Observation
 
The transit starts at about 5:13 Universal Time (UT) and ends at about 11:16 UT. If you’re watching the transit from the northern hemisphere, Venus will move from left to right on the lower half of the Sun, at a slight downward angle from start to finish. If you’re in the southern hemisphere, Venus will move from right to left at a slight upward angle on the top half of the Sun. (People in the southern hemisphere are "upside down" relative to people in the northern hemisphere, and vice versa.)
 
The transit starts with the first contact, when Venus’s outside edge first appears to touch the outside edge of the solar disk, or "solar limb." It will then take about 19 minutes for Venus to arrive at the second contact, when Venus touches the inside of the solar limb. The time between the first and second contacts is known as the "ingress." A little over six hours later, the third and fourth contacts will occur, incorporating the "egress." (The egress repeats the process of the ingress, only in reverse.)

View of the transit path from the northern hemisphere. Depending on what your viewing location is, the path’s position will vary slightly.

A simulation of the black-drop effect, projected onto a real image of the Sun.

If you’re able to observe either the beginning or end of the transit with a telescope (equipped with a special filter!), you’ll be able to observe something called the "black-drop effect": Venus seems to develop a smeared appendage as it enters and leaves the solar disk. The black-drop effect impeded astronomers from accurately timing Venus’s ingress and egress; without exact times, their calculation of the astronomical unit (AU), the distance from Earth to the Sun, was inexact.
For centuries, no one knew what caused the black-drop effect, although suggestions included the idea that Venus’s substantial atmosphere was responsible. In 1999, however, NASA’s Transition Region and Coronal Explorer (TRACE) observed the black-drop effect during a transit of Mercury. This was significant because TRACE was beyond Earth’s atmosphere, and Mercury has no atmosphere to speak of. It now appears that the black-drop effect may be due to limitations in telescopes, combined with the fact that the disk of the Sun appears darker at its edge.
(For more information, go to http://nicmosis.as.arizona.edu:8000/POSTERS/TOM1999.jpg.)
 
Another thing to watch for as you observe the transit is a "halo" around Venus. Mikhail Lomonosov, a Russian scientist observing from St. Petersburg, noticed this halo during the 1761 transit. He proposed that the halo occurred because Venus had an atmosphere; he was later proven correct by the astronomer William Herschel.
 
Complication
 
Even the most sophisticated telescope won’t be of any use if Mother Nature decides to send in the clouds. If clouds are covering the Sun in your neck of the woods (or at the Exploratorium’s Webcast venue in Greece), the transit won’t be visible. The map on the previous page shows what kind of cloud cover might occur in various locations.