Transit of Venus

   Every One-hundred years or so, observers on Earth can watch Venus at inferior conjunction pass across the face of the Sun. These rare events are called transits of Venus, and they occur in a pair, 8 years apart. The last two transits were in 1874 and 1882; the next two take place on 8 June 2004 and 6 June 2012. In 2004, Venus crossed the southern part of the Sun, and in 2012, the northern part.
   In earlier times, transits of Venus let astronomers measure the distance from Earth to Venus, and, by extension, the scale of the Solar System. After the telescope was invented, a handful of astronomers made individual efforts for the 1639 transit. But for the 1761 and 1769 events, the British and French sent out expeditions all over the globe, among them the famous exploring voyage of James Cook, which went to Tahiti for the 1769 transit.
   Better distances for the Solar System did emerge from these efforts, but the most notable finding was about Venus itself. Observing the 1761 transit, the Russian scientist Mikhail Lomonosov discovered that Venus has an atmosphere. He noted the halo it produced around the black dot of the planet as it slipped onto the solar disk and off again.

Transit of Venus in 2004