Lecture 12: The Wanderers

Key Ideas

The Planets: Planetary Configurations: Retrograde Motion

The Naked-Eye Sky

Celestial objects visible to the naked-eye include
Bright disk ~1/2 across

Pale disk ~1/2 across that goes through monthly phases.
Pinpoints of light that appear fixed relative to each other on the Celestial Sphere.

Planets: (Greek: planetai = wanderers)
Points of light that move relative to the "fixed" stars.
Stay within a few degrees of the Ecliptic
Follow complex paths that take between 88d (Mercury) and 30y (Saturn) to complete a circuit through the Zodiac.

Five Classical Planets:

Mercury, Venus, Mars, Jupiter, & Saturn

Inferior & Superior Planets

Early astronomers recognized that the 5 classical planets could be divided into two groups:

Inferior Planets: Mercury & Venus

Superior Planets: Mars, Jupiter, & Saturn

Inferior Planet Configurations

(Click on the image to view at full scale [Size: 9Kb])

Inferior Conjunction:

Superior Conjunction:

During either conjunction, the inferior planet appears to rise and set with the Sun.

Maximum Eastern Elongation:

Maximum Western Elongation:

Superior Planet Configurations

(Click on the image to view at full scale [Size: 9Kb])



Eastern Quadrature:

Western Quadrature:

Retrograde Motion

In general, the planets move eastward relative to the "fixed" stars.

Sometimes, however, the planets appear to

[Click Here to see an image (16Kb) of the retrograde motions of Mars during 1994/95 when it made a particularly characteristic loop. A 688Kb QuickTime movie shows this figure in motion (beware! it is large!)]

Apparent retrograde motion is observed in all planets.

Inferior Planets (Mercury & Venus):

Superior Planets (Mars, Jupiter, & Saturn):

For both, the paths are complex, making loops and S-curves due to an additional North and south wandering about the Ecliptic. [Click Here to see an montage (18Kb) of the recent (1998-2000) retrograde motions of Mercury, Venus, Mars, Jupiter, and Saturn. Note that Jupiter & Saturn, currently visible after sunset high in the Eastern sky, are just now (Jan 18) ending a period of retrograde motion that started back in October of last year.]

Who Ordered That?

Unlike all of the other motions we've seen thus far, planetary motions are dauntingly complex.

The struggle to understand these motions took nearly 3000 years:

Explaining planetary motions poses a formidable challenge to any theory of the heavens.

A Question of Approach

How do we explain the motions of the planets?

Two approaches have been taken:

Phenomenological Description

Physical Description

From Myth to Science

Any satisfactory theory of planetary motions had to have the following characteristics:

Internal Consistency:

Predictive Power:

The effort to come up with a self-consistent, predictive theory of planetary motions marks the true birth of science.

Return to [ Unit 2 Index | Astronomy 161 Main Page ]
Updated: 2006 September 23
Copyright Richard W. Pogge, All Rights Reserved.