Inferior and Superior Planets

The inferior (or inner) planets are those whose orbits are between Earth's orbit and the sun. The superior (or outer) planets have orbits beyond Earth's.

The inferior planets (Mercury and Venus) are never seen very far from the sun in the sky; Mercury's greatest elongation is about 28, Venus' is about 46. (A planet's elongation is the angle made at Earth by straight lines drawn to the planet and the sun.) When a planet is between Earth and the sun, it is at inferior conjunction; when the sun is between it and Earth, it is at superior conjunction. At both conjunctions the elongation is 0. The inferior planets show phases like the moon's. The phases cannot be seen without a telescope.

The superior planets (Mars, Jupiter, Saturn, Uranus, and Neptune) are said to be at conjunction (elongation 0) when the sun is between Earth and the planet. They are at opposition (elongation 180) when Earth is between the sun and the planet. At eastern and western quadrature, the elongation is 90. Superior planets do not show phases like the moon's. They are full both at conjunction and at opposition, and show only a slight flattening of one side at other times.

Superior planets usually move eastward as seen against the background of stars. When opposition approaches, they appear to slow down, stop, and begin moving westward. The westward motion is called retrograde motion. After a time, the planet appears to reverse its direction again and it then moves eastward until the next opposition approaches. The retrograde motion is due to Earth passing the superior planet as each travels in its orbit.

Bode's Law

Bode's Law, also known as the Bode-Titius Law, was worked out by Johann Titius in the 1760's and published in 1772 by Johann Bode. It gives the approximate distances of all the planets known at that time and of the planet Uranus, discovered in 1781. The law also indicates a planet should exist between the orbits of Mars and Jupiter, and in the early 1800's the asteroids were discovered in that region. However, the distances of the last bodies to be discoveredNeptune and Plutodeviate greatly from Bode's Law and most astronomers today do not believe that the law has any physical justification.

Bode's Law
ObjectDistance by Bode's LawActual Distance
Mercury 0.40.39
Venus 0.70.72
Earth 1.01.00
Mars 1.61.52
Main Belt of asteroids 2.82 to 3
Jupiter 5.25.20
Saturn 10.09.54
Uranus 19.619.18
Neptune 38.830.06