Yep, we are less than 24 hours away from the last Venus transit to take place for the next 105.5 years! So you had better get out and see it.
Okay, as many of you may know, Venus orbits closer to the Sun than the Earth. It also orbits the Sun faster than the Earth so every now and then Venus will lap the Earth and pass between the Earth and the Sun. However, we don't get a transit every time Venus passes between the Earth and the Sun. This is due to the fact that Venus and the Earth don't orbit in exactly the same plane. Imagine each of our orbits is a hula hoop but they are not quite lined up. The hoops are tipped relative to each other. Venus might pass us when its orbit is slightly above Earth's or slightly below Earth's orbit. Therefore most of the time we see Venus pass slightly above the Sun or slightly below the Sun.
Transits occur in pairs separated by eight years. There was a transit of Venus in 2004 which I saw from Wisconsin rising over Lake Michigan. This is the second transit in the pair. The next pair won't start until 2117!
But not on June 5th/6th (depending on which side of the International Date Line you are on), 2012, Venus will pass directly between the Earth and the Sun. Venus will appear as a small dot on the surface of the Sun. The entire transit takes over six hours. In Tucson, Venus touches the edge of the Sun around 3:06pm MST and leaves the Sun around 8:47 MST...but the Sun sets around 7:30MST so we miss the end. That is true for most of North America...we see the first part of the transit only. You can find times for your location here.
People in Hawaii and East Asia get to see the whole transit. As you move into Europe, they see the transit at sunrise. Most of south America and west Africa are out of luck this time (hope they watched in 2004!) Here is a map showing who sees what.
First rule of observing the transit: NEVER LOOK AT THE SUN WITHOUT EYE PROTECTION. Sunglasses don't cut it. Get #14 welder's glass (any welder's shop will know why you are asking for it...they won't look at you funny, I promise), eclipse shades or a properly filtered telescope. If in doubt, leave it to the pros. You can also make a pinhole viewer quickly and cheaply (if you are in Tucson, I have solar eclipse glasses, just drop me a line!)
If you are clouded out, there are lots of online streams you can watch. The National Solar Observatory will have a stream and Universe Today promises to have a lively feed. Look for you local astronomy clubs...many are having viewing events. In Tucson, there is a big event on the University of Arizona Mall tomorrow with free telescope viewing. There are also free events at Tumamoc Hill and Ventana Canyon Resort in Tucson. I recommend going out and seeing it while the Sun is up and then joining the streams after the Sun goes down where you are to see the end.
I will be at NOAO with for the first part and then probably hit the east side of Tucson where I normall photograph sunset to take pictures with my scopes.
This is the last time you will get to see a transit of Venus unless you are planning on having your head frozen...don't rely on them figuring out how to reanimate you!
Reprinted with permission from the Half-Astrophysicist Blog.