There are five (excluding the Earth) naked eye planets, Mercury, Venus, Earth, Mars, Jupiter, and Saturn ( sky view from Eugene ). Uranus is actually just bright enough to be seen with the unaided eye at a dark sight if you know where to look and someone points it out. The planets' motions share some common features:

• The motions of the planets are roughly confined to the ecliptic. This implies that the orbits of the planets are roughly co-planar with the orbit of the Earth about the Sun.

• Not as obvious is the fact that all planets orbit the Sun in the same sense as does the Earth. This means that they usually move in the west-to-east direction on the Celestial Sphere (CCW as viewed from the NCP). However, an exception to this motion is the retrograde motion sometimes exhibited by the planets (see below).

• The planets' orbits are all roughly circular in shape.

The preceding facts are interesting (and profound) because they suggest that the planets in our Solar System formed as a single unit rather than as individual planets which were later assembled into the Solar System.

What are the interesting properties of the apparent ( observed) motions of the planets?

Retrograde motion

The planets generally move west-to-east through the stars (prograde motion) but, peridiocally, the motion changes and they move east-to-west through the stars (retrograde motion). Below is the 2014 retrograde of Mars:


The retrograde motion continues for a short time and then the motion switches back to prograde. This seemingly inexplicable behavior is easily understood within the context of a Sun-centered (helio-centric) Solar System. This motion is harder to understand in the context of an Earth-centered (geo-centric) model. We return to this issue later.

Morning and Evening Stars

Venus and Mercury are observed only in the early morning or early evening; Venus and Mercury can not be seen at midnight in Eugene. The explanation for this phenomenon is rather simple:


Because the orbits of Mercury and Venus lie inside the orbit of the Earth, we see that both planets have to stay rather close to the position of the Sun as they move through the sky. Venus must be within ~46 degrees of the Sun at all times while Mercury (because of its smaller orbit) must remain within 28 degrees of the Sun (Note -- due to the non-circular shape of Mercury's orbit, this limit can be as small as 18 degrees). This close proximity to the Sun means that we should be able to see Mercury and Venus only in the early evening (after the Sun has just set) or in the early morning (just before the Sun rises):

Phases of Venus

Venus and Mercury both go through the full set of phases like the Moon; Mars does not. Why?