IV. PLANETARY MOTIONS
There are five (excluding the Earth) naked eye planets, Mercury,
Venus, Earth, Mars, Jupiter,
and Saturn
(
current positions of the planets).
Uranus is actually just barely bright enough to be seen 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 obviously, the
planets all orbit about the
Sun in the same sense as does the Earth, namely, in the west-to-east direction
(CCW as viewed from the NCP). (However, see the discussion of retrograde
motion below.)
- The planets' orbits are all roughly circular in shape.
The preceding are interesting and somewhat
profound results as they suggest that the planets
of our Solar System
formed as a
unit rather than as individual planets which were
then assembled into our Solar
System, after the fact.
Now what are some interesting
properties of the apparent ( observed) motions of the
planets?
1. 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):
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.
The explanation for retrograde motion
in a helio-centric model is that retrograde occurs roughly
when a faster moving planet catches up to and passes a slower moving
planet (Copernicus circa 1500).
Assume:
- all orbits are in ecliptic
- all orbits are in the same sense (CCW)
- all orbits are nearly circular
- Would an observer on Mars see the Earth undergo retrograde motion?
- How often will a given planet show retrograde motion?
2. 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 at most sites
on the Earth
(e.g., in Eugene we could not see Mercury at midnight).
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):
3. Phases of Venus
