Orwell Astronomical Society (Ipswich)
Guide to Observing Venus
Venus is our nearest planetary neighbour and the most conspicuous of the planets. It is the second planet in order out from the Sun and, existing within the orbit of the Earth, passes through a cycle of phases like the Moon. But the phase is almost the only feature visible through an amateur telescope. All Venus ever shows to the optical astronomer is the upper deck of a thick layer of cloud, beneath which the solid globe of the planet rotates unseen; the cloud deck appears bright, slightly yellowish, with only the most subtle of detail apparent.
Confounded by a brilliance that dazzles the sight and exaggerates every imperfection of the telescope (Sir John Herschel) it is not unknown for the would-be observer of Venus to give up in sheer frustration and turn to a more amenable object. And yet, under favourable conditions when its brilliance is tempered by haze or a man-made filter, Venus is a different subject and the practised eye is quick to spot a pattern of shadings, less distinct than those of Mercury or Mars but as certain. With experience, still more will be seen; scrutinising the image, noting every nuance of shade and irregularity, accurately recording that which is affirmed, indicating that which is elusive, is a gradual process. Patience, care, determination and a methodical approach are the elements which in time will unite into a knowledge of the object which transcends textbooks.
By definition, an inferior planet such as Venus can never be seen all night long. The maximum angular separation of Venus from the Sun is 47° but, when the planet is well placed, e.g. when greatest elongation east occurs near the vernal equinox, it may be visible for more than four hours after sunset for observers resident at northern temperate latitudes. Near this date the ecliptic presents a steep gradient to the western horizon. A similarly favourable presentation in the morning sky occurs when Venus comes to greatest elongation west near the autumnal equinox. Conversely, when greatest elongation east occurs near the autumnal equinox the ecliptic makes a shallow gradient to the horizon and Venus is poorly seen in the evening sky; when greatest elongation west occurs near the vernal equinox; the ecliptic makes a shallow gradient to the horizon and Venus is poorly seen in the morning sky. The situation is reversed for observers in the southern sky.
The period between superior conjunction and greatest elongation is about 31 weeks; that between greatest elongation and inferior conjunction is about 10 weeks. The apparent diameter of the disk ranges from about 10 arcseconds at superior conjunction to over 60 arcseconds at inferior conjunction but it should be remembered that increasing diameter of the disk means a decreasing phase. Venus reflects 76% of the incident sunlight which explains the brilliant spectacle that it makes in the twilight sky. The planet reaches magnitude -4.5 at greatest brilliancy, 35 days after eastern elongation and 35 days before western elongation. The brilliance of the disk means that telescopic observation of the planet in a twilight or dark sky is hampered by glare which subdues subtle detail. For this reason, Venus is best observed against a bright sky foreground: sunset or sunrise are ideal times. During the winter months, Venus may be usefully observed at any time during daylight hours but this is not practical during the summer when the seeing is often poor while the Sun is above the horizon.
An equatorial mount fitted with setting circles will be an advantage in this respect. There are various methods by which Venus, when an evening object, may be found using setting circles but perhaps the easiest way is simply to sweep in the planet's declination, the planet coming into view in the finder. When a morning object, Venus may be found first with the naked eye and followed through the telescope as dawn gets under way. However, whatever method is used, extreme caution should be exercised when Venus is in proximity to the Sun.
Features sometimes visible on Venus.
The observed phase may differ slightly from the predicted value, the phenomenon being known as the Schröter effect. Dichotomy is seen to occur a few days early at eastern elongation and likewise late at western elongation. There are three ways in which the observed phase may be determined:
Care should be taken when attempting to determine the phase from a photograph as the outline of the planet varies with the duration of exposure and the processing.
Although often appearing completely devoid of detail to the novice, with patient practice subtle brighter or shaded areas may be discerned on the disk. The terminator often appears shaded and diffuse but bright areas have been observed. The apparent poles are often bright, these being known as the cusp caps, and these in turn may be bordered by dark cusp collars. All these features are nearly always of an ephemeral nature and an intensity scale can be used to describe them. The scale, due to Patrick Moore, is as follows:
It is a good idea to record intensity estimates on a sketch. Cloud shadings recorded carefully can often show the four-day retrograde rotation of the atmosphere.
Night-side events on Venus can include:
Both appearances are accessible to modest telescopes but the latter is so feeble that great care must be exercised in its observation. The illuminated atmosphere usually shows after greatest elongation east. It consists of faint cusp extensions that gradually creep around the dark limb until at inferior conjunction they link to encircle the planet in a ring of pearl-like light. The cycle is reversed after inferior conjunction.
The visibility of the dark side is divided into three phases:
This is an irregularity in the apparent outline of the telescopic image. The terminator can have an uneven curvature and the cusps can have a slightly different shape (especially around dichotomy when a blunting of the southern cusp is regularly seen). Projections are occasionally seen along the limb but more often down the line of the terminator. A careful watch should be maintained and the observer should not expect to see the geometric norm.
Colour filters form a significant extension to the programme of the Venus observer. At the time of writing (early 1990s), the range most widely used is the Kodak Wratten series of gelatine-based filters. These come in various colours and densities and may be conveniently cut to fit eyepiece filter adapters, etc. Filters are useful in three areas:
Sketches of Venus by OASI member Carl Cornish in 1983 (from our archives!) illustrate several of the above features.
Astronomical photographic techniques can be applied to Venus with successful results. The observer should experiment with a range of exposures to determine what is best for his or her individual method and instrumentation. The techniques of prime-focus and eyepiece projection are the most popular.
Ultra-violet (UV) photography is difficult but may be attempted. The rewards for success are substantial, since Venusian cloud features, delineated by UV absorbing compounds in its atmosphere, become apparent. The main difficulty is caused by the absorption of UV by ordinary glass. Thus UV photography must be undertaken at the prime focus or with a quartz eyepiece (available from suppliers as single-element eyepieces). For optimum results a UV filter should have its maximum transmission between 320 nm and 370 nm. The Kodak W18A filter, among others, has this characteristic. Ordinary emulsions such as Kodak Panatomic X, Plus-X and Tri-X are sensitive in the UV.
Observers wishing to pursue further observations of Venus should consider joining the BAA Mercury and Venus Section.
Adapted from an article by D Graham, R M Baum, J L Benton, Jnr, J Nichol & J McCue