Phases of the Moon
The orbit of the Moon is very nearly circular
(eccentricity ~ 0.05) with a
mean separation from the Earth of about 384,000 km, which is about 60 Earth
radii. The plane of the orbit is tilted about 5 degrees with respect to the
Revolution in Orbit
The Moon appears to move completely around the celestial sphere once in about
27.3 days as observed from the Earth. This is called a sidereal
month, and reflects the corresponding orbital period of 27.3 days
The moon takes 29.5 days to return to the same point on the celestial sphere
as referenced to the Sun because of the
motion of the Earth around the Sun; this is called a synodic
month (Lunar phases as observed from the Earth are correlated with the
synodic month). There
are effects that cause small fluctuations around this value that we will not
discuss. Since the Moon must move Eastward among the constellations enough to
go completely around the sky (360 degrees) in 27.3 days, it must move Eastward
by 13.2 degrees each day (in contrast, remember that the Sun only appears to
move Eastward by about 1 degree per day). Thus, with respect to the background
constellations the Moon will be about 13.2 degrees further East each day.
Since the celestial sphere appears to turn 1 degree about every 4 minutes, the
Moon crosses our celestial meridian about 13.2 x 4 = 52.8 minutes later each
The Moon appears to go through a complete set of phases as viewed from the
Earth because of its motion around the Earth, as illustrated in the following
Phases of the Moon
In this figure, the various positions of the Moon on its orbit are shown
(the motion of the Moon on its orbit is assumed to be counter-clockwise).
The outer set of figures shows the corresponding phase as viewed from
Earth, and the common names for the phases.
Here is an animation of actual
lunar phases, and here is a
illustrating the orbit of the moon around the Earth and the corresponding
phases of the Moon as viewed from Earth. Notice that you can set this applet
to a top view, an Earth view, or both on a split screen, and that you can
start and stop the animation with a button. Also, note that in this applet the
position of the Sun is shown to the left, whereas in the above figure the view
is such that the position of the Sun is to the right.
Perigee and Apogee
The largest separation between the Earth and Moon on its orbit is called
apogee and the smallest separation is called perigee.
Here is an online Lunar Perigee and Apogee
Calculator that will allow you to determine
the date, time, and distance of lunar perigees and apogees for a given year
Rotational Period and Tidal Locking
The Moon has a rotational period of 27.3 days that (except for small
fluctuations) exactly coincides with its
(sidereal) period for revolution about the Earth.
As we will see later, this is no coincidence; it is a consequence of
coupling between the Earth and Moon.
Because of this tidal locking of the periods for revolution and rotation,
the Moon always
keeps essentially the same face turned toward the Earth
(small fluctuation mean that
over a period of time we can actually see about 55% of the Lunar surface
from the Earth).