11/7/2023 0 Comments Planetary retrograde motionIn the case of Uranus, the whole retrograde part of its path covers only 4° of ecliptic longitude. The other outer planets are less dramatic. When I observed it every two or three days, Mars’s movement against the stars was obvious to the naked eye. In the summer of 2016, for example, Mars traced out its retrograde path against a background of fairly bright stars. The word “planet” comes from the Greek word meaning “wanderer.” The retrograde motion caused many headaches for early astronomers until Copernicus explained it in 1543, in his monumental book “On the Revolutions of Heavenly Spheres.” The apparent backward motion of the planets is caused by the relative motion of the Earth and the other planets as they all go around the Sun.įor the superior planets, the retrograde motion is more or less easy to see. For most of the time they move from west to east, but for a fraction of their periods they move from east to west. Roger CowleyĪs seen from the Earth, the other planets move in an irregular way with respect to the stars. In this photo, taken just before sunrise on April 4, 2017, Venus is low on the horizon while the measurable stars are near the top of the photograph. Inspired by a "Sun, Moon & Planets" column, the author spent the spring of 2017 tracking Venus's retrograde motion across the sky, exploring the relationship between the orbits of Venus and the Earth pointed out by Copernicus.
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