Mercury Transit on November 8, 2006
News story originally written on November 6, 2006
The planet Mercury will cross in front of the disk of the Sun on Wednesday, November 8, 2006. 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 second of just 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 only a few years ago on May 7, 2003. However, the next Mercury transit won't be until May 9, 2016!
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. You may have been lucky enough to view the last one, which occurred on June 8, 2004. In case you missed it, there will be another on June 6, 2012.
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 angle between Venus and the Sun 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.