What is a transit? It is a precise alignment of the Sun, Earth and another celestial body. In this case, that body is Mercury, the closest planet to the Sun and a fairly small planet. It orbits the Sun every 88 days, and when we have this special alignment with a transit, we are witnessing the proper motion of the planet as it progresses in its orbit around the Sun. Mercury's diameter is 194 times smaller than the Sun so it will in fact appear as a tiny, perfectly round black dot against the surface of the Sun (the Photosphere) and will take approximately 5 1/2 hours to cross from solar limb to solar limb.
|Time Lapse of Mercury Transit in 2016|
If you want to see this, you will need special equipment that incorporates the correct level of filtering to reduce the Sun's intensity to a safe level, and magnification to make it possible to see the shape of Mercury against the disk of the Sun. Many astronomy clubs and public observatories will host viewing events. Here in Munich, the Volkssternwarte München will be open for the entire duration that the transit is visible from here, starting at 1:35 pm and continuing through sunset which is at 4:39 pm on Monday. We just need clear skies and a proper filter and we can enjoy this unique and rare event.
For general information about the transit and more detailed timing for US-based locations, check out this article from Space.com.
Warning: do not stare directly at the Sun for any length of time, and in particular do not look at the Sun through a telescope or binoculars with "eclipse glasses" as these will not protect your eyes sufficiently. Only observe the Sun directly with a high quality solar filter built especially for a telescope or binoculars.
Image courtesy of NASA.