to project the solar image onto white card. 400 mm focal length. Altazimuth mount. Not driven. Nikon FM camera with Fujicolour Superia 200 ASA film used to capture images.

The following is a summary of our observations.

We did not observe the teardrop effect. Our estimates of contact times are below (UT), obtained using an Oregon radio controlled watch and a digital stopwatch, with no allowance for personal reaction time:

• 1^st contact 05:21:56
• 2^nd contact 05:39:08
• 3^rd contact 11:03:31
• 4^th contact 11:22:31

Figures 52-58 show our equipment and results. Figures 52-54 show our telescopes. Figure 55 is our first clear photo of the TOV, taken with the ETX70 at 06:24:30 UT, once the initial haze had dispersed. Figures 56-59 show later stages of the transit. Figures 52-57 were taken at our home; figures 58 and 59 were taken at Queen Elizabeth High School. (The ruler visible in figure 58 is for measuring the scale of the image as part of project work by pupils at the school.)

7. Peter Grimer Observing From Woodbridge

I observed from Woodbridge together with a friend, Rev Fr Peter Wynekus. We adopted four approaches to observation, described below; see also figures 60 - 63. We did not time our observations.

1. Naked eye using a no. 13 welder's glass. We were able to identify Venus in transit, and were surprised at how small it appeared - nowhere near as large as some sunspots I have seen by this method in recent years.
2. Direct vision through a 7x50 World War II gunsight with welder's glass permanently fixed over the OG. This method provided a very clear view of the TOV, with terrestrial image orientation.
3. Solarscope. The Solarscope is a very good, inexpensive instrument (circa £45 at Broadhurst, Clarkson and Fuller's) which uses a long-focus, non-achromatic objective lens without an eyepiece, but with a surface-silvered convex mirror to reflect the image. I missed 1^st contact because I was searching for it in the wrong quadrant of the Sun. (Unfortunately, I did not have the opportunity of testing the Solarscope in advance to establish the quadrant in which Venus would first appear.)
4. Projection in my observatory using a home-constructed 76 mm refractor (Swift Acro objective with focal length approximately 900 mm coupled with a 26 mm Kellner eyepiece). This provided very good views. At no time could I discern the teardrop effect. Even though I kept the lens cover on the objective when not in use, I was surprised and rather alarmed at how hot the whole instrument became!

All in all it was a most enjoyable morning's observing, benefiting immensely from the perfect weather.

In commemoration of such a successful set of observations by so many members of OASI, Peter subsequently commissioned a limited edition run of plates depicting the ToV, available for purchase exclusively by members of OASI.

Note that a welder's glass does not provide full protection against solar radiation and its use as described above is not recommended. Safe methods to observe the Sun are via projection or via a certified solar filter.

8. Mike Harlow Observing From Bucklesham

Although a frequent traveller to observe astronomical phenomena from exotic locations, Mike Harlow stayed at home (Bucklesham, near Ipswich) to observe the TOV. Figures 64 and 65 show his observing equipment, and figure 66 shows a sample of his results, Venus in mid-transit.

9. Alan Smith Observing From Grundisburgh

I carried out a trial observing run during the week before the TOV (using the predicted event times). This demonstrated that the best place to observe 1^st and 2^nd contact through to mid-event was from inside my garage, which happened to face directly towards the rising Sun. It also showed that from mid-event onwards, the observational angles would make it necessary to move the telescope out of the garage, and that the wind would probably freshen around mid morning, necessitating a windbreak.

My equipment consisted of a 7.5 cm refractor atop a home-made equatorial mount based on a World War I gunsight driven by a stepper motor. I used eyepiece projection onto a translucent screen permanently mounted behind the eyepiece (figure 67). The screen was at the end of a lightweight framework which was covered in paper to keep out spurious light. I attached a large but very lightweight screen to the OG end of the telescope to cast a shadow backwards over the whole assembly (figure 68). The image of the Sun on the screen was about 150 mm in diameter, and I video-taped it from a camera externally mounted on a tripod (figure 69). I observed the TOV visually (using the screen) and "on television" using a TV set to display the output of the camera (giving an effective image of about 48 cm diameter). I also used the video camera to record an audio track as event log.

