7. Venus: The veiled planet

The veiled planet

The view from Earth

Fig. .. 

When viewed through a telescope, Venus brightens and fades, and also changes in apparent size, during its dance around the Sun. As noticed by Galileo Galilei (1564-1642) in 1610, the planet exhibits a complete sequence of Moon-like phases. Its apparent illumination goes from a full round disk to a narrow crescent and back to rotundity again every19 months. This was one of the earliest indications that the planets move around the Sun rather than the Earth. Venus also appears to grow when it approaches us in its orbit and shrinks as it recedes. When Venus is furthest from the Earth, on the opposite side of the Sun, it is fully illuminated and smallest. As the planet comes closer to Earth, it looks partly illuminated and larger.

Venus rotates backwards, at an exceptionally slow rate

Although no human eye has ever seen the surface of Venus, radio waves can penetrate its obscuring veil of clouds and touch the landscape hidden beneath. By bouncing pulses of radio radiation off the surface, radar astronomers discovered in 1967 that Venus spins in the backward direction, opposite to that of its orbital motion. That is, unlike the other terrestrial planets, Venus does not rotate in the direction in which it orbits the Sun. The radar observations also showed that Venus spins with a period longer than any other planet, at 243.025 Earth days. This rotation period is even longer than the planet's 224.7 Earth-day period of revolution around the Sun, so the day on Venus is longer than its year.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University