Venus is the second planet from the Sun, with a nearly circular orbit having an average radius of 0.7 A.U. This gives it an orbital period of 225 days. Venus is peculiar in that its rotation is retrograde (in the opposite sense of the Earth and all other planets except Uranus) and because it is very slow: a day on Venus corresponds to 243 Earth days. At present, we have no solid explanation for why this is so. The most plausible theories invoke the collision of two large masses to form Venus in just such a way to cancel most of the rotation for the two masses. Like Mercury, but unlike the other planets, Venus has no moons.

The radius of Venus is almost exactly that of the Earth. Its average density is 5.2 g/cc, which is slightly less than that of the Earth or Mercury, but suggests a dense iron core and internal structure similar to that of the Earth. Venus has an extremely weak magnetic field, but that may be a consequence of its having such a slow rotational velocity.

Venus is always covered by a thick layer of clouds that make it impossible to see the surface for light in the visible part of the spectrum. Light at radar wavelengths penetrates the cloud deck and allows us to study the surface. A comparison of the motion of the surface with that of the upper clouds indicates that while the surface takes about 8 months to rotate, the clouds rotate all the way around the planet in about 4 days. This indicates that there are very high velocity winds in the upper part of the Venusian atmosphere.

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