Zodiacal MotionAn Introduction

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Johann Bayer, Uranometria (Ulm, 1661)

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Kerry Magruder and Brent Purkaple

Against the background of the daily diurnal motion,the outer planets have an additional slow motioncalled their Zodiacal (or !direct") motion. Stars arefixed in relative position, but planets move slowlyfrom west to east.

As the planets move in their direct, eastward motionfrom night to night, they travel near the path of theSun (the ecliptic). Any constellation containing thepath of the Sun is a Zodiac constellation. A planet#seastward motion along the Zodiac is its Zodiacal ordirect motion.

On a celestial globe, find the path of the Sun, or ecliptic.(See the Introduction to the Celestial Globe.)

Imagine that a planet lies at any point on the ecliptic.Each night, when the planet rises in the east, it willmove a little farther eastward along the ecliptic.In other words, it falls a little behind the daily motionof the background stars. For that reason, it will riseslightly later each evening, taking more than 24hours to appear above the horizon from one risingtime to the next. For example: if a planet rose withthe bright star Regulus this morning, then tomorrowmorning Regulus will rise first, and the planet will fallbehind it, rising a minute or so later.

Zodiacal MotionDemonstration

1. Hold the celestial globe so that the equator is inboth your left hand (east) and your right hand (west).You are facing south. Stars and planets will appear torise in the east and set in the west as you rotate theglobe from left to right, as in the exercise on diurnalmotion.

2. This time, imagine that the planet falls behind thefixed star it rose with the day before. Find Regulus inthe Leo the Lion. Regulus lies almost exactly on theecliptic. Turn the celestial globe so that Regulus isrising on the eastern horizon (left). Imagine that theplanet is rising very near the ecliptic, at the very sametime as Regulus.

3. Turn the globe from left to right one full turn, sothat Regulus is rising very near the ecliptic, on theeastern horizon (left) once more. Now, the planet hasfallen behind Regulus so that it is still invisiblebeneath the horizon.

4. Turn the globe from left to right just a slightamount, so that Regulus is just a little above theeastern horizon. Now the planet is rising on theeastern horizon.

Alternatively, imagine that the planet is a tiny antwalking on the ecliptic from your right hand towardyour left. The ant marches less than half an inch witheach turn of the globe. You see the ant rise above theeastern horizon a little later with each turn of thecelestial globe. This is the direct, eastward motion ofthe outer planets (Mars, Jupiter and Saturn) as theytravel around the Zodiac.

Regulus on the ecliptic

Taurus the Bull on the ecliptic