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
Planet is between the Earth and the Sun.

Superior Conjunction
Planet on the other side of the Sun from Earth.

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 a large file)]

Apparent retrograde motion is observed in all planets.

Inferior Planets (Mercury & Venus):
Undergo retrograde motion around inferior conjunction.

Superior Planets (Mars, Jupiter, & Saturn):
Undergo retrograde motion at around opposition.
In all cases the apparent paths followed by the planets as seen from the Earth are very complex. They make various loops and S-curves during their retrograde motions that are due to the tilts of their paths relative to 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
Must follow the same basic rules, no special cases or special pleading is allowed.

Predictive Power
Must provide measurably accurate predictions of future behavior.
The effort to come up with a self-consistent, predictive theory of planetary motions marks the true birth of science.
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Updated: 2007 September 29
Copyright Richard W. Pogge, All Rights Reserved.