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Venus' orbit is slightly inclined relative to the Earth's orbit; thus, when the planet passes between the Earth and the Sun, it usually does not cross the face of the Sun. However, [[transits of Venus]] do occur in pairs separated by eight years, at intervals of about 120 years, when the planet's [[inferior conjunction]] coincides with its presence in the plane of the Earth's orbit. The most recent transit was in 2004; the next will be in 2012. Historically, transits of Venus were important, because they allowed astronomers to directly determine the size of the [[astronomical unit]], and hence of the solar system. [[Captain Cook]]'s exploration of the east coast of Australia came after he had sailed to [[Tahiti]] in 1768 to observe a transit of Venus.
A long-standing mystery of Venus observations is the so-called 'ashen light'—an apparent weak illumination of the dark side of the planet, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made as long ago as 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated that it may result from electrical activity in the Venusian atmosphere, but it may be illusory, resulting from the physiological effect of observing a very bright crescent-shaped object.<ref>Baum, R. M. (2000), ''The enigmatic ashen light of Venus: an overview'', Journal of the British Astronomical Association, v.110, p.325</ref>venus
==Studies of Venus==
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