©2005 Bob Riddle

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Sky Explorations

An Exploration of the Earth in Space

Retrograde Motion of Mars - 2005

The retrograde motion of Mars occurs each time that the Earth catches up with and passes Mars as both planets orbit the Sun. This is somewhat like a faster   runner traveling on the     inner lane of a elliptical track passing a slower    runner traveling in an outer lane.

Since each planet remains in its own "lane", the Earth never really intercepts or crosses the orbital path of Mars. The Earth simply catches up with Mars (E1,P1-E2,P2) and then passes by (E3,P3-E5,P5), leaving the Red Planet behind (E6,P6). As a result, when viewing from Earth, Mars appears to come to a stop (E3,P3), begin to move backward, come to a stop again (E5,P5), and then begin to move forward.

At opposition, 28 August, (E4,P4) Mars will be visible all night. It rises at sunset and sets at sunrise local time.

By: George Reed