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Zodiacal motion of the Planets
Distinguishing Planets from Fixed Stars
If we are going to study the zodiacal motion of the planets, then we will need to first learn how to distinguish planets from fixed stars, since only planets (including Sun and Moon, but not fixed stars) undergo zodiacal motion. In fact, the zodiacal motion of the planets is recognizable in reference to the stable background of the "fixed" stars. We can note several ways in which planets change their relative positions against the background of fixed stars.
Study Questions.
1. Only seven celestial bodies seem to change their relative positions against the background of other "fixed" stars. What are these seven "wandering" bodies? 1. 2. 3. 4. 5. 6. 7.
2. What was the Greek name for these "wanderers"?
Note: Two wanderers (the "morning star" and "evening star") were often recognized as being different appearances of the same body, namely, Venus. (They were never seen in the sky together, and never appeared very far from the Sun.)
As with the Sun, the other planets move into different constellations at different times; sometimes a given planet is visible in the nighttime sky and sometimes it appears in constellations nearer the sun in the daytime sky (and thus is usually invisible).
Furthermore, when a planet returns to a given constellation it will not always be at the same month or season of the year.
The planets, like the Sun, lie within a particular constellation of the zodiac at any given time.
Sidereal Period
Since the direction of a planet's motion is roughly consistent, eventually it will return to about the same location at the same time of year with respect to the fixed stars (in the Zodiac). The time it takes to do so is called the planet's "sidereal" (or "zodiacal") period. Remember that the term "sidereal" literally means "related to the stars."
The term "zodiacal period" is even more specific for this kind of period of planetary motion because it refers to the zodiac constellations along which planets move.
Planet Sidereal Period Modern Value Sun about 365 1/4 days 365.256366 days; "sidereal year" Moon about 27 1/3 days 27.32166 days; "sidereal month" Mercury about 1 year (not applicable) Venus about 1 year (not applicable) Mars about 687 days 686.98 days Jupiter about 12 years 11.862 years Saturn about 30 years 29.458 years