Other Planets Seen Through the Telescope: Venus Jupiter Saturn Uranus

Venus Through the Telescope

by Martin J. Powell

A lthough any telescope (or steadily-held binoculars) will allow the phases of Venus to be seen, the planet is a notoriously difficult one to observe, for three main reasons. Firstly, whenever the planet is visible (shortly before sunrise or after sunset) it is positioned at a low altitude (angle above the horizon) where it is immersed in haze and atmospheric turbulence, which adversely affects the quality of the image seen in the telescope. Consequently most telescopic observers prefer to view the planet in full daylight - when its altitude is much higher - taking special care to shield the Sun from view. Secondly, the planet's brilliance - although making it a very obvious and beautiful object to the naked-eye - causes a menacing glare when seen through a telescope. Thirdly, the planet itself is permanently cloaked in thick cloud, so its surface features are never visible through telescopes. Most amateur astronomers can therefore only expect to observe its characteristic phase changes, but little else.

Telescopic Venus A simulated and much-magnified view of Venus as it appears through a telescope under moderate viewing conditions. The planet is seen in crescent phase, some 15% illuminated (phase = 0.15). Two more simulated views of the planet are shown below (all views on this page are shown at the same scale). The planet was filmed on May 15th 2004, during an evening apparition, when Venus was shining brilliantly in the Western sky after sunset, at an apparent magnitude of -4.5. The unsteady appearance of the planet through the telescope is caused by a combination of turbulence in the Earth's atmosphere and rising heat currents from the ground and buildings (for more details see the section ' Seeing Conditions ' below ). The images were obtained by pointing a tripod-mounted videocamera directly into the eyepiece of an 8-inch Schmidt-Cassegrain telescope; the eyepiece view was then magnified. The image is orientated with North upwards and towards the right, which approximately matches the Northern hemisphere view through a terrestrial telescope.

Experienced observers using medium and larger-sized telescopes often observe detail in its cloud features by attaching ultraviolet (UV), infrared (IR) or colour filters to the telescope eyepiece. In the latter category, yellow , blue and violet filters help to reduce the glare of the planet and improve the contrast of its cloud features whilst orange and red filters help to reduce the brightness of the sky during daylight observations. Even when using large telescopes, observers often report that Venus does not take high magnification well; magnifications over 300x are rarely beneficial, the image succumbing to the effects of low contrast and atmospheric turbulence.

Phases of Venus as seen through the Telescope This series of Venus images were captured using the writer's videocamera during the planet's 2004 evening apparition (click on the image for a larger version). The information shown beneath each image are as follows: Phase = the illuminated percentage of the disk, Apparent Diameter = the angular diameter of the full disk (in arcseconds ), Elongation = the planet's angular distance from the Sun, Apparent Magnitude = the apparent (visual) brightness of the planet and (delta) = the planet's distance from the Earth (in Astronomical Units ). In 2004, Venus reached greatest elongation East (46°E) on March 29th, greatest brilliance on May 2nd and inferior conjunction on June 8th. All images are shown at the same scale and are orientated with North towards the upper right and East towards the upper left (note that astronomical telescopes will usually show the image inverted, with South up). ( Images: Copyright Martin J Powell 2004 )

Features observed in the planet's atmosphere include 'Y', ' ' or 'C' shaped cloud patterns, bright spots, cusp caps, cusp extensions and irregularities along the terminator (the line separating the light and dark sides). With patience, perseverance and considerable experience, detailed images of the planet can be obtained using CCD cameras and modified webcams (excellent examples of which can be seen at Damian Peach 's website and on the ESA site). Although modern technology has allowed much detail to be resolved and captured electronically, many observers prefer to record their observations of the planet with more traditional sketches and drawings made at the eyepiece, a time-consuming but rewarding experience (see for example a selection of sketches produced by observers at the Astronomy Sketch of the Day website).

Apart from slight variations caused by light-scattering in the Earth's atmosphere, Venus normally appears white through telescopes. Its clouds are actually pale lemon-yellow but this can be difficult to discern because the coloration is subtle and is easily lost in the planet's glare.

There are two frequently observed but anomalous effects which are well-known to observers of Venus . They occur at very different times during the planet's apparition; namely, when it reaches half-phase (around the time of its greatest elongation) and when it displays a large, thin crescent phase (in the days leading up to - or following - inferior conjunction). More details about Venus ' varying orbital configurations and phases can be found on the Planet Movements page.

