Telescopically, Venus is a brilliant white disk that shows phases, which wax and wane like the Moon. As Venus is an inferior planet, these observed phases exhibit great variations in angular diameter. Conjunctions are deemed as either at inferior conjunction, when the orbit places Venus between the Earth and Sun, or superior conjunction, when Venus appears on the opposite side from the Sun. Near inferior conjunction, the disk appears slightly greater than one arcmin, and displays just a very thin crescent. As distance is close to the Earth, the apparent sky motion is high, and for only two short weeks, the observed phase changes dramatically. Seeing these phases during evening or morning twilight is either difficult to impossible due to the closeness to the horizon, and proximity to the Sun at these times makes daytime telescopic observations of these wafer-thin phases fairly dangerous for mere novices. At superior conjunctions, Venus appears as an uninteresting small 10 arcsec disk with only a slight deformation of its otherwise uniform round disk. This finds a four month period where Venus is within ±12° either side of superior conjunction. Here any visual observations of Venus are unimportant and mostly deemed quite unnecessary – especially from the dangerous perspective of accidentally exposing the optics to the Sun. (See My Disclaimer Below.)

As stated before, all visual observations of Venus become quite restricted at night when the planet is close to the horizon, so it is best to make them during daylight hours when the planet is nearest to the local meridian. This is mostly done to reduce the overwhelming nighttime or twilight glare of the brilliant white disk.

Serious Amateur Observations

Sometimes, with some gained experience, a few elusive dark or bright cloud features may be seen, but these are usually faint and difficult to discern. Debate continues on the reality of these cloud patterns, with some still contending that they might be just optical illusions. Prior to spacecraft visitations, visual observations were the only means of investigation Yet such early amateur observations were first used to detect the true 4.3 day atmospheric rotation, being found since the late-1980s. I have seen these marking myself, and have made numerous observations over the years. Also interesting are the sometimes irregular shape of either of the two cusps or the planetary terminator. Some suspect that these too may be optical tricks or illusions created in observers mind.

It has been usefully suggested that such effects can be checked by rotating the field by 90°, and looking at the disk from a different perspective. WHilst this is ideal using refractor or catadioptic designed telescopes, where the star diagonal can be easily twisted by the required amount, however, this is not normally easily done in Newtonian refractors (Though there is nothing wrong using a star diagonal here, it is still very cumbersome task. Also having your back to the sun, in some circumstances, can be dangerous if the optics stray onto the full power of the Sun. It is not a recommended practice to do so.)

Most observations are ideally made in aperture exceeding 15cm (6-inch), though larger telescopes are certainly advantageous – mostly displaying a larger apparent diameter disk. I have used a 20cm f/10 Celestron 8, and have found it more than adequate to make the necessary observations.

Useful observation and timings of the moment of exact the dichotomy half-phase are made by a number of amateurs through the world. Predictions show that the observations are consistently out by about four days, which depends only which side of the Sun the planet lies. The effect is probably due to Venus’s upper atmosphere, though the true reason is still elusive. Other filter observations may also improve visually discerning cloud features, while the polar cusp-caps or seeing the ashen light — a night-side illumination only occur near inferior conjunctions when Venus only appears as a very thin crescent.

HOW TO FIND VENUS IN DAYLIGHT

Firstly, and most importantly, the observer should only attempt serious observations of Venus during daylight hours. This necessary because overwhelming brightness of the planet especially at low elevation — making either nighttime or twilight observation nearly impossible. It is much preferably to observe each successive day between either the hours of 3pm to 4pm, when Venus is east of the Sun (Evening Star), OR between 9am or 10am, when Venus is west of the Sun (Morning Star). This always makes Venus close to the observer’s meridian.

Trying to find Venus for the first time during daylight may seem fairly intimidating. This can be quite tricky because the planet’s visual position is not surrounded by any reference field stars that is usually so helpful at night. Once you know where to look, you can always easily see Venus with the naked-eye. Venus then can be quickly found each successive day, especially if seen at similar times. This can also be simply remembering using the planet’s positions from the nearby references like the corner of some roof or building, etc. If you cannot use setting circles to dial up Venus position directly, it is best to use the Sun’s position then look for the required location up to 45° East or 45° West of the ecliptic. Alternatively, you can use the following methods;

Note: If you don’t observe Venus at the time of transit, the planet should lie on an arc on the meridian.

