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Apologies for a silly question engendered by a silly book, i.e. Dan
Brown's Da Vinci Code. It claims that Venus traces a "perfect pentagram" through the sky, a claim echoed in many other sources online including Wikipedia. Unfortunately, I do not understand how this is possible in any meaningful way. Venus passes the Earth in its orbit (inferior conjunction) five times in eight years (less two days). (As seen from Venus, each conjunction is five days apart; the same side of Venus faces the Earth each time) Trivially, yes, this means that the Earth is roughly 8/5 of a year ahead each time, so the Sun (with Venus close by) is in a different part of the zodiac each time, and these points can conceptually be linked up as a five-pointed star with the Earth in the middle. However, the claim seems often to be taken to mean more - i.e. that the position of Venus makes some symmetric figure when considered in a geocentric frame of reference against the fixed stars. I find myself doubting this now, because the Earth and Venus are in different positions in their orbits at each conjunction, and the orbits should be differently inclined with some change in orbital speed due to eccentricity. I'd think this would mean that even the positioning of the points of the zodiac would be imperfect, and when actually looking at the position of Venus in the sky at different "points" of the pentagram that it'd be all over the place. But I can't readily prove that to myself from first principles. It would be curious to see what the apparent position of Venus really traces out as over time. Is there a program that can readily calculate and display this? Also it would be nice to mark out where Venus is located in this figure at the end of each Venereal day. |
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