The moment at which Venus attains an exact half-phase is referred to as the dichotomy, at which point the terminator appears perfectly straight. In theory, Venus should appear exactly half-phase (phase = 0.50 or 50%) when it reaches greatest elongation, however it frequently does not because of eccentricities in the orbits of Venus and the Earth. Even when these eccentricities have been factored in, the date of the predicted dichotomy and that of the observed dichotomy often differ by up to several days; this is known as the phase anomaly or Schröter's effect (the latter coined by the late Sir Patrick Moore after the German astronomer Johann Schröter, who first noted the phenomenon in 1793). The date of dichotomy for evening elongations often occurs earlier than predicted, whilst that of morning elongations occurs later than predicted. The exact reason for the phenomenon was not fully understood until quite recently; it was often considered to be simply due to errors in observation. Today it is thought to be due to Venus ' dense atmosphere scattering the sunlight. Blue light scatters more readily than red light (which is why the sky on Earth appears blue) and this effect is also seen on Venus when it is observed using coloured eyepiece filters. The phase anomaly is much more evident when the planet is observed through a blue filter, whilst the anomaly is less evident when seen through filters of other colours, e.g. red or yellow . It is worth noting that the terminator normally appears straight for about four days in each elongation.

V enus at Half-Phase and Crescent Phase Two very different views of the planet, in terms of both appearance and viewing conditions: ( Left) Venus at 51% illumination (phase = 0.51) on March 29th 2004, when at greatest Eastern elongation and close to the moment of dichotomy (note the straightness of the terminator ). The telescopic image is fairly steady (with some undulations) because the planet was filmed at a relatively high altitude of about 30°. The same cannot be said of the image filmed on May 29th 2004 ( right ) when Venus was 4% illuminated (phase = 0.04), 15° from the Sun and only ten days from inferior conjunction . In this case Venus was just 7° above the horizon (hence the severe 'boiling' of the image). The lighter background reflects the fact that the planet was observed in twilight. Note how dispersion has split the thin crescent into the spectrum colours. Both images are orientated with North upwards and towards the right.

When Venus appears as a thin crescent, many telescopic observers begin to look out for a mysterious phenomenon known as the Ashen Light. First observed in 1643 by the Italian astronomer Giovanni Riccioli (1598-1671) , it is a faint glowing of the unlit (night) portion of Venus . Observers searching for the Light will normally use ultraviolet and/or infrared filters in order to help reveal it. For many decades the phenomenon was dismissed by many as illusory, possibly a contrast effect. However, with the arrival of the Space Age and the newly-acquired knowledge that Venus has a very high surface temperature and atmospheric pressure, it became increasingly likely that the phenomenon had either a geological or a meteorological cause. One theory was that the Light was due to electrical activity in Venus ' atmosphere, another that it was due to airglow . More recently, it has been suggested that Venus ' extremely high surface temperature (averaging 460°C) causes the surface of the planet to glow red hot. Recent studies comparing visual observations of the Light with contemporary infrared images, together with observer descriptions of the Light as appearing 'coppery' or 'reddish-grey' in colour, seem to provide evidence in favour of this theory. Intriguingly, reports of the Ashen Light have been rare in recent years, though it is difficult to say whether this is the result of improved observing equipment (i.e. eliminating contrast effects caused by poor optics) or a true reduction in activity of a genuine phenomenon.

Perhaps the most difficult Venusian phenomenon to observe, the Ashen Light can only be detected when the distractingly-bright crescent is out of view. To this end, an occulting bar is often attached to the telescope eyepiece, blocking the lit portion of the planet from view. Occulting bars are not commercially available but Sky & Telescope magazine recommends using a piece of wire or aluminium foil cut to the correct size and shape, which should then be attached to the rear end (field stop) of the eyepiece with sticky tape. For Venus , a specially curved occulting bar is recommended for attempting to view the Ashen Light.

Whenever Venus is showing a crescent phase, it is never very far from the Sun , i.e. its solar elongation is narrow. Unless the planet is observed in daylight (a highly dangerous activity which must only be attempted by experienced observers!) telescope users have little option other than to observe it at low altitude in the sky, soon after sunset or before sunrise. When Venus is viewed at low altitude through a telescope, Earth's turbulent, dense atmosphere acts like a prism on the planet's thin crescent, breaking up its light into the spectrum colours: this effect is known as dispersion. The lower the altitude of the planet, the more pronounced the dispersion (the final image taken on May 29th above was filmed when the planet was just 7° above the horizon, hence the obvious dispersion). Venus and Mercury are particularly prone to unstable seeing conditions (see below) and dispersion because of the low altitude at which these planets are typically observed. Indeed, patience is required when observing any of the planets through a telescope; whilst blurry and rippling images are mostly typical, considerable detail can often be seen during occasional moments of steady seeing.

