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.
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