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) of the Royal Swedish Academy of Sciences. The telescope is located at La Palma in the Canary Islands.
Click on image for full size
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 disk of the Sun on May 7, 2003. Astronomers call the event a transit. A transit is similar to a solar eclipse. However, a transit occurs when a planet, instead of Earth's Moon, passes between the Sun and Earth. Because planets are so much further away than the Moon, the planet does not completely cover the Sun during a transit like the Moon does during most eclipses.

This transit is the first of only 14 transits of Mercury during the 21st century. On average, transits of Mercury occur about once every seven years. The most recent previous transit of Mercury was on November 15, 1999; the next will be on November 8, 2006.

Only two planets, Mercury and Venus, ever transit the Sun as viewed from Earth. All of the other planets orbit the Sun further from Earth, and thus never pass between Earth and the Sun. Since the orbits of the planets are tilted, transits occur only some of the times when Mercury or Venus pass between Earth and the Sun. Usually the planets pass above or below the Sun as viewed from Earth, instead of directly crossing in front of 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 accurate measurements of the distance between Earth and the Sun. By noting the precise time of the transit as viewed from different locations on Earth and by using their knowledge of the way planets orbit, astronomers were able to calculate the distance to the Sun. Sir Edmund Halley, of comet fame, was the first astronomer to realize that transits could be used to make that calculation.

Last modified July 2, 2003 by Randy Russell.

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