What is a transit? Mercury and Venus have orbits closer to the Sun than we do -- occasionally they will cross between the Earth and the Sun (across the face of the Sun as we look at it). We call that passage (the "black dot" of the planet crawling across the face of the sun) a "transit".
Why don't they happen all the time? The orbits of the planets and the orbit of the Earth have to line up exactly to get a transit. But, more importantly, both Mercury and Venus have orbit planes that are tilted relative to ours -- so there are only two places in their orbit where they can be seen to cross the Sun -- the Earth would have to be in the right place at the right time!
When do we get Mercury transits? For those two places where Mercury rises above the Earth-Sun plane, or sinks below, the Earth has to be also in line with that point. That occurs in November or in May. But, certainly not every year! We get between 13-14 Mercury transits in a century - the one on Monday is the last one of the 20th Century! There are rough periods of 6, 7, 46, 217 years (to name but a few of the many strong periods).
Why is this called a "grazing" transit? Most transits can be seen by any place in the hemisphere that faces the sun during the transit (the maximum time for a Mercury transit is about 5 1/2 hours, so some parts of the Earth might lose the end in sunset, or miss the beginning in sunrise - the one on Nov 15th, if you could see all of it, is only about 1 1/2 hours). If the transit is very near the top or the bottom of the Sun, then some parts of the hemisphere that faces the Sun will not be able to see the transit (essentially, their "angle" is wrong ... they would perceive Mercury as going "past" the Sun instead of in front of it). These grazing transits are much more rare, in fact, this is the first grazing transit since the invention of the telescope ! The next one won't be before the 23rd Century!
Can we see this transit on November 15th, 1999? Here in Detroit, the transit should start about 4:11 PM, and reach the midpoint at about 4:40 PM ... but we probably won't see the end of it because of the Sun setting. You also have to have a reasonably clear view to the West.
What is needed to see the transit? First of all - DO NOT LOOK DIRECTLY AT THE SUN WITHOUT PROPER SUN FILTERS!!! Since the "dot" of Mercury will be about 1/200th the size of the Sun, you will need a telescope to see it (but a small one would work - WITH SOLAR FILTERS!). "Pinhole projection" methods (as used for eclipses) probably won't work - the Mercury dot will be too small.
Will LTU be doing anything to see the transit? We are going to try! (Assuming the equipment cooperates and Mother Nature cooperates!) We have at least two telescopes that can be used - one with a solar filter for direct viewing and one that will project an image onto a "screen" for groups of people to watch. We will be setting up sometime after 3:00 PM on the quad-side of the main Science Building entrance. Before the transit, we should have opportunities to see sunspots (during also). Weather permitting, all are welcome to join us!
What about SohO? There is a solar observatory that is between the Earth and the Sun that takes some amazing pictures of the Sun in all different types of wavelengths. Will that be used to observe the transit? Unfortunately, NO! SohO will not experience this transit, because it is closer to the Sun, and thus Mercury will appear to be going "above" the Sun [this is the same reason that the lower part of the hemisphere doesn't see the transit .. it is "worse" for SohO since it is closer!] This is explained here (also follow the link for the "updated plot showing Mercury ...), and there is more information about what SohO did see here!
Any more information? I recently gave a talk on campus about the transit. I will soon post that talk here, as well as the supporting programs, and fill in some of the details. Until then, here are some excellent links to follow to get more information on this transit :
If you find other useful links - let me know!