Goddard/NASA IMAGE 5 JUNE 2012

Observing the Transit:  From NASA

Since the apparent diameter of Venus is nearly 1 arc-minute, it is just possible to see without optical magnification (but using solar filter protection) as it crosses the Sun. Nevertheless, the planet appears to be only 1/32 of the Sun’s apparent diameter so a pair of binoculars or a small telescope at modest power will offer a much more satisfying view.  All binoculars and telescopes must be suitably equipped with adequate filtration to ensure safe solar viewing.


Frequency of Transits

The orbit of Venus is inclined 3.4° with respect to Earth’s orbit. It intersects the ecliptic at two points or nodes that cross the Sun each year during early June and December. Since the invention of the telescope (1610), there have only been seven transits as listed in Table 5.

Table 5

Transits of Venus: 1601-2200

Date Universal Separation

1631 Dec 07 05:19 939 ”
1639 Dec 04 18:26 524 ”
1761 Jun 06 05:19 570 ”
1769 Jun 03 22:25 609 ”
1874 Dec 09 04:07 830 ”
1882 Dec 06 17:06 637 ”
2004 Jun 08 08:20 627 ”
2012 Jun 06 01:28 553 “

Still to Come
2117 Dec 11 02:48 724 ”
2125 Dec 08 16:01 733 “

The 2004 and 2012 transits form a contemporary pair separated by 8 years. More than a century will elapse before the next pair of transits in 2117 and 2125. During the 6,000-year period from 2000 BC to AD 4000, a total of 81 transits of Venus occur.

2004 and 2012 Transits NASA

History of Transits

When Johannes Kepler published the Rudolphine Tables of planetary motion in 1627, they permitted him to make detailed and fairly accurate predictions of the future positions and interesting alignments of the planets. Much to his surprise, he discovered that both Mercury and Venus would transit the Sun’s disk in late 1631. Kepler died before the transits, but French astronomer Pierre Gassendi succeeded in becoming the first to witness a transit of Mercury. The following month, he tried to observe the transit of Venus, but modern calculations show that it was not visible from Europe. Although Kepler’s predictions suggested that the next Venus transit would not occur until the following century, a promising, young British amateur astronomer named Jeremiah Horrocks believed that another transit would occur in 1639. His calculations were completed just a month before the event so there was little time to spread the word. Horrocks and his friend William Crabtree were apparently the only ones to witness the transit of Venus on 1639 Dec 04 which allowed them to accurately measure the apparent diameter of the planet. Unfortunately, Horrocks and Crabtree both died young before reaching their full potentials.

The distance to the Sun and planets can now be measured extremely accurately using radar, so the 2004 and 2012 transits are of minor scientific importance. Still, they are remarkably rare events that were of great value during the early the history of modern astronomy.