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PLANETARY MOVEMENT

On any given night, the planets look stationary against thebackground stars. But watch over a period of a few days forVenus or Mars (with orbital periods of 225 days and1.88 years) and you can easily see why they arecalledplanets, from Greek, meaningwanderers. Farther out,Jupiter andJupiter and Saturn (11.9 and 29.4 years) takelonger for movement to be noted, but move they do. Farther out yetare much slower movingUranus and Neptune (83.7and 163.7 years), but even these slowly trace their orbital coursesagainst the stellar scene. Most of the times the planets movedirectly, to the east through the constellations of theZodiac. But as our Earth laps the slower-moving outer planets (oras the faster inner planets lap us), they seem to stop and gobackward intoretrograde. Direct motion is shown below by Venusand Mars and by Uranus, while retrograde is seen for both Jupiterand Saturn.

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) moveverynoticeably to the east against the stars ofAries andTaurus.

Jupiter in Retrograde

Jupiter moved some 10 degrees to the west inretrograde throughTaurusbetween October of 1988 (left) and January of 1989 (right) againstthe background of theHyades (bottom)andPleiades (upper left). This pair of identically-exposed photographs also shows just howmuch we have lost of the nighttime sky. The one on the left wastaken from town, the one on the right from a dark mountaintop. Atleft, none of the faint stars can be seen.

Jupiter and Saturn inRetrograde

The movements of Jupiter (the brightest body in the picture) andSaturn (the bright one to the right of Jupiter) are easy to seeagainst the background ofTaurus, bothclose to the Hyades and thePleiades. At left, on October 12,2000, the planets have just begunretrograde; at right, February 16, 2001, they have alreadyresumed direct motion, but are still far west of their previousposition.

Journeys of Uranus and Neptune

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

Then follow Uranus as it passed throughAquarius some four years later, seen here on October 9,2004.

It took Uranus another six years to make it to southwesternPisces, where it accompanied Jupiter, asseen on October 2, 2010. On the other hand, during the 10 yearssince it was last in Taurus (as seen above), Jupiter had moved morethan three-fourths of the way around the sky.

Uranus is speedy compared to Neptune. After a full nine years, themore distant planet (see here on November 8, 2010) had not evenmade it out of Capricornus. Neptune is seen close to its 1846discovery 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.