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Why do some planets seem to move differently?
Home » Astronomy » Observing the Planets » Why do some planets seem to move differently?Above all, the planets follow the rules of retrograde motion, sidereal time, and their position on the ecliptic plane. The only exception is Pluto, whose orbit is slightly inclined to the ecliptic.
Keep in mind Mercury is so close to the Sun that we can only see it briefly. Both it and Venus appear before the Sun rises or after the Sun sets. Both of these planets orbital rings are smaller than Earth’s, so they will never go completely across the sky.
Unless Earth is behind the Sun from the viewpoint of the outer planets, they are always there—following sidereal time. Remember, each night the sky changes by four minutes... If an outer planet begins in the morning sky at 5:00 a.m., it will be 75 days before it rises at midnight, or 150 days before it appears at 7:00 p.m.!
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