Planetary Motions

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There are other celestial objects that drift eastward with respect to the stars. They are the planets (Greek for ``wanderers''). There is much to be learned from observing the planetary motions with just the naked eye (i.e., no telescope). There are 5 planets visible without a telescope, Mercury, Venus, Mars, Jupiter, and Saturn (6 if you include Uranus for those with sharp eyes!). All of them move within 7 degrees of the ecliptic. This tells you something about of the orientation of the planet orbit planes with respect to the ecliptic---the figure below shows how flat the solar system is when viewed along the ecliptic plane. The planet positions, of course, do change as they orbit the Sun, but the orbit orientations remain the same.

solar system side view

The arrow pointing to Polaris in the solar system picture is tilted by 23.5 degrees because the Earth's rotation axis is tilted by 23.5 degrees with respect to the ecliptic. As viewed from the Earth, two of the planets (Mercury and Venus) are never far from the Sun. Venus can get about 48 degrees from the Sun, while Mercury can only manage a 27.5 degrees separation from the Sun. This tells you something about the size of their orbits in relation to the Earth's orbit size—their orbits are smaller and inside the Earth's orbit. When Venus and/or Mercury are east of the Sun, they will set after sunset so they are called an ``evening star'' even though they are not stars at all. When either of them is west of the Sun they will rise before sunrise and they are called a ``morning star''.

geometry for inferior planet phases

Planets produce no visible light of their own; you see them by reflected sunlight. True stars produce their own visible light. The planets inside the Earth's orbit are called the ``inferior'' planets because their distance from the Sun is less than (or inferior to) the Earth's distance from the Sun. Their closeness to the Sun enables us to see them go through a complete set of phases. Because they can get between us and the Sun, Venus and Mercury can be seen in a crescent or new phase. This also explains why the planets outside the Earth's orbit, called the ``superior planets'', are never seen in a crescent or new phase. When Venus is in crescent phase, it is the brightest object in the sky besides the Moon and the Sun. Even though you see a small fraction of its sunlit side, it is so close to us that you see it appear quite bright. At these times, Venus is bright enough to create a shadow! The fact that you can see Venus and Mercury also in gibbous and nearly full phase proved to be a critical observation in deciding between a Earth-centered model and a Sun-centered model for the solar system.

Because Mercury and Venus are closer to the Sun than we are (i.e., their orbits are inside the Earth's orbit), they are never visible at around midnight (or opposite the Sun). The superior planets can be visible at midnight. At midnight you are pointed directly away from the Sun so you see solar system objects above the horizon that are further out from the Sun than we are. If you want to see where the planets are in their orbits today or any other date, then go to the Solar System Live site (will display in another window).

Ordinarily the planets ``wander'' eastward among the stars (though staying close to the ecliptic). But sometimes a strange thing happens---a planet will slow down its eastward drift among the stars, halt, and then back up and head westward for a few weeks or months, then halt and move eastward again. The planet executes a loop against the stars! When a planet is moving backward it is said to be executing retrograde motion. Perhaps it seemed to the ancients that the planets wanted to take another look at the stars they had just passed by.

The figure below shows Mars' retrograde loop happening at the beginning of 1997. Mars' position is plotted every 7 days from October 22, 1996 (the position on November 12, 1996 is noted) and the positions at the beginning and end of the retrograde loop (February 4 and April 29, 1997) are noted. An animation of this is available here. What causes retrograde motion? The answer to that question involved a long process of cultural evolution, political strife, and paradigm shifts. You will investigate the question when you look at geocentric (Earth-centered) models of the universe and heliocentric (Sun-centered) models of the universe in the next chapter.

Mars' retrograde loop in 1997

Vocabulary

retrograde motion

Review Questions

  1. How do the planets move with respect to the stars?
  2. What does the fact that all of the planets visible without a telescope move within 7° of the ecliptic imply about the alignment of their orbital planes? What would an edge-on view of our solar system look like?
  3. Why are Venus, and Mercury never seen at midnight while the other planets can be visible then?
  4. What phase would Venus be in when it is almost directly between us and the Sun? Where would it be in its orbit if we see in a gibbous phase?
  5. Are the planet motions random all over the sky or are they restricted in some way?

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last updated: January 8, 2013

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel