Timing, Timing

On November 15th, 1999 a moderately rare event occured. The planet Mercury positioned itself between the earth and the sun. This has happened only 14 times this century. This was the first time in 25 years that a transit of Mercury was seen in the United States, so it was a very special astronomical event. This November event was an extra-special transit. Mercury passed very close to the edge of the sun - so close that some places on the earth only saw the planet graze the edge of the sun's disk. The last time this happened was hundreds of years ago, before the invention of the telescope! That means that this is the first time that this has ever been observed. The next grazing transit won't be until May 11th, 2391. This is a great time to be alive!

If you missed this transit, there will be more as listed below, but you might have to travel. In particular, you won't want to miss the transit of Venus coming up in 2004. Because the orbit of Venus is larger, and Venus is much closer to us when it is between us and the sun, the tilt of its orbit almost always causes the planet to cross above or below the sun's disc. This is only the sixth transit of Venus since the telescope was invented. The 2004 transit of Venus will be the first in 121 years.

Future Transits

	Object		Date		Visibility
	______		______________	________________________________________	
	
	Mercury		May 7, 2003	Europe, East Africa, Central & West Asia
	Venus		June 8, 2004	Europe, East Africa, Central & West Asia
	Mercury		Nov 8/9, 2006	Australia, Pacific, N & S America
	Venus		July 5/6, 2012	Australia, Pacific, N & S America & E Asia
	Mercury		May 9, 2016	N & S America, W Africa
	Mercury		Nov 11, 2019	NE & S America, W Africa

Contact!

To emphasize the definition of the contacts, Mercury's size has been exaggerated in this diagram. The actual size of Mercury's disk relative to the sun is indicated.

As mentioned in the " What Will I See? " section, there are four important points of time during the transit that astronomers would like to measure. These are called 1st, 2nd, 3rd, and 4th contact. 1st contact, which is very hard to observe, is the point in time when Mercury first touches the disk of the sun. 2nd contact is the time at which Mercury is fully engulfed within the sun's disk. 3rd contact is when the disk of Mercury just begins to leave the face of the sun, and 4th contact is when the disk of Mercury has just completely left the sun's disk. Every location on earth had it's own contact timings. If you have a telescope of your own, and an accurate clock such as a WWV radio or GPS receiver, you can do these timings and send them in to the Association of Lunar and Planetary Observers (ALPO) . They would appreciate getting your observations. The observations will be used to refine orbital figures and the diameter of the sun.

For a table of approximate contact timings for many locations, you can go to Fred Espanek's transit pages .