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The Moon orbits the Earth but they orbit the sun
as system
They both revolve around a common center of gravity
called the barycenter which is below Earth’s surface
Both have their own real motion that is affected
by the presence and motion of the other
Formation
Most accepted theory
-When the earth was cooling so that only the outer surface
had cooled to form a crust it was struck by a Mars sized
object
-This caused part of the still molten center to be shot
out of the Earth into space
-Gravity and molecular forces would have pulled this mass together
- it would have cooled from outside to in causing a crust to
form on the outside
-As the inside cooled and contracted, it would pull the rigid
exterior in causing folding that resulted in the mountains called
highlands which appear much lighter to us
- The large flat, dark areas were thought to be seas so they were given the
name maria (mare is singular) but they are now known to be large lava
flows from cracks caused by meteor impacts
The moon has mountains, valleys and plains just like Earth.
Since the moon can’t hold an atmosphere, the processes that
formed them and change them over time are very different
The outermost covering is called regolith
- it is made of, dust, rock fragments, and boulders
- it is formed by meteoroids crashing and exploding
Micrometeoroids are the only cause of erosion on the moon
Besides highlands and lava flows the moon has other
features
Impact craters – since there is no atmosphere to burn them up,
all meteoroids reach the surface of the moon
These crashes cause the much cratered surface we are familiar with
There is no weathering on the
moon so these craters do not
change until another meteoroid hits
that area
Even the craters have features visible with telescopes from the
Earth, The picture below labels some of these features
Terraced crater walls –
stair step features on
large craters where the
walls have slumped
Central crater uplift
only found on craters of
least 40km diameter
- Formed by rebounding
of highly compressed
area
Some meteoroids were so large or hit so hard the cause multi-ring
basins
These are rings of mountains around the outside of the
crater shoved up by the strong impact
Terraced wall
Rings of
mountains
Ejecta
Material thrown out of crater
during impact
A
You can actually tell the
angle of impact by how the
ejecta was thrown out
Crater A was made by a fairly
vertically falling meteoroid
B
Crater B’s meteoroid hit at an
angle to the surface
Rays
Bright lines of material
ejected out of craters when
they are formed
The brighter they are the newer
they are
You can also tell the relative
age of craters close to each
other because the rays of the
newer crater will cover the
older crater’s rays
Moon layers
Crust – outermost layer
- covered with regolith and maria
Mantle – middle
layer
-where moonquakes
originate
Outer core – may be
partially molten
Inner core is thought
to be iron rich and
solid
Effect of Earth on Moon Structure and Motion
The pull of Earth’s gravity while
the moon was still molten pulled
the denser parts towards Earth
This makes the moon egg shaped
with the pointy end towards Earth
The crust is the least dense
portion and it is 60km thick on
the Earth side and 100km thick
on the backside
The gravity also pulled the heavier interior material this way, this
causes the near side to be 50/50 highlands and maria while maria
are fairly rare on the far side
Motion of the moon
Since one end is heavier, it is always pointed at Earth
and makes the moon rotate and revolve in the same
amount of time
Moon rotates and revolves in about 27.5 days
That is why we always see the same side of the moon
If you observe very closely, you may see past the poles due to
the angle of the moons orbit in relation to Earth
Procession of the moon
The moon is orbiting the Earth from east to
west and the Earth is also rotating in that
direction
Since the moon has moved eastward since the last
moonrise, the Earth has to turn more than 1 turn to
catch it and the moon rises later each night
The moon orbits 3600 in 27.5 days so it moves
about 130 per day 3600/27.5da = 130/da
The Earth rotates at 150/hr
13/15 x 60 min = 50min, so the moon rises 50min
later each day
Caused by the relative positions of the Earth, Moon and Sun
The moon rotates and revolves in 27.5 days, how
long does it take to go through the phases?
29.5 days, 2 days longer
New
moon
27.5 days
later
Depend on the relative position of the sun,
moon, and Earth
They are the result of the moon or Earth
passing into the shadow of the other
Since the orbit of the moon is tipped, they
do not occur every month
When the moon blocks the sun
Because of large
distances there are
actually two types of
shadows
The inner complete shadow of an eclipse is called the Umbra
The partial shadow of an eclipse is the Penumbra
Total solar eclipse
Bright cloud of light
is the corona of the
sun
Annular eclipse- orange
to red ring visible is
called the annulus
Occur when the moon is
further from Earth
Partial solar eclipses are
seen from the penumbra
This area is next to the
umbra and gives a dimmed
and maybe partially blocked
view of the sun
Occur when the
moon moves
into the Earth’s
shadow
They can only occur
on full moons
The Earth
shadow also
has an umbra
and penumbra
so the eclipses
may be total or
partial
This shows the progression as the shadow
crosses the moon, it appears red while totally
blocked
During a partial eclipse, the moon never becomes completely
covered
Interesting Tidbit
Solar eclipses are actually much more
frequent but are seen by many less people
Solar eclipse paths are very narrow and often
pass over only water
Lunar eclipses may be rarer but are seen by
everyone on the night side of the Earth
Another phenomenon caused by positions of
the Earth, Sun and Moon
One area of the ocean is
pointed at the moon and the
gravity pulls the surface of
the water higher
On the opposite side, the
inertia of the water from
the Earth’s rotation makes
it bulge out
Since these are occurring at the same time there are
two high tides and two low tides each day except at
higher latitudes where there is only one
High tides occur 12:25 hrs. apart, 12 for half of 24
and 25 min for amount of movement of the moon in
its orbit
The Sun Tide
there is also a gravitational attraction between the
earth’s oceans and the sun. The effect of the sun
upon the tides is not as significant as the moon’s
effects. Basically, the sun’s pull can add to or take
away from the moon’s effect, depending on where the
moon is in relation to the sun.
Spring tides with the
largest differences
between high and low
tides occur on new and
full moons when the
three are in a line
Forces acting together
Neap tides have the
smallest differences
between high and low
tides. They occur
when the three make a
right angle
Forces working against
each other
Origin of a solar system
As a star is forming the forces causes it to spin
The material outside of the star is flattened
into a disk that also is rotating
The materials in the disk are not traveling at the same rat
and they begin to collide with each other
As they collide, the form larger masses with greater
gravity to pull in more materials
They keep colliding until they form bodies large
enough to be called protoplanets which later become
the planets and moons
The larger ones have gravity great enough to trap the
smaller ones into and orbit if and when they pass
Evidence of this shown by the planets
1. The planets revolve in the same direction as the
sun rotates
2. Almost all the planets rotate in the same directions
3. Most of the planets have moons that revolve in the
same direction that revolve in the same direction as
the sun rotates
Theories of solar system organization
Geocentric theory - Earth centered
Because bodies seem to rise and set as we watch them,
people thought that the Earth was the center of the solar
system
Some bodies did not follow a regular path but seemed
to wander around
They were called planets which
means wanderers
Ptolemy explained this irregular
motion with a big orbits called a
deferent with occasional little
loops called epicycles
These explained the backward
motion of the planets
Heliocentric theory – Sun centered
Copernicus, a Catholic priest from Poland, discovered
a system that made the movements much easier to
explain with the sun at the center of our solar system
Galileo then made two discoveries that
helped prove Copernicus’ theory
1. The Galilean moons of Jupiter
showed not all bodies revolved
directly around Earth