For Earth being rather small, we have been given an impressive Moon!
We will study the Moon more directly in 4 weeks. Today we will discuss
the effect such a nearby planetary body has on Earth.
The moon orbits the Earth every 27.3 days (sidereal period), which is also equal to the time it takes for it to rotate once on its axis. Consequently, we see only one hemisphere of the Moon from Earth, the near side. The far side is sometimes termed the `dark side'. But beware! This is a misnomer!
The Moon lies in the same plane as the Sun-Earth system (or nearly so with just a 5o relative tilt), so we only see it lie along the ecliptic.
 New Moon |
 Quarter Moon |
 Full Moon |
The Lunar Phases we see from Earth
Our view of the Moon is greatly complicated by the fact that we
are sitting on a planet which is spinning about its axis once every
day and orbiting the Sun once per year, while trying to observe a
Moon which is orbiting the Earth once per month while spinning
on its axis at the same rate. It's enough to make you dizzy.
A few assumptions go into understanding the Moon phases:
1) The Moon is spherical.
2) The Moon s not self-luminous, but rather reflects the light of the distant Sun.
3. The Moon circles the Earth once per month.
To understand the Phases of the Moon, we must look at the location
of the Moon relative to the Sun and our view from Earth.