Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 10: Planetary Motions

Key Ideas

The Planets: "Wandering Stars" that follow complex paths near the Ecliptic
Planetary Configurations:: Inferior Planets; Superior Planets; Conjunction and Opposition
Retrograde Motion

The Naked-Eye Sky

Sun: Bright disk about 0.5 degrees across; Daily motion East to West; Annual motion North and South
Moon: Pale disk about 0.5 degrees across that goes through phases; Similar but not identical motion to the Sun; Rises later every day
Stars: Pinpoints of light that appear fixed onto the Celestial Sphere

The Wanderers

Planets: (Greek: planetai = wanderers): Objects that move relative to the "fixed" stars; Stay within a few degrees of the Ecliptic; Follow complex paths that take between 88 days (Mercury) and 30 years (Saturn) to complete a circuit through the Zodiac; Change in brightness
Five Classical Planets:: Mercury, Venus, Mars, Jupiter, and Saturn

Inferior and Superior Planets

Early astronomers divided the planets into Inferior and Superior
Inferior Planets: Mercury and Venus: Appear to follow the Sun across the sky; Less than 28 degrees for Mercury, less than 47 degrees for Venus
Superior Planets: Mars, Jupiter, and Saturn: Move along the Ecliptic independently of the Sun

Inferior Planet Configurations

Inferior Conjunction:: Planet is between the Earth and the Sun
Superior Conjunction:: Planet is on the other side of the Sun from the Earth
During either conjunction, the inferior planet rises and sets with the Sun.
Maximum Eastern Elongation:: Furthest East of the Sun (Mercury 28 degrees, Venus 47 degrees); Rises and sets after the Sun ("Evening Star")
Maximum Western Elongation:: Furthest West of the Sun; Rises and sets before the Sun ("Morning Star")

Superior Planet Configurations

Opposition:: Opposite the Sun in the Sky; Rises as the Sun sets; Highest at midnight
Conjunction: Same side of the sky as the Sun; Rises with the Sun; Does not appear at night
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

In general, the planets move eastward relative to the fixed stars: Called "Direct Motion"; Motion is non-uniform (not at the same speed)
But sometimes the planets appear to: Slow down and stop; Start moving westward, or Retrograde; Slow down, stop, and resume moving eastward
Inferior Planets:: Retrograde motion at inferior conjunction
Superior Planets:: Retrograde motion at opposition
For both, the paths are complex: Loops and S-curves due to additional north/south motions about the Ecliptic

Summary of Planetary Motions

Daily motions about the celestial poles
Generally eastward motion near the Ecliptic at different speeds for each planet
Westward "retrograde" motions at opposition (Superior Planets) or inferior conjunction (Inferior Planets)
Superior planets are brighter at opposition, fainter at conjunction
Any successful description of the Solar System must explain all of these facts.

Disorder in the Heavens?

Planetary motions are considerably more complex than others we have seen so far
The struggle to understand these motions took nearly 3000 years: The motions are subtle and defy poetic or metaphorical description; Their complexity defies simple geometric description

A Question of Approach

How can these motions be explained?
Phenomenologically: Find a way to compute the motions without worrying about why they work this way; Make good predictions
Physically: Discover the underlying physical principles behind the motions - ask "why?"; Predictions then follow from first principles

From Myth to Science

Any satisfactory theory of planetary motions has to have the following characteristics:
Internal Consistency:: They must follow the same basic rules, no special cases or special pleading.
Predictive Power:: Must provide measureably accurate predictions of future behavior
This effort marks the true birth of science

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.