II.

Observation from Earth

When viewed through a telescope, Venus exhibits phases like the Moon. This effect occurs because Venus orbits closer to the Sun than Earth does. An observer on Earth can see Venus illuminated from the side and from behind as well as face-on—planets further away from the Sun than Earth only show a nearly fully lit, face-on view toward our planet. The phases of Venus were one of the proofs that the 17th-century astronomer Galileo used to show that Earth itself orbits the Sun. Maximum brilliance (a stellar magnitude of -4.4, 15 times as bright as the brightest star) is seen in the crescent phase when the planet is closer to Earth. Venus’s full phase appears smaller and dimmer because it occurs when the planet is on the far side of the Sun from Earth. The phases and positions of Venus in the sky repeat every 1.6 years (see Time; Year).

Transits of Venus (when the planet moves across the face of the Sun as seen from Earth) are rare, occurring in pairs at intervals of a little more than a century. The most recent transit occurred in 2004. The second transit of the pair will be in 2012. The next pair of transits will occur in 2117 and 2125.

The thick cover of clouds around Venus meant that earlier generations of astronomers using telescopes had little information about conditions on the surface. Some researchers speculated that Venus might be a lush tropical world or an ocean planet drenched by thick rain clouds. Other scientists predicted a dry desert swept by dust storms, or with petroleum seas. The first clues that conditions on Venus might be extremely hot came from microwave observations in 1956. Earth-based radar studies in the 1960s discovered the planet’s slow retrograde rotation. It would take space probes to provide much more detailed information.