I assembled and tested the equipment in the garage the night before the TOV. On the day of the TOV itself, I was up very early and had the equipment calibrated, running and ready to make observations by 5.30am (04:30 UT). With less than five minutes to go before 1^st contact, the electricity supply to the whole street failed! After a few seconds panic, I realised what had happened and fortunately, the power soon came back. However, all the observing hardware had reset! The audio log at this time should not be heard in the company of young children! A few desperate minutes later and I was able to resume observations.

Despite having worked out in advance which quadrant of the Sun would show 1^st contact, I was persuaded, in a pre-event phone call with another member of OASI, that I was wrong. Accordingly, I concentrated on the solar limb 180° opposite the original point and waited for 1^st contact. Yes - I had been right the first time! So, like some others I missed 1^st contact, but later concluded that in any case my equipment would not have revealed it until some time later.

Once the trauma of the ingress phase was complete, my observations proceeded more smoothly. As indicated by the trial observing run, after I moved the telescope out of the garage at mid-transit, the wind did freshen and I had to erect various ad hoc windbreaks.

Throughout my observations, I recorded timings with the onboard camera clock synchronised to a Rugby radio clock. I ensured that the video recording captured the Rugby clock in many frames throughout the observation to facilitate any adjustments necessary to the camera's timing after the event. In practice, the camera clock was perfect.

Apart from missing 1^st contact, everything worked perfectly and I was able to record the whole event on 8 mm and VHS tapes. Figures 67 - 71 show aspects of the observations.

Alan estimated the following contact times (UT). Visual estimates were obtained by off-line analysis of the event log on the audio channel of the video tape, Video estimates were obtained by frame-by-frame analyhsis of the video tape.
 

10. Ted Sampson Observing From Offton

Having seen my neighbour at Castle Lane, Offton, trying to project an image of the TOV onto a hand held card, with hand held binoculars, all without success, I quickly assembled my binoculars (7x50) onto a camera stand, setting up in my front garden. From about 8.30am until I set off for Orwell Park Observatory at 9.30am, six or seven of my neighbours came to have a look - some summoning others by phone to come round and see. Not very dramatic, but made for a bit of local interest!

11. Harold Watters Observing From Woodbridge

Harold Watters attended Orwell Park Observatory for the ingress phase of the ToV, and later returned home to Woodbridge, where he captured the following mid-transit image with a handheld digital camera at the eyepiece of his Meade ETX90.

12. Analysis Of Event Times By Horrocks' Method

While sketching the position of Venus against the solar disk (see above), Patrick Cook measured the diameters of the projected disks of the two bodies as 15 mm and 511 mm respectively. (In fact, the measurement of Venus' diameter quoted here is taken at 90° to the radius to the centre of the field of view; measuring in this way should avoid inaccuracy due to the radial distortion of the eyepiece). The following are treated as known quantities:

Using Patrick's measurements with the above data, Horrocks' method gives an estimate for the Earth - Sun distance of 124,000,000 km. The fact that it is more accurate than Horrocks' estimate of 1639 is likely due to the use of accurate modern estimates of the quantities tabulated above.

13. Analysis Of Event Times By BBC/OU Calculator

The BBC and OU (Open University) ran a website supporting observations of the TOV. It hosted a calculator for estimating the Earth - Sun distance based on observers' timings of 3^rd contact. The calculation is based on De Lisle's method and utilises an additional, unspecified timing of the TOV made in South Africa.

The following table lists estimates of the Earth - Sun distance calculated by the BBC/OU website from timings of 3^rd contact by OASI observers. It lists estimates in km and as a percentage of the currently accepted value (CAV).
 

Unfortunately, a question mark hangs over the accuracy of the calculator on the website. To see this, consider the following:

1. The USNO provides definitive predictions of astronomical phenomena, based on the latest and most accurate ephemeris data, making them available on its web site. The USNO gives a predicted time of 3^rd contact at Orwell Park of 11:04:03 UT.
2. At the time of 3^rd contact predicted by the USNO, the Earth - Sun distance, computed by the NASA JPL reference ephemeris DE-405, was 151,851,436 km (1.0150642 AU).
3. The BBC/OU web site gives an Earth-Sun distance of 150,460,000 km (0.99084 AU) for the USNO's predicted time of 3^rd contact. This is clearly different from both the currently accepted length of the AU and the instantaneous Earth-Sun distance (from DE-405) at the time.

