The dark disk of Mercury seen against the backdrop of the Sun during the May 2003 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 (33 Kb) Image courtesy of The Royal Swedish Academy of Sciences.

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.

The dark disk of Mercury seen against the backdrop of the Sun during the May 2003 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. Click on image for full size (33 Kb) Image courtesy of The Royal Swedish Academy of Sciences.

The planet Mercury will cross in front of the Sun on Wednesday, November 8, 2006. 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 second. The last transit of Mercury before this one was on May 7, 2003. The next won't be until May 9, 2016.

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. Maybe you saw the one on June 8, 2004. If not, the next transit of Venus is coming along in just a few years on June 6, 2012! 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.

Mercury (the dark circle) near the edge of the Sun during the May 2003 transit. Click on image for full size (33 Kb) Image courtesy of The Royal Swedish Academy of Sciences.

The planet Mercury will cross in front of the Sun on Wednesday, November 8, 2006. When that happens, astronomers call it a transit. A transit is like a solar eclipse. An eclipse happens when the Moon passes in front of the Sun. A transit happens when a planet passes in front of the Sun. Planets are much further away than the Moon, so they look much smaller. A planet does not completely cover the Sun during a transit like the Moon does during an eclipse.

There will be 14 transits of Mercury this century. This one will be the second. The last transit of Mercury before this one was in 2003. The next won't be until May 9, 2016.

Only two planets ever transit the Sun. The two planets are Mercury and Venus.

Transits of Venus are very rare. They only happen about two times every 100 years! In the 1700's astronomers used transits of Venus to make the first good measurement of the distance between Earth and the Sun.