On November 8th 2006, amateur astronomers with small telescopes and solar filters will be able to observe Mercury's tiny silhouette moving slowly across the face of the Sun as it makes a rather feeble attempt to eclipse the Sun. Astronomers call this event a "transit".
A transit occurs when one of the inner planets, Mercury or Venus, passes between the Earth and the Sun. Most of the time the inner planets pass either above or below the Sun. There are only certain points in these planets' orbits, called nodes, when they are in the plane of the Earth's orbit. For Venus, these nodes occur in June and December. For Mercury, they occur in May and November. Transits can occur only in those months.
Because Mercury orbits the Sun much more rapidly than Venus, it's in the right place for a transit far more often.
Transits of Mercury occur 13 times a century or roughly once every 7.4 years, whereas transits of Venus occur less than twice a century, or roughly once every 62.5 years.
In fact, transits of Venus usually occur in pairs 8 years apart, separated by a gap of 125 years. We're right in between a pair of Venus transits, one having occurred on June 08, 2004 and the next one on June 06, 2012.
Mercury transits also occur in clumps. The last one was 3.5 years ago on May 07, 2003, and the next one will not be until 9.5 years from now, on May 09, 2016.
This year's transit will be a late afternoon event for most of North and South America and a morning event for East Asia and Australia. The Mercury transit is not visible from Europe, Africa, and most of Asia.
Choose an Observing Location
In eastern North and South America the Sun will set before the transit is over, so it's important to observe from a site with a good western horizon.
The one imponderable in observing a transit is of course the weather. Unlike a solar eclipse, where observers are restricted to a narrow band, transits are visible over a wide area. In case of inclement weather, you should be prepared to travel; the Clear Sky Clock will be a useful guide to where to go:
Start with the clear sky clock closest to your chosen location. If it shows poor weather prospects, check the clocks farther away, within 100 km (60 miles) and 200 km (120 miles) radius. If you can see a clear patch, hop in your car and drive! Check the Sun's setting point a day or two beforehand, to verify that trees or buildings do not block your view. This is especially important in North America, where the Sun will set before the transit is over.
To read the rest of this article, please download our special six-page Mercury Transit PDF [468KB].