A Transit of Venus, one of the rarest astronomical events, will take place June 5 as the disk of the planet Venus passes like a moving sunspot across the face of the Sun.

Venus transits occur eight years apart, then don’t repeat again for another century. The last transit before 2004 took place in 1882. The next will be in 2117.

Observations of these events helped 18th-century scientists learn about the solar system. Venus transits provided astronomers with data that eventually led to a very close estimate of the distance between the earth and the sun.

British astronomer Edmund Halley observed a transit of Mercury in 1677 and in 1716 published his ideas for using such events to measure the distance from the Earth to the Sun, known as the Astronomical Unit. By then he had determined it was more useful to study a transit of Venus, with two people at different latitudes measuring the shift in the planet’s position, its parallax.

In 1761, France and Britain sent expeditions across the world time the exact moment when Venus and the Sun initially touch. Unexpectedly, these observations were hampered by the appearance of a bright ring around Venus as the planet entered and exited the solar disk, blurring the precise lines of contact with the Sun. This led Russian scientist Mikhail Lomonosov to speculate that the glow around Venus meant it had an atmosphere.

Captain James Cook stopped in Tahiti to observe the next transit of Venus in 1769 while on a voyage to circumnavigate the Earth.

Data from both transits allowed 18th-century scientists to calculate the Astronomical Unit within one percent of the 93,000,000 mile distance determined using modern technology.

On June 5, the transit begins at 22:00 UT. The Exploratorium’s webcast will have a telescope feed plus audio commentary every 30 minutes. The duration of the program will be about six-and-a-half hours, beginning at 22:00 UT (noon in Hawaii) on June 5. First contact is at nine minutes past the hour.

[Liberally drawn from The Exploratorium website.]