By Roy Bishop; from the 2012 Observer's Handbook of the Royal Astronomical Society of Canada (RASC)

From the third planet, only Mercury and Venus can be seen silhouetted against the Sun. When an inferior conjunction takes place, the appreciable orbital inclinations of Mercury and Venus usually cause them to pass north or south of the solar disk and no transit occurs. Transits are uncommon for Mercury, rare for Venus. During the 20th century, there were 14 transits of Mercury, and 0 transits of Venus. Currently, transits of Venus occur in pairs, with 8.0 years separating the members of a pair, and the pairs separated alternately by 105.5 and 121.5 years, resulting in a 243.0-year period for the pattern.

Johannes Kepler, extraordinary astronomer and author of the Rudolphine Tables of planetary positions, predicted the Venus transit of 1631. Unfortunately, he died in 1630. Jeremiah Horrocks and his friend William Crabtree, in England, were the first to see a transit of Venus, on 1639 Dec. 4. Beginning with the transit pair in that century, years having Cytherean transits include:

Read more: RASC 2012 Transit of Venus