Classification of the
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 System
The 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 Ancients
The term "planet" originally meant "wanderer": it was observed long ago
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 Planets
Planets (and the Sun and Moon) have some
observational characteristics that distinguish them from what we would now call
|Observational Differences between Planets &
|The planets move
relative to stars on celestial sphere
positions of the stars are fixed on celestial sphere
|The nearer and larger planets appear as disks in telescope
||The stars appear as "points" of light, even through the
|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 sphere
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 modern astronomy.