Much of our concern this semester will be with the development of our present
understanding of the Solar System. We begin with a brief overview of the modern
and ancient classifications of the planets.
The Modern Solar SystemThe planets of the modern solar system are
grouped into several different and sometimes overlapping classifications, as
illustrated in the following figure:
- The planets inside the orbit of the earth are called the Inferior
Planets: Mercury and Venus.
- The planets outside the orbit of the earth are called the Superior
Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
- The planets inside the asteroid belt are termed the Inner Planets
(or the Terrestrial Planets): Mercury, Venus, Earth, and Mars.
- The planets outside the asteroid belt are termed the Outer
Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto.
- The planets sharing the gaseous structure of Jupiter are termed the
Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune.
The 7 Planets of the AncientsThe term "planet" originally meant
"wanderer": it was observed long ago that certain points of light wandered
(changed their position) with respect to the background stars in the sky. In
ancient times, before the invention of the telescope and before one understood
the present structure of the Solar System, there were thought to be 7 such
wanderers or planets: Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the
Sun. This list is different in several respects from our modern list of planets:
A central theme of our initial discussion will be how the "7 planets
of the Ancients" (only 5 of which are really planets) evolved into our present
list of Solar System planets.
- The Earth is missing, because it was not understood that the points of
light wandering on the celestial sphere and the Earth on which we stood had
anything in common.
- Uranus, Neptune, and Pluto are missing because they would only be
discovered when the telescope made them easily visible.
- Uranus is barely visible to the naked eye; it was discovered in 1781.
- Neptune and Pluto are too faint to see at all without a telescope; they
were discovered in 1846 and 1930, respectively.
- The Sun and the Moon were classified as planets because they wandered on
the celestial sphere, just like Mars and Jupiter and the other planets.
Stars Look Different from PlanetsPlanets (and the Sun and Moon) have
some observational characteristics that distinguish them from what we would now
call the stars:
|Observational Differences between Planets
|The planets move relative to stars on celestial
||The relative positions of the stars are fixed on
|The nearer and larger planets appear as disks in
||The stars appear as "points" of light, even through
|The brighter planets do not "twinkle"
||The stars appear to "twinkle"|
|The planets are always near the imaginary yearly path
of the Sun on the celestial sphere (the ecliptic)
||Stars can be anywhere on the celestial
These observational differences, particularly the "wandering" of the planets
on the celestial sphere, attracted a lot of attention from ancient observers of
the sky. The attempt to explain these differences ultimately led to the birth of