JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Retrograde motion of Mars. Diagram of the zodiacal constellations of the ecliptic plane, showing the retrograde motion of Mars as seen from Earth. The Earth orbits the Sun in less time than Mars does. This means that at certain times in its orbit it overtakes Mars. This is illustrated by showing the positions of Earth and Mars in August (right) and October (left). The blue arrows show the position of Mars against the night sky as seen from Earth. This shows how Mars appears to reverse its motion in the night sky. The zodiacal constellations mark the ecliptic plane of the planetary orbits and the changing position of the Sun against the sky as the Earth orbits the Sun.
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