Jay H. Beder
A transit of Mercury (or Venus) is the passage of the planet across the disk of the sun. The planet looks like a dot crossing the sun.
Mercury transits occur 13 times per century. The next transit after this will be May 9, 2016. Since it is usually hard to observe Mercury, this is a nice opportunity to see it. Of course, what you'll see is its silhouette. (Normally Mercury looks like a star, but it's near the sun, and so is easily washed out by sunlight. Thus, ordinarily you have to view Mercury shortly before sunrise or after sunset, and you need a clear horizon. If you're lucky enough to see a total solar eclipse, that's another good time to view Mercury.)
This year's transit will begin at 1:12 pm CST and end at 6:10 pm, reaching the midpoint at 3:41 pm. However, the sun will set in the Milwaukee area before the end of the transit. Residents in the western part of the country will be able to see the whole transit (weather-permitting).
The same viewing rules apply as for Venus transits or for other viewing of the sun: DO NOT LOOK DIRECTLY AT THE SUN without a proper filter such as welder's glass (No. 14 or darker). You may be able to see the transit by projecting the image of the sun on a surface, but the planet will be a very small dot, and if the sun's image is small, it may be very hard to see Mercury.
Viewing in the Milwaukee area will be a bit iffy, for a couple of reasons. First, of course, is the weather in early November. Second, the sun will be in the west, heading towards the horizon, and so people will need to find a viewing area with an unobstructed view to the west. Of course, the first part of the transit will be in the middle of the afternoon, which is very convenient.
The excellent book by Eli Maor also has some information on transits of Mercury:
June 8, 2004: Venus in Transit, Princeton University Press, 2000. 186 pages.
Kepler was the first (1631) to predict a transit of Mercury (as well as of Venus).Back to my home page