Motions of the Moon
 You can make 4 observations if you
watch the moon move through the sky
over several months:
 First, the apparent size of the full moon
varies from month to month.
Motions of the Moon (2)
 From the different sizes of the full moon,
we can conclude that the moon’s orbit is
quite elliptical.
 In fact it varies by 13.3% of the average
distance.
 Perigee or closest approach is 356,000
km. Apogee or farthest approach is
407,000 km, with an average of 384,000
km.
Motions of the Moon (3)
 2nd Observation: the moon doesn’t orbit the
earth on the ecliptic (the plane of the earth’s
orbit and the apparent path of the sun against
the background of stars), but rather spends
half its time above and half its time below the
ecliptic.
 From this, we can conclude that the moon’s
orbit is tilted or inclined (5o with respect to
the ecliptic.)
http://www.mmscrusaders.com/newscirocks/eclipse/eclipses.htm
Motions of the Moon (4)
 Later, we’ll use the inclination of the
moon’s orbit as a clue to its origin. The
tilt is evidence that the moon was NOT
formed at the same time and in the
same region of space as the earth was
formed, but that another process
caused the origin of the moon.
Motions of the Moon (5)
 3rd Observation: The moon moves
eastward against the background of
stars, ~0.5o (the width of the moon) per
hour, 13o per day. This causes the
moon to lag behind the stars, rising 53
minutes later every evening.
 The orbital motion results in the
changing lunar phases.
http://www.sumanasinc.com/webcontent/anisamples/astronomy/moonphase.html
The hyperlink above illustrates the changing
lunar phases as the moon orbits the earth.
Motions of the Moon (6)
 The 4th observation is that the same
side of the moon always faces the
earth. This means that the moon
rotates on its axis at exactly the same
rate at which it orbits the earth. We call
this “locked” rotation synchronous
rotation.
 Later, this synchronous rotation will give
us a clue about the construction of the
moon’s interior.
http://www.sckans.edu/~gangwere/LAS170a2/_27.html
Synchronous Rotation of the Moon