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Seen from the earth, Venus moves in a tricky fashion, appearing, disappearing, then reappearing, first as a morning “star,” then as an evening "star." (Venus is a planet, of course, but observers in the past—persisting in some cases to the present—mistook it for an unusually bright star.) So complicated is Venus’s disappearing act that the ancient Greeks misconstrued it as two different stars. The Mayans knew better, and they recognized Venus in both the morning and evening skies as one and the same.

If you were to watch the horizon at sunrise, you might first see Venus appear over the horizon just at daybreak. Each day, Venus would rise a little earlier, climb a little higher, and shine a little brighter, before its light was drowned out by the sun. Then the pattern would reverse, and Venus would rise a little later each day, until finally it would fail to rise.

Mayan pictograph of Venus

This rise and fall of Venus as a morning star takes 263 days. For the next 50 days, Venus disappears and cannot be seen in the sky at all.

Then, Venus reappears in the evening sky, where it remains for another 263-day phase before disappearing below the horizon for 8 days. At the end of these 8 days, Venus reappears as a morning star, and the cycle begins again.

The entire cycle—263 as a morning star, 50 days absent, 263 days as an evening star, and finally, 8 days absent—takes 584 days, an interval also known as the synodic period of Venus.

It just so happens that the synodic period of Venus (584 days) relates to the orbital period of the earth (365 days) in a relatively simple ratio of 5:8. Five times the synodic period of Venus (5 x 584 days) is 2,920 days. If you divide 2,920 by the 365 days in our year, you get 8. In short, the motions of Venus relative to our sun repeat almost exactly every 8 years. Eight years would have been a useful span of time for ancient sky-watchers, who relied heavily on natural cycles to mark time.

There is a simple way to track this 8-year cycle. Like the sun, the position of Venus against the horizon appears to shift with the seasons. Venus shifts north in the summer and south in the winter. Exactly how far Venus shifts each year depends on where it is in its 8-year cycle, but the extreme points in the back-and-forth oscillation are known as the northern extreme and the southern extreme of Venus. An observer will see Venus rise at each of these extreme points once every 8 years.

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