THE MOTIONS
OF VENUS
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.
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Mayan
pictograph of Venus
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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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