General

Venus is the second out from the sun of the two ‘inferior’ planets. It has a diameter at the surface of 7521 miles (12104 km), only a shade less than that of the Earth (fig. 6.1). Measured at the cloud tops the diameter is 7606 miles (12240 km). Because of this close similarity in size to the Earth, Venus has often been called ‘the Earth's twin sister’. The masses and therefore the densities and gravities are also similar. The resemblances end here, however, because the two planets are vastly different in other respects.

The mean distance of Venus from the sun is 67.2 million miles (108.2 million km) and varies from a perihelion distance of 66.8 million miles (107.5 million km) to the aphelic distance of 67.7 million miles (108.9 million km). The orbital eccentricity is 0.007 so that the orbit is very nearly circular in contrast to Mercury's orbit.

Venus comes closer to the Earth than any other celestial body except for the moon, a comet, an occasional meteor or an asteroid. At its nearest it is only about 100 times the moon's distance from us.

Venus journeys around the sun at a mean orbital velocity of 21.8 miles (35.1 km) per second and completes one circuit of the orbit in 224.7 Earth days, its sidereal period. Its mean synodic period, say from one inferior conjunction to the next, is 584 days.