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 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.
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.
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):