The above discrepancy prompted me to re-examine techniques for estimation of the AU from observers' estimates of contact times.

14. Analysis Of Event Times By Direct Method

Using a modern personal computer (PC), it is possible to apply a more direct approach to estimating the Earth - Sun distance than Halley or De Lisle and to calculate an estimate of the AU directly from every single recorded contact time. A modern PC can utilise a high-precision reference ephemeris (e.g. DE-405) to compute an accurate prediction of the four contact times for the observer's location. By repeating the calculation for different assumed values of the AU, it is possible to compile a table of predicted contact times as a function of the latter. To estimate the AU, the observer need only look down the table and select the assumed value which provides a predicted contact time equal to the measured contact time.

Note that the above approach estimates the AU directly, not the instantaneous value of the Earth - Sun distance like Horrocks' method. Further, it is based upon accurate values for all other astronomical quantities involved in the calculations, with the only unknown being the value of the AU. It therefore offers the promise of a more accurate estimate than other approaches which rely on estimation of intermediate quantities together with the AU itself.

I applied the above approach to the OASI estimates of contact times of the TOV, using the algorithms given in [3] and the ephemeris DE-405 to compute predictions of the contact times for each observer's location. I first performed two checks as follows on the basic accuracy of the direct method using predictions from the USNO as reference data. Both checks showed good agreement with the reference data.

1. Comparison of predicted contact times from the USNO with those of the direct method (based on the currently accepted value of the AU) for Orwell Park Observatory. The following table lists the results, with a worst-case discrepancy of two seconds.

2. Application of the direct method to the time of 3^rd contact predicted by the USNO for Orwell Park Observatory (11:04:03 UT). This gives a value for the AU of 149,590,400 km, equal to 99.995% of the currently accepted value, clearly very close to the nominal value of 100%.

The following table gives the results, listing the estimate of the AU, expressed as a percentage of the currently accepted value, corresponding to each observer's recorded event time. Figure 73 is a graph of the results.
 

Estimates associated with 1^st contact show the greatest range. However, this is largely due to the presence of two outliers; if they are excluded, the estimates associated with 1^st contact would, in fact, show the smallest range. The outliers, which approximately cancel one another out, are a high-side visual estimate and a low-side video estimate. The latter corresponds to the timing by Alan Smith obtained by backwards extrapolation of later video frames (see above for an explanation of why Alan perforce adopted this approach). The process of extrapolation appears to have introduced some inaccuracy. All but one of the estimates associated with 1^st contact are too large: this corresponds to estimates of the event time which are too late, probably due to the need for the disk of Venus to partially overlap the solar disk before it is visible to the observer.

Estimates associated with 2^nd contact show a significant range with, arguably, two outliers (one low-side, one high-side). All estimates except the high-side outlier are low, corresponding to estimates of the event time which are too early. Some observers reported the teardrop effect around the time of 2^nd contact (see figure 13) and this was likely responsible for the wide spread of results.

Estimates associated with 3^rd contact show the smallest range. This may be in part because most observing projects associated with the TOV (including the BBC/OU project) were associated with 3^rd contact, and observers therefore focussed most of their efforts on estimating it accurately. Further, by the time of 3^rd contact, most observers had already witnessed 1^st and 2^nd contact and were reasonably confident in what to expect, which again helped to improve the accuracy of timings. However, the teardrop effect was again very noticeable around the time of 3^rd contact (see figures 18-19) and this created difficulties in estimating the contact time accurately.

Estimates associated with 4^th contact are all too large, which corresponds to estimates of the contact time which are too early. This suggests that the observers were too hasty in judging the instant at which the last segment of Venus left the solar disk.

