Venus

Venus orbits the Sun on a near circular orbit at mean distance of 0.723 Astronomical Units (AU). At aphelion the planet is at a maximum distance of 0.728 AU from the Sun, while at perihelion Venus it is 0.718 AU away. Coming as near as 0.272 AU, no other planet can approach the earth as close as Venus does.

Since it circles the Sun in an orbit smaller than that of Earth's, Venus is never very far away from the Sun for terrestrial viewers. The greatest angular distance between the Sun and Venus, as seen from Earth, is 47 degrees. This means that even under the most favorable of conditions Venus will set at most three hours after the Sun, or rise no earlier than three hours before the Sun. Observed since the most ancient of times, Venus is often called the "morning star," if it rises before dawn or the "evening star" if it sets after the Sun. The Greek philosopher Homer referred to Venus in his Illiad as "the most beautiful star set in the sky."

The time required for Venus to complete one orbit about the Sun, its sidereal period, is 224.701 days, whereas the time for Venus to repeat alignments with respect to the Sun and Earth is 584 days. The best times for viewing Venus are when it is near greatest eastern, or greatest western elongation. Greatest western elongation follows about five months after greatest eastern elongation, and greatest eastern elongation's repeat about every 19 months.

As it circles the Sun, Venus shows phases just like the Moon, and the planet Mercury. The phase changes of Venus were first recorded by Galileo Galilei in 1610. Galileo's observations of Venusian phase changes were important since they strengthened his belief in the heliocentric model of the solar system which had been proposed by Copernicus in his now famous book, De Revolutionibus, published in 1543. Galileo reasoned that if Venus circled Earth, as the then accepted geocentric model of the Universe decreed, it would not show the full range of phases that were observed. Likewise, the correlation between angular size and phase would also be different to that observed if Venus orbited Earth.

Transits of Venus across the disk of the Sun, as seen from Earth, are not common. If a transit is to occur, however, it will take place in either June or December, when Earth is at the line along which Venus's orbit cuts the ecliptic—the line of nodes for Venus. A very precise geometrical alignment of Earth, Venus and Sun is required for a Venusian transit to take place. The last Venusian transit occurred on December 6, 1882. The next transit will take place on June 8, 2004. In general, Venusian transits are seen in pairs. Each of the transits in a pair are eight years apart, and the pair-cycle repeats on an alternating cycle of 121.5 and 105.5 years.