Timeline
1608 Telescope invented.
1629 Kepler predicts that a transit of Mercury and a transit of Venus will occur in 1631.
1631 November 7 At least four people see the transit of Mercury; they are the first witnesses to a planet's transit across the Sun.
1631 December 7 First predicted transit of Venus; no reports of any observations.
1639 December 4 Horrocks and Crabtree are the first to see a transit of Venus.
1672 Astronomers attempt to calculate the Earth-Sun distance by triangulating the Earth-Mars distance, with mixed results.
1679 Halley believes a transit of Venus can provide an accurate Earth-Sun distance.
1716 Halley develops a method to use transits of Venus to measure the Earth-Sun distance.
1725 Delisle improves on Halley's measurement technique.
1761 June 6 Transit of Venus.
1763-67 Mason-Dixon Line measured.
1769 June 3 Transit of Venus.
1776 American colonies declare independence from Britain.
1840 First astronomical photograph taken - a daguerreotype of the Moon.
1844 U.S. Naval Observatory established.
1846 Smithsonian Institution established.
1871 Congress establishes U.S. Commission on the Transit of Venus.
1874 December 9 Transit of Venus.1882 December 6 Transit of Venus.
1895 Official value of Earth-Sun distance standardized at 92,867,000 miles; transit-of-Venus observations given little weight.
1976 Earth-Sun distance determined to be 92,955,859 miles by timing echoes of radar signals sent to Venus from Earth.
2004 June 8 Transit of Venus.
2012 June 6 Transit of Venus.