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 The
Many Faces of Venus
Although
Venus is the closest planet to Earth, it has long remained a mystery
due to the thick clouds that completely hide its surface.
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What
Is a Transit of Venus?
A
transit of Venus occurs when the planet Venus passes directly between
Earth and the Sun. Venus appears as a black dot moving across the
surface of the Sun.
Only
six transits of Venus occurred between 1600 and 2000, but two will
take place this century. Click
here to see the timeline.
Lining
Things Up
Venus
actually passes between Earth and the Sun every 584 days. Usually
when this happens, Venus appears to us to be above or below the
Sun, because Venus's orbit is tilted slightly with respect to Earth's
orbit.
On
very rare occasions, however, Venus lines up directly between us
and the Sun, just as the Moon does during a solar eclipse. But because
Venus covers only a tiny part of the Sun, we don't notice any change
in the Sun's brightness. Unless we knew where to look, we would
miss the transit entirely.
A
Yardstick to the Sun
For
ages, astronomers looked for a way to measure the distance between
Earth and the Sun. In the 1700s they realized that the transit of
Venus might help them do it.
By
comparing the different times of the start and/or end of a transit
of Venus from various locations on Earth, astronomers could use
surveying methods to measure the distance from Earth to Venus. Since
Kepler in the 1600s had determined the relative distances among
the planets and the Sun, astronomers could use the Earth-Venus distance
to calculate the Earth-Sun distance.
The
Earth-Sun distance - called the astronomical unit (AU) - was a yardstick
that could measure the entire solar system. In pursuit of the AU,
astronomers began chasing the transit of Venus in earnest.
Contact!
The
key to finding the Earth-Sun distance using the transit of Venus
was in timing the exact moment when the edge of Venus's disk appeared
to make contact with the edge of the Sun.
There were four contact points that astronomers could measure as Venus passed in front of the Sun. However, an optical trick called the black drop effect prevented astronomers from obtaining the precise time of contact.
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