Friday, July 05, 2002

Synodic periods

Feedback from Skippy prompted me to do a bit of research (thanks Skippy, and the rest of you, keep those cards and letter a-comin'...), and I can now tell you that Venus' "year" is 224.7 days of Earth time (meaning that's how long it takes Venus to go all the way around the sun).

This is not, however, the amount of time it takes Venus to return to the same point in Earth's sky, i.e. greatest elongation in the evening sky. That figure is larger, since the Earth chases Venus around the sun, albeit more slowly, as Earth is slightly more massive (4.87x10^24 for Venus vs. 5.98x10^24 for Earth) and Earth's orbit is larger than Venus', (more on this later).

The name for this length of time is synodic period of Venus wrt Earth. It is calculated via the formula 1/T_synodic = (1/T_Venus) - (1/T_Earth). (I found this on a Google post, and the planetary year figures were garnered from a JPL page.)

Plugging in the proper numbers and doing the math yields the fact that it takes 583.9 days on Earth for Venus to cycle from its highest point in the evening sky, around to the morning sky, and back to its highest point again in the evening sky. (Doing the same for Mercury yields a much smaller 115.88 days, given Mercury's year of 87.97 days.) Sorry, Skippy, but you asked.

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