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Seeing Conditions

I n astronomy, the seeing is a measure of the quality of the observing conditions experienced by an observer at the telescope. Good seeing is characterised by sharp, steady images over prolonged periods of time, the stars appearing as pinpoints of light. Under poor seeing conditions the planets appear blurry and 'boil' frequently whilst the stars appear as blobs. Whether good or bad seeing is experienced depends upon the amount of turbulence which is present in the Earth's atmosphere at the time of observation, and this originates from three potential sources:

(1) from the immediate vicinity of the telescope (e.g. telescope tube currents, heat from people standing nearby or from a nearby house), (2) from the lower atmosphere (e.g. from rising heat currents from the ground and buildings) and (3) from the upper atmosphere (caused by high-altitude fast-moving air currents).

The telescopic observer only has control over the first two sources, the upper atmosphere turbulence varying from day to day and from one location to the next. To maximise one's chances of obtaining good seeing, an observing site should be selected which minimises sources (1) and (2) - namely, where heat currents are minimal.

To minimise tube currents (source 1), an observer should always ensure that the telescope is allowed to reached thermal equilibrium with its surroundings; most amateur astronomers allow 30 - 45 minutes for this to take place. Doors and windows should also be closed to prevent heat escaping from a nearby house.

Favourable observing sites (source 2) include broad hilltops, grassy fields and alongside lakes. Sites with views over extensive tarmac, ploughed fields, valleys and shorelines are best avoided because they absorb heat during the daytime and radiate it into the sky after sundown. As a general rule, seeing conditions are best in the early morning hours, when most of the ground heat has been radiated into the atmosphere and temperatures across the local landscape have reached equilibrium. If seeing is still poor despite finding a good location, observing should be deferred until another night.

In order to provide a measure of consistency between telescopic observations of planets, it is always necessary to state the seeing conditions which prevail at the time of any given observation. Several numeric scales have been devised over the years to measure seeing, but the Antoniadi Scale, devised by the Greek astronomer Eugène Antoniadi (1870-1944) has become the default system for astronomers worldwide. Under this system the seeing is assigned a Roman numeral from I to V (1 to 5), as follows:

Antoniadi Seeing Scale Description I Perfect seeing, without a quiver II Slight undulations, with moments of calm lasting several seconds III Moderate seeing, with larger air tremors IV Poor seeing, with constant troublesome undulations V Very bad seeing, scarcely allowing a rough sketch to be made

Using this scale, the simulated telescopic images on this page would be assigned Antoniadi numbers of approximately III (for Venus phase 51%, centre-left of page), IV (for phase 15%, at top of page) and V (for phase 4%, centre-right of page). Clearly, much sharper images of the planet are within reach, as evidenced by the highly-detailed results obtained by other amateur astronomers (referred to in the links above). In practice, however, most observers will experience moderate seeing (around III) for most of the time and perfect seeing (I) will be experienced only very rarely.

It is important not to confuse the atmospheric seeing with atmospheric transparency, which is a measure of the clarity of the atmosphere. Transparency is determined by the amount of cloud, moisture or aerosols present in the atmosphere at any given time, each of which reflect light to a greater or lesser extent. In general, transparency is less important for planetary observers since most of the planets are sufficiently bright to be seen through hazy or light-polluted skies; in this instance seeing is the more important factor. Transparent clear nights may initially seem ideal for planetary observing, however they are often accompanied by much turbulence, causing the planets to 'ripple' when seen through the telescope. Hazy skies, on the other hand, are unsuitable for observing extended and diffuse objects (e.g. galaxies and nebulae) but are usually ideal for planetary observing because of the relative steadiness of the air. Transparency diminishes with altitude, so that a celestial body appears dimmer the closer it is positioned to the horizon (an effect known as atmospheric extinction). In any case telescopic observation of celestial bodies is not recommended at altitudes of less than about 30°, unless the observer has no other alternative.

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Naked-eye Venus : Apparitions, Conjunctions and Elongations, 2010-2020

Sources

" British Astronomical Association Observing Guide " (BAA, Piccadilly, London, 1995).

" Norton's 2000.0 " edited by Ian Ridpath (Longman Group UK Ltd., Harlow, 1989) .

" Celestron 8 Instruction Manual " (Celestron International, Torrance, CA, 1994).

Copyright Martin J Powell April 2010