Moving Planets

Planets are always on the move, and are easier to see at some times than at others. The inner (or inferior) planets, Mercury and Venus, never appear very far from the Sun. Their maximum separation from the Sun is termed greatest elongation, either east of the Sun (in the evening sky) or west (morning sky). They are invisible around the times of superior or inferior conjunction, although occasionally they cross in front of the Sun at inferior conjunction to cause a transit. The outer (or superior) planets can lie anywhere along the ecliptic, and are best seen when near opposition. They then lie iNfERiOR ORBiT

directly opposite the Sun in the sky and are visible all night. At opposition the outer planets are closest to us and appear brightest. Conversely, at conjunction they are behind the Sun and invisible.

superior conjunction conjunction greatest eastern elongation greatest eastern elongation

angle of, elongation greatest western elongation greatest western elongation

\ Earth / \ planet opposition / (closest to Earth)

angle of, elongation inferior conjunction

\ Earth / \ planet opposition / (closest to Earth)

SUPERiOR ORBiT

Planispheres are easy to use, cheap, portable, and never go out of date. Their disadvantages are that they do not show the positions of moving objects such as the Moon and planets, they work only for the latitude for which they are designed, and they depict only the brightest objects. A planisphere can be used up to about 5° from the latitude for which it is designed, after which a discrepancy with the sky will become noticeable.

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