In general terms, estimates based on video recordings of the event were not significantly more accurate than those based on the best visual timings. This result is somewhat unexpected, as prior to the TOV there had been much interest in using frame-by-frame analysis of video recordings to obtain highly accurate estimates of contact times. It had been hoped that the video frames, of very short duration, would not be degraded by atmospheric turbulence and would exhibit sharp images of the Sun and Venus. This would enable the use of interpolation over several consecutive frames to obtain an accurate estimate of the time of contact. However, in practice, video recordings turned out to suffer greatly from atmospheric turbulence: the limbs of the Sun and Venus were indistinct and suffered from considerable frame-to-frame jitter. Estimates of contact times based on video recordings suffered from as much subjectivity as did those of visual observers. The video sequences in figures 74-76 illustrate the difficulty. They are respectively clips of a few seconds duration around the times of 2^nd, 3^rd and 4^th contacts, extracted from David Payne's video recording. Note the jitter in the solar limb and in the position and shape of the silhouette of Venus. (Details of the clips are as follows: figure 74: 2^nd contact, 06:42:47 - 06:43:05, 448 frames; figure 75: 3^rd contact, 12:07:37 - 12:08:08, 762 frames; figure 76: 4^th contact, 12:26:24 - 12:27:03, 982 frames. Frame rate: 25 per second.)
 

Averaging all the OASI estimates above gives an estimate of the AU of 149,622,000 km or 100.02% of the currently accepted value. Clearly, a very creditable result for a group of amateur astronomers!

15. Teardrop Effect

The teardrop effect can render timings of 2^nd contact and 3^rd contact very inaccurate and thereby vitiate estimates of the length of the AU. The cause of the phenomenon has been something of a mystery since it was first observed during the TOV in 1761. Astronomers have ascribed it to the presence of an atmosphere around Venus, disturbance in the Earth's atmosphere, imperfections of the eye, poor instruments, diffraction effects, and combinations of factors.

The following table summarises the experiences of the teardrop effect reported by OASI observers.
 

On the basis of the OASI observations, atmospherics and optical quality would appear to be prime determinants of whether or not the teardrop will be visible.

At the BAA Exhibition Meeting on Saturday 26 June 2004, Nick James presented a series of photographic results of the TOV observed from Sharm El Sheikh and in the subsequent discussion it was agreed by all that those who observed the event with really sharp (state of the art) optics both in Sharm and in the UK saw little if any teardrop effect. This view appears to coincide with the experience of the OASI observers. However, there is as yet no generally accepted, definitive explanation of the teardrop phenomenon.

16. Concluding Remarks

OASI observers of the TOV on 08 June 2004 count themselves fortunate indeed that most enjoyed near-perfect weather conditions which enabled observation of the TOV in its entirety. It is to be hoped that observers of future TOVs will be as fortunate. Details of forthcoming ToVs:

• 06 June 2012. The second ToV of the current pair, a descending node transit. Unfortunately, only the final hour of this TOV will be visible from the UK, after Venus rises at approximately 03:45 UT.
• 11 December 2117. An ascending node transit which will not be visible at all from the UK.
• 08 December 2125. The ingress phase of this TOV will be visible from the UK.
• 11 June 2247. Will be visible from the UK in its entirety.

Orwell Park Observatory has an association with the TOV of 1882 through J I Plummer, resident astronomer at the observatory 1874 - 1890, who led an official expedition to observe the phenomenon from Bermuda. The observatory also has the association as described above with the TOV of 2004. It is intriguing to speculate on the part that the facility may play in future TOVs. The saying of William Harkness of the USNO following the ToV of 1882 has a resonance in relation to future TOVs:

There will be no other [TOV] till the twenty-first century of our era has dawned upon the Earth and the June flowers are blooming in 2004. What will be the state of science? God only knows.

Additional images of the ToV by members of OASI.

17. Acknowledgement

Thanks are due to the following members of OASI who contributed to this article: Garry Coleman, Martin Cook, Patrick Cook, Nigel Evans, Roy Gooding, Ken Goward, Peter Grimer, Mike Harlow, Monica Lustig, Dave McCracken, Neil Morley, David Payne, Paddy O'Sullivan, Gerry Pilling, Dave Robinson, Ted Sampson, Alan Smith, Harold Waters, Paul Whiting.

18. References

James Appleton

Original: Newsletters October, November, December 2004; January, February, March 2005
Updated: 26 March 2023