Portal:Astronomy/Featured/August 2005

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and the Earth, obscuring a small part of the Sun's disc. During a transit, Venus can be seen from the Earth as a small black disc moving across the face of the Sun. Before modern astronomy, observations of transits of Venus helped scientists measure the distance between the Sun and the Earth using the method of parallax.

Transits of Venus are rare and occur in a pattern that repeats every 243 years, with pairs of transits 8 years apart separated by long gaps of 121.5 years and 105.5 years. The pattern repeats every 243 years because after this time both Venus and Earth have returned to very nearly the same point in each of their respective orbits. This period of time corresponds to 152 synodic periods of Venus.

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