On any given night, the planets look stationary against the background stars. But watch over a period of a few days for Venus or Mars (with orbital periods of 225 days and 1.88 years) and you can easily see why they are called planets, from Greek, meaning wanderers. Farther out, Jupiter and Jupiter and Saturn (11.9 and 29.4 years) take longer for movement to be noted, but move they do. Farther out yet are much slower moving Uranus and Neptune (83.7 and 163.7 years), but even these slowly trace their orbital courses against the stellar scene. Most of the times the planets move directly, to the east through the constellations of the Zodiac. But as our Earth laps the slower-moving outer planets (or as the faster inner planets lap us), they seem to stop and go backward into retrograde. Direct motion is shown below by Venus and Mars and by Uranus, while retrograde is seen for both Jupiter and Saturn.

Venus and Mars

Venus and Mars Venus and Mars
Between March 9, 2004 (left) and just two days later, March 11, 2004 (right), Venus (at bottom) and Mars (below the Pleiades star cluster) move very noticeably to the east against the stars of Aries and Taurus.

Jupiter in Retrograde

Jupiter moved some 10 degrees to the west in retrograde through Taurus between October of 1988 (left) and January of 1989 (right) against the background of the Hyades (bottom) and Pleiades (upper left). This pair of identically-exposed photographs also shows just how much we have lost of the nighttime sky. The one on the left was taken from town, the one on the right from a dark mountaintop. At left, none of the faint stars can be seen.

Jupiter and Saturn in Retrograde

Jupiter and Saturn Jupiter and Saturn
The movements of Jupiter (the brightest body in the picture) and Saturn (the bright one to the right of Jupiter) are easy to see against the background of Taurus, both close to the Hyades and the Pleiades. At left, on October 12, 2000, the planets have just begun retrograde; at right, February 16, 2001, they have already resumed direct motion, but are still far west of their previous position.

Journeys of Uranus and Neptune

Uranus Uranus and Neptune
Between August 18, 2000 (left) and October 6, 2001 (right), Uranus slowly moved easterly against the stars of Capricornus. Neptune, much farther away and fainter, is just visible at right.

Uranus Then follow Uranus as it passed through Aquarius some four years later, seen here on October 9, 2004.
Uranus It took Uranus another six years to make it to southwestern Pisces, where it accompanied Jupiter, as seen on October 2, 2010. On the other hand, during the 10 years since it was last in Taurus (as seen above), Jupiter had moved more than three-fourths of the way around the sky.
Neptune Uranus is speedy compared to Neptune. After a full nine years, the more distant planet (see here on November 8, 2010) had not even made it out of Capricornus. Neptune is seen close to its 1846 discovery position (the actual anniversary on July 12, 2011). While it has gone around but once since it was found, Jupiter has circled the Sun some 14 times.

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Copyright © James B. Kaler.