Figure 1. Star field around Pluto, 1984.
Orwell Astronomical Society (Ipswich)
Pluto, 26 April 1984 - 18 May 1986
The best time to observe Pluto, the outermost planet1 of the solar system, is when it as opposition (opposite the Sun in the sky). Successive oppositions are separated by a period of a little over a year and, in the mid-1980s, occurred in late April. At this time, with the planet close to perihelion on 05 September 1989, its opposition magnitude was 13.7, visible in the 26 cm Orwell Park refractor.
My attempts to observe this most elusive world are summarised below.
Opposition in 1984 was on 20 April and, during the following week, we were blessed with excellent weather and clear evening skies, making it an opportune time to undertake the search. I began the search on 25 April at Orwell Park, together with Stewart Dedman and Darren Payne. We started at about 21:30 UT and used the excellent finder chart in Astronomy magazine. We first identified with binoculars the pattern of brighter stars in the vicinity of Pluto, then used the 7.5 cm finder on the Orwell Park refractor to point the telescope to the predicted position of Pluto, and then searched for the body itself with the refractor, using an eyepiece giving x70 magnification and a field of view of around ½°.
Figure 1 shows the star field around Pluto on 25 April. It was difficult initially to identify the stars visible in the 26 cm refractor with the stars shown in the finder chart. We first identified the obvious asterism of the triangle of stars (in the dotted box, below) and then "star hopped" to position the field of the refractor over the predicted position of Pluto. We then increased the magnification to x150 in order to darken the sky background. The circle on the diagram below shows the resulting field of view. The seeing was clear and steady (although there was considerable glow from the lights at Felixstowe Docks). The star numbered 1 on the chart was the most easily seen, and the star numbered 2 was next most easily seen, together with the object marked A. The latter was not shown on the finder chart - so was it Pluto? Careful observation of the star field revealed the star numbered 3 but we were unable to see the star marked 1a on this occasion.
Although we were fairly certain that object A was Pluto (Stewart and Darren agreed with me on this) it could only be confirmed positively by further observations on successive evenings, to see if it moved as expected. Luckily the clear weather held and the following night (26 April) was clear. I made a further observation with my 25 cm reflector at home. I found the area of sky quickly, easily recognising the star patterns from the previous evening. I could easily see stars 1, 2, 3 and 1a. Object A had moved to position B, confirming that it was indeed Pluto!
I was unable to observe again until 29 April, by which time, Pluto had moved a considerable distance to the position marked C on the chart and now formed a faint, close pair with a star. Unfortunately, the following day, the weather deteriorated, remaining poor for many days, and I was unable to confirm which of the two points of light at position C was Pluto. I believe it to be the one nearest to the A and B positions but cannot be certain.
Pluto was clearly visible with direct vision using my 25 cm reflector on Thursday evening and would probably have been visible in a 20 cm reflector with averted vision.
Figure 1. Star field around Pluto, 1984.
Abysmal weather during April 1985 considerably hindered attempts to observe Pluto. I made a couple of attempts to observe it in early April but failed due to hazy skies. My first successful observation of the planet in the year was with the Orwell Park refractor on 24 April, one day after opposition. Two other members of OASI, Roy Lobbett and Stuart Dedman, observed with me and we found it difficult to positively identify the body.
Unfortunately the following night was cloudy, preventing a second observation to confirm that the object observed was indeed Pluto and not a field star. Continuing bad weather and lack of opportunity delayed further observations until late May. On 24 and 25 May, using my 25 cm reflector I managed to make two definite observations of Pluto. (I did try for a third observation on 26 May, but was thwarted by moonlight and poor seeing.)
The star charts below show the location of Pluto as follows.
Figure 2. General position of Pluto, from Norton's Star Atlas. Red rectangle corresponds to star field in fig. 2.
Figure 3. BAA finder chart for Pluto. Red rectangle corresponds to star field in figure 4.
Figure 4. Plot of stars surrounding Pluto, 24 & 25 May 1985. (David Payne.)
The middle of May 1986 brought some good, clear skies that were ideal for searching for Pluto. I started the search on the night of 15 May. I began with binoculars and easily located the magnitude 6.5 star2 near the centre of the BAA finder chart shown in figure 5; this star is approximately 2° west of the naked eye star 109 Virginis and is easy to find with the aid of Norton's Star Atlas or Sky Atlas 2000. I centred my 250 mm reflector on the magnitude 6.5 star. Using a low magnification eyepiece (x80 Erfle) I was able to identify the fainter stars in the area surrounding the Pluto. The Moon was approaching first quarter and proved troublesome so, having located the area where Pluto was located, I increased the magnification to darken the sky background and increase contrast. I used a range of magnifications from x80 to x280 to draw the star chart below. Unfortunately, because I had convinced myself that I knew the position of Pluto, I drew only the stars close to the position of interest, and this turned out to be wrong! I had thought that Pluto was close to the star marked "A" and, because I could see several stars in addition to those on the finder chart, I thought that one of them must be Pluto. So, preconception and careless plotting meant that I did not observe Pluto on this occasion; it was in fact situated a few minutes of arc further west. Unfortunately I didn't discover my error until the following day when I re-checked the star charts.
The following night was also clear, but the Moon was older and its light more troublesome. I quickly found the appropriate area of the sky and again using various magnifications was able to extend my star chart westwards. I found several stars visible in the telescope which did not appear on the finder chart: one of them must be Pluto!
The next night was cloudy and I was unable to confirm the observation. One night later still, 18 May, was again clear. The Moon was a problem but with high magnification I could discern the faint stars seen on previous nights (albeit with great difficulty!) and one of them had moved - this had to be Pluto!
Figure 5 shows the BAA finder chart for Pluto. Figure 6 recreates my sketches of the star field made over the three nights 15, 16 and 18 May. The rectangle shown in figure 5 corresponds approximately to the area of figure 6. During this period, OASI member Mike Harlow took a photograph on 15 May of the star field around Pluto which recorded the planet. Unfortunately, the photograph would not reproduce well so is not shown below.
Figure 5. BAA finder chart for Pluto.
Figure 6. Sketch of the star field around Pluto, 15, 16 and 18 May 1986.
[1]
At the time, the solar system was understood to contain nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. On 24 August 2006, the IAU redefined the term planet in a way which excluded Pluto and reclassified the body as dwarf planet 134340 Pluto.
[2]
Approximately a decade after David's observations, the star was catalogued as Hipparcos 71510.
David Payne