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The dark disk of Mercury seen against the backdrop of the Sun during the transit. The transit is almost over, and Mercury appears near the edge of the Sun. This picture was taken with the Swedish 1 meter Solar Telescope (SST), which is located at La Palma in the Canary Islands.
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Image courtesy of The Royal Swedish Academy of Sciences.

Mercury Transit on May 7, 2003
News story originally written on July 2, 2003

The planet Mercury appeared to cross in front of the Sun on May 7, 2003. Astronomers call the event a transit. A transit is like a solar eclipse. However, a transit occurs when a planet, instead of Earth's Moon, passes between the Sun and Earth. Planets are much further away than the Moon, so the planet does not completely cover the Sun during a transit like the Moon does during an eclipse.

Transits of Mercury happen about once every seven years on average. There will be 14 transits of Mercury this century. This one was the first. The last transit of Mercury before this one was on November 15, 1999. The next will be on November 8, 2006.

Only two planets ever transit the Sun as viewed from Earth. The two planets are Mercury and Venus. All of the other planets orbit the Sun further from Earth and never pass between Earth and the Sun.

Transits of Venus are much rarer than transits of Mercury. Venus transits happen only twice per century. We're in luck though - the next transit of Venus is coming along pretty soon on June 8, 2004! Astronomers in the 1700's used transits of Venus to make the first good measurements of the distance between Earth and the Sun. They did that by carefully measuring the time of the transit from different places on Earth.

Last modified July 2, 2003 by Randy Russell.

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