Astronomy/Retrograde of Venus
I understand that the retrograde of Venus lasts approximately 40 days.
Is this something that can be observed and measured with the naked eye given clear skies? or does the proximity of Venus to the Sun as viewed from Earth make this impossible?
I am wondering whether the ancients [Greeks, Babylonians and Sumerians] who were skilled observers of the stars and planets, would have been likely to have observed it.
The beginning and ending of Venus' retrograde motion CAN be seen with the "naked" eye, but due to being in "crescent" phase at the time it passes between the Earth and Sun, it is not as bright as during the weeks before and after its retrograde motion. In order to give you a prompt answer to your question I have not done a thorough search of all possible references, so I can't say whether the retrograde motion WAS observed by the ancients; but I'm sure it COULD have been.
To see the extent of Venus' retrograde motion take a look at "Transits of Mercury and Venus" (at http://cseligman.com/text/sky/transit.htm
). A little over halfway down the page, a "Review of Retrograde Motion" shows a superposition of multiple images of Venus and an image of the Sun at the time of Venus' 2004 transit of the Sun on an image of the stellar background as it would have been seen at the time of the transit, if stars had been visible at that time (they wouldn't have actually been visible at that time, because the transit could only be seen on the "day" side of the Earth). The image shows that the brightest images of Venus occur before and after the retrograde motion, but that Venus is still much brighter than the stars in its vicinity at the start and end of the motion, so it would have been possible to see the "stationary" stages of the retrograde motion, if nothing else. However, even if the retrograde motion itself were not observable, the fact that the brightest portion of Venus' post-retrograde motion is well to the west of the brightest portion of its pre-retrograde motion would have made it obvious to ancient observers that it had been in retrograde motion in between those times, whether they actually observed the retrograde motion itself or not.