A transit of Venus is a rare event in which the planet Venus is directly between the Earth and the Sun. The black circle near the top of the photograph is the disk of Venus, blocking the Sun's light. (The smaller dark areas lower in the photo are sunspots.)
The transit lasted more than six hours, as Venus made its way across the visible disk of the Sun. Unfortunately, the sky was completely overcast here almost the entire time, but the Sun peeked out from behind the clouds briefly about 37 minutes after the transit began. That's when this photograph was taken; you can see clouds in front of the Sun still.
The next transit of Venus will be in the year 2117!
