A transit of Mercury is a very small solar eclipse. It’s never a total eclipse because the disk of Mercury is about 200 times smaller than the diameter of the sun as seen from the earth. Instead, you’ll see a small black dot cross the face of the sun during a five-hour journey.
There are four contact times during the transit that astronomers would like to measure. First contact, which is very hard to observe, is the point in time when Mercury first touches the disk of the sun. Second contact is the time when Mercury becomes fully engulfed within the sun’s disk. Third contact is the mirror image of second contact and occurs just before the planet begins to leave the face of the sun. And fourth contact is when Mercury has completely left the sun’s disk.
The image below shows the four predicted contact times for Kitt Peak. For a table of approximate contact timings for many other locations, you can go to Fred Espanek’s transit pages.