The dark circle in this picture is the planet Mercury. Mercury is in front of the Sun. This picture was taken during a transit in 2003. The transit is almost over and Mercury is near the edge of the Sun.
Click on image for full size
Image courtesy of The Royal Swedish Academy of Sciences.
"Transit" is a special term astronomers use. A transit is like an eclipse
of the Sun. When an eclipse of the Sun happens, the Moon gets between us
and the Sun and blocks the sunlight. When a transit happens, a planet gets
between us and the Sun. The planet doesn't cover up the Sun like the Moon does,
though. Planets are very far away. The planet looks like a small black dot
on the Sun when a transit happens.
There are only two planets that can have transits. Only planets that are closer
to the Sun than Earth is can have transits. Can you guess which planets those
two are? The transit planets are Mercury and Venus!
Transits don't happen very often. There are about 13 or 14 transits
of Mercury every 100 years. Transits
of Venus are very rare. The last Venus transit was in 1882! There will be
another transit of Venus in June 2004.
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