Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 12: The Wanderers

Key Ideas

• "Wandering stars" that follow complex paths near the Ecliptic.

• Inferior Planets
• Superior Planets
• Conjunction & Opposition

The Naked-Eye Sky

Sun:

Bright disk ~1/2º across

Moon:

Pale disk ~1/2º across that goes through monthly phases.

Stars:

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 88^d (Mercury) and 30^y (Saturn) to complete a circuit through the Zodiac.

Five Classical Planets:

Mercury, Venus, Mars, Jupiter, & Saturn

Inferior & Superior Planets

Inferior Planets: Mercury & Venus

• Appear to follow the Sun across the sky.
• <28º for Mercury, and <47º for Venus.

Superior Planets: Mars, Jupiter, & Saturn

• All move along the Ecliptic on paths seemingly independent of the motions of the Sun.
• All can appear in the sky any time (e.g., around midnight)

Inferior Planet Configurations

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:

• Planet is at its furthest East of the Sun as seen from the Earth (28º for Mercury, 47º for Venus)
• Rises & sets after the Sun ("evening star")

Maximum Western Elongation:

• Planet is at its furthest West of the Sun as seen from the Earth (same angles as Eastern Elongation).
• Rises & sets before the Sun ("morning star")

Superior Planet Configurations

Opposition:

• Planet is opposite the Sun in the sky.
• Rises as the Sun sets
• Highest in the sky at midnight.

Conjunction:

• Planet is on the same side of the sky as the Sun.
• Rises with the Sun
• The planet does not appear in the the night sky.

Eastern Quadrature:

• Planet at right angles to the Earth-Sun line.
• Planet rises at noon, sets at midnight.

Western Quadrature:

• Planet at right angles to the Earth-Sun line.
• Planet rises at midnight, sets at noon.

Retrograde Motion

• Called "Direct Motion".
• Motion is non-uniform (not at the same speed).

Sometimes, however, the planets appear to

• Slow down & stop!
• Start moving westward, or RETROGRADE,
• Slow down & stop again,
• Resume moving eastward again.

Apparent retrograde motion is observed in all planets.

Inferior Planets (Mercury & Venus):

• Undergo retrograde motion at around the time of inferior conjunction.

Superior Planets (Mars, Jupiter, & Saturn):

• Undergo retrograde motion at around the time of opposition.

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?

The struggle to understand these motions took nearly 3000 years:

• The motions are subtle and defy poetic or metaphorical description.
• Their complexity similarly defies simple geometric description.

A Question of Approach

Two approaches have been taken:

Phenomenological Description

• Find a way to compute the motions without worrying about the details of why they work this way.
• Goal is to "preserve appearances" (i.e., make reliable predictions)

Physical Description

• Discover the underlying physical principles behind the planetary motions (i.e., ask "why" they move).
• Predictions of the motions should then follow from first principles

From Myth to Science

Internal Consistency:

• Must follow the same basic rules, no special cases or special pleading 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.