OBSERVATIONAL TECHNIQUES

Many techniques have been suggested over the years to measure the phase of Venus. Visually estimating this by eye is very subjectively, so drawing the phase on some standard blank form with a drawn circle exactly 50mm across. Mark the north and south points on the sketch. Next draw the planetary phase, which is recommended to be rotated, so that the observed terminator of the planet appears horizontal to the line of sight — thus improving the eye’s means of interpreting the shape of the phase. If this is impossible, say with an equatorially mounted Newtonian, just be a little bit more careful when drawing what you see, as the fixed orientation can be tricky.

Each of the grey areas mentioned above are difficult to see. Observers grade these various intensities from 0 to 5, where 0 is bright than the general background, 1 is the colour of the normal colour of the Venusian atmosphere, while 5 is darkest of features. Selection of the values is quite arbitrary, and it is only after dozens of observations that the observer gets the feel for the degrees of possible shading. Observation finds that the features change significantly day by day. Experience has also found a rough four-day “period” of these shadings, corresponding to the true rotational period of the planet’s atmosphere. Early in the 1950s, this was independently determined by amateurs well before arrivals of interplanetary spacecraft!

ADVANCED TECHNIQUES and FILTERS

Advanced observers sometimes use coloured filters, often allowing the surface features to appear more distinct. Application of Yellow Wratten 12 filters often improves the appearance of the general phase, eliminating the blue part of the spectrum and darkening the sky background to improve contrast. More often, Wratten 25 (Red) and 44A (Light Blue) filters are recommended, as these have been found to significantly affect the apparent shape of the terminator. For still unknown reasons, the red filter makes the phase 1% or 1.5% larger, and in blue 1% or 2% smaller, compared with white light images. Such colour dependancy hints that the effect is some real atmospheric phenomenon and not just an optical or observational illusion.

If you do have any of these filters, it is best to draw three separate images. In my own experiences, I find that drawing Venus favours the average measure of the phase and is more relevant when determining the time of dichotomy than by using the white light image alone.

The ASHEN LIGHT

An interesting visual observation, like one made during August 2003, lead to the appearance of the so-called ashen light, where some background lighting appears opposite to the side of the phase on the “night” side of the planet. (See Side Figure. →) It only will occur prior to, and after, inferior conjunction when the planet is less than about 10° from the Sun. Telescopically, Venus appears like a thin sliver of light when it subtends over one arcmin across. The window of opportunity is quite small, lasting perhaps several days against the motion across the sky is quite rapid.

Observing the phenomena, it is best to see its weak light by using a purple Wratten 35 coloured filter over the illuminated phase. Sometimes it is also recommended that the crescent should be obscured with an occultation bar, as to eliminate some of the planet’s brightness. You could do this with some aluminium foil covering at least half the field stop of any positive eyepiece, and place the planet behind the partition in the field.

Another useful device is the ashen eyepiece, which has several different sized circular indents, allowing only partial disk segments of Venus enabling seeing any possible additional nebulous light above the Venusian surface. This works as it greatly darkens the field from the sky, improving contrast and therefore enhancing the faintness of the ashen light. Its use requires a positive eyepiece, with the shape of the placed on the eyepiece field stop. (← Figure 3A). This eyepiece is used with an optical telescope of 15cm or greater aperture, under high magnification, preferably with a clock drive, as to keep steady enough of the examined portion of the Venusian disk. The size of the circles arc must roughly correspond to the curvature of the disk, where the darken boundary appears. (← Exampled in Figure 3B) Although this sounds easy to do, it takes a little practice to get the field adequately aligned and centred long enough to see the phenomena.

Disclaimer : The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. Onus of its use is placed solely with the user.

Last Update : 1st January 2013

Southern Astronomical Delights © (2013)

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