VENUS : Part 2


Telescopically, Venus is a brilliant white disk shows the phases, which wax and wane like the Moon. Observations of Venus are quite restricted but are best made during daylight hours when the planet is near the meridian - mainly to reduce the night time or twilight glare of the brilliant white planet. Sometimes, with some experience, a few elusive cloud features may be seen, but these are usually faint and difficult to discern. Yet amateur observations were first to detect the true 4.3 day atmospheric rotation.

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.

!! W A R N I N G !!
When using any optical aid or telescope daylight observing of either the Sun or close to the Sun should ONLY be made by projecting the image onto a white screen or card - and even this should only be for short durations.
Any direct or accidental viewing of the Sun, by either eye or any other optical equipment is VERY DANGEROUS without proper protection. Otherwise, TOTAL BLINDNESS WILL RESULT. Even glancing through a telescope will blind you in less than a ten-thousandth of a second.
Also if your telescope has something called a sun filter - You should NEVER use. If this filter cracks while you are observing, blindness is the only outcome.
When observing Venus or Mercury in daylight (as described below), if the safest to place to put the telescope is in shadows either behind some building or wall. Only then is it really safe to point your telescope towards Venus. If this is not possible, then make sure the telescope’s mounting cannot be accidentally knocked into the field containing the Sun. Observations of Venus is always recommended, even for the most experience observers, when the planet lies less than 20 degrees from the Sun.

(NOTE : Please Read My Disclaimer Below)


Firstly, and most importantly, the observer should only attempt serious observations of Venus during daylight hours. This necessary because of the planet’s overwhelming brightness and 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) - making 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 45o East or 45o West of the ecliptic. Alternatively, you can use the following methods;

Equatorial Mounted Telescopes (without circles) can use the following method;
1) Cover the main optics from the rays of the Sun.
2) Projecting the finder’s solar image onto a piece of white paper.
3) Using the setting circles, roughly offset the telescope to Venus’s present declination
4) Read from the “Elong” column in the “Venus 2006” Table. 2 has the given value of the nearest date.
5) Move the telescope by the number of degrees listed either east or west of that position.
6) Venus may not be centred in the telescope field, but should be placed within the finder’s field.
Non-Equatorial Telescopes can use the following method.
1) Observing Venus as close as possible to the local meridian.
2) Read the given value of the nearest date from the “Trans” column in the “Venus 2003” Table
3) Estimate the transit time of Venus for the current day between the two listed transit dates. This roughly calculates the time of Venusian transit.
4) Face the telescope due north (south in the northern hemisphere), and make sure the top of the telescope tube is level to the horizon. (A small spirit level might help here.)
5) To find the planet in the southern hemisphere, simply raise the telescope by either;
5a) If Venus has a positive declination take : 90o minus your latitude in degrees minus Venus’s Declination. [90o - Lat o - Dec o (Venus)] or 5b) If Venus has a negative declination take : 90o minus your latitude in degrees plus Venus’s Declination [90o - Lat o + Dec o (Venus)]
6) If you are in the northern hemisphere, then;
5a) and 5b) are reversed.
7) This is Venus’s rough altitude above the horizon on the meridian.
8) Venus should be near the centre of the finder.
9) If not, slightly moving the telescope slightly up and down the meridian, and you should see Venus somewhere in the finder.
Note: If you don’t observe Venus at the time of transit, the planet should lie on an arc on the meridian.


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 planet's phase, which is recommended to be rotated, so that planet’s observed terminator 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 bit more careful when drawing what you see, as the fixed orientation can be tricky.

Procedure to Draw Venus’s Disk and Phase

a) First draw the position of the ‘poles’ of the blank form.
b) Note the time and the seeing conditions.
c) Note any deformation of either or both observed cusps.
d) Slowly “fill-in” the phase using an HB or H pencil, until it meets in the middle of the figure.
e) Look for any of the subtle greyish atmospheric features, using a soft 4B/5B or 6B pencil.
f) Sketch each individual feature by size, shape and position.
g) Then smooth and soften the draw image, by smudging the pencil marks with your finger or a scrap piece of paper. Doing this often obtains a far more realistic effect.
h) Tidy up the image, especially smudges outside the fixed circle using a pencil eraser.
i) With either Indian ink, charcoal, black pencil, or black texta, and then fill in the dark portion of the phase. Care is especially needed when drawing near the terminator.
j) Write a short summary of what you have seen.

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 a dozen or so 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. This was independently determined by amateurs in the early 1950’s - even before the arrival of interplanetary spacecraft!


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.


An interesting observation during August 2003 is the appearance of the Ashen Light, where some background lighting appears opposite to the side of the phase on the “night” side of the planet. It only occurs prior to, and after, inferior conjunction when the planet is less than about 10o from the Sun. Telescopically, Venus appears like a thin sliver of light when it subtends over 1'arcmin across. The window of opportunity is small, lasting perhaps seven to ten days as the motion in the sky is quite rapid.

Observing the phenomena, it is best to see its weak light by using a purple Wratten 35 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 a positive eyepiece, and place the planet behind the partition in the field.

Another device is the Ashen Eyepiece, which has circular indents allowing only segments of Venus’s disk to be view and look above the Venusian surface for any additional nebulous light.

If you do have 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.


Last Update : 17 July 2006

Southern Astronomical Delights © (2005)


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