CHAPTER 17- MOONS AND SMALL SOLAR SYSTEM
BODIES
General Features
The moon is the second brightest object in the sky. It reflects
sunlight and produces no light of its own. It is so bright
because it is only 384,000 km (240,000 mi) from the Earth.
It is the only other celestial body where humans have walked.
"That's one small step for man, one giant leap for mankind"
Neil Armstrong (1969)
The Moon revolves around the Earth in 29.5 solar days and
rotates at the same rate. Because of this, the same side of the
Moon always faces the Earth.
The Moon is nearly spherical with a diameter of 3476 km,
about one fourth of the Earth's diameter.
The mass of the Moon is only 1/81 of the Earth's mass and its
average density is about .6 that of the Earth. The surface
gravity on the Moon is about 1/6 that of the Earth.
The Moon is thought to be geologically inactive with a small,
solid, iron rich core, a solid mantle and a crust about 64 to 125
km thick.
The Moon has no magnetic field, but there is evidence in Moon
rocks that at one time it did.
The surface features of the Moon include craters, basins,
plains, rays, rills, mountain ranges, and faults.
Craters are small and large diameter depressions surrounded
by raised areas. They are believed to have been formed by
meteoroid collisions with the Moon. Since the Moon has no
atmosphere, meteoroids do not burn up before they strike the
surface. Over the years there have been many impacts.
Also, since there is no atmosphere, once a crater forms, it
remains relatively intact unless it is destroyed or covered up by
another meteoroid strike.
Plains were originally called Maria (meaning seas) by Galileo.
They are thought to be areas filled with lava flows after
meteoriod impacts. They are darker than the surrounding
material and are visible to the naked eye during a full moon.
Rays are streaks that extend outward from some craters. They
are believed to be pulverized rock thrown out when the crater
was formed. The pulverized rock appears lighter than the
undisturbed rock.
Rills are long, narrow trenches or valleys. They vary from a
few meters to 5 kilometers in width and extend hundreds of
kilometers in length. They have very steep walls and fairly flat
bottoms. They are thought to have been caused by
Moonquakes.
Mountain ranges on the Moon have peaks as high as 6100
meters. They probably were not formed by the same processes
that form mountain ranges on the Earth.
A fault is a break or fracture in the surface along which
vertical or horizontal movement has occurred. The Straight
Wall is a vertical fault 113 km long and 244 m high.
Composition and History of the Moon
Astronauts have collected and brought back 379 kg of lunar
material.
Rocks from the highlands were formed between 4.4 and 3.9
billion years ago. Rocks from the plains have ages of 3.1 to 3.9
billion years. No rocks with ages outside of this range have
been found.
Most of the craters on the Moon's surface were probably
formed between 4.4 and 3.9 billion years ago when there was
more space debris from the formation of the new solar system.
Lunar Motions
Figure 17.7
The Relative Motions of the Moon and Earth
The moon revolves around the Earth from west to east. The
Moon completes a sidereal month once every 27.33 days. This
is the time for the moon to revolve through 360 degrees.
The Moon completes a synodic month once every 29.5 days.
This is a month of phases and returns the Moon to the same
place in the sky relative to the Sun. This path covers more than
360 degrees.
Figure 17.8
Differences between Sidereal and
Synodic Months
On the Earth, the Moon appears to rise in the east and set in
the west. This apparent motion of the Moon is due to the
rotation of the Earth from west to east.
The Moon rises about 50 minutes later each day. This happens
because the Moon moves in its orbit as the Earth rotates. At
the end of 24 hours, the Moon has moved far enough that the
Earth must rotate for another 50 minutes to catch up.
Phases of the Moon
Figure 17.9
Phases of the Moon
Copyright © Houghton Mifflin Company
17-2
One half of the surface of the Moon is always illuminated. The
changing relative positions of the Earth, Moon and Sun cause
only a part of the illuminated half to be visible from the Earth
most of the time.
The new moon occurs when the moon is on the side of the
Earth facing the Sun. Since the illuminated side always faces
the Sun, we could only see the dark side if it were visible. Solar
eclipses sometimes occur during this phase. It is highest in the
sky at 12:00 noon.
A waxing crescent phase occurs when we can see a crescent
shaped slice of the Moon on the western side that appears to be
increasing in size. Less than ¼ of the Moon's surface appears
to be illuminated as seen from the Earth. It is highest in the sky
around 3:00 PM.
The first quarter phase occurs when the Earth, Moon and Sun
form a 90 degree angle with the Moon east of the Sun. The
western one fourth of the Moon appears to be illuminated as
seen from the Earth. It is highest in the sky at 6:00 PM.
A waxing gibbous phase occurs when more than one fourth of
the Moon is illuminated on its western side as seen from the
Earth. It is highest in the sky at about 9:00 PM.
A full moon phase occurs when the Moon is on the opposite
side of the Earth from the Sun. The entire half of the Moon
facing the Earth is illuminated. The full moon is highest in the
sky at 12:00 midnight.
The waning gibbous phase is next. It appears the same as the
waxing gibbous phase except that the eastern side of the Moon
is illuminated. It is highest in the sky at 3:00 AM.
The last quarter phase is similar to the first quarter phase but
the 90 degree angle with the Earth and Sun is now west of the
Sun and the eastern side of the Moon is illuminated. This phase
is highest in the sky at 6:00 AM.
A waning crescent phase is similar in appearance to a waxing
crescent but the eastern side of the Moon is illuminated. This
phase is highest in the sky about 9:00 AM.
Figure 17.10
Phases of the Moon
Copyright © Houghton Mifflin Company
17-2
Did you know that a full moon can happen twice in one month? When this happens, the
second full moon of the month is called a Blue Moon!
Eclipses
There are two types of eclipses involving the Moon. They are
solar and lunar eclipses.
Figure 17.15
A Total Solar Eclipse
Copyright © Houghton Mifflin Company
17-2
During a solar eclipse, the Moon passes between the Earth and
the Sun during daytime hours so that the Sun is completely or
partially blocked out by the Moon. The Sun is much larger in
diameter than the Moon, but the Moon is much closer to the
Earth. This coincidence of distance and diameter makes the
apparent size of the Moon almost identical to that of the Sun.
When the Moon is closer to the Earth in its orbit, it blocks out
the disk of the Sun completely, allowing us to view the solar
corona(hot gasses surrounding the Sun). When the Moon is
farther from the Earth in its orbit, a ring of the Sun may still
be visible around the Moon. This is called an annular(ringed)
eclipse.
The darkest part of the shadow cast by the Moon on the
surface of the Earth is called the umbra. In this region direct
light from the Sun is completely blocked out and we experience
a total eclipse.
Outside of the umbra, a larger circular region of partial
darkness called the penumbra, exists. In this region part of the
Sun's disk is still visible and we call this experience a partial
eclipse.
In order for a solar eclipse to happen, the Moon must be in the
New phase and it must be on or near the ecliptic plane. We
don't have an eclipse every new moon because the Moon's orbit
takes it slightly above and below the ecliptic plane.
The average total solar eclipse lasts only 3 to 4 minutes since
the Moon's shadow moves at about 1600 km/hr and its
diameter averages less than 270 km.
Figure 17.18
A Lunar Eclipse
Copyright © Houghton Mifflin Company
17-2
A lunar eclipse occurs when the Sun, Earth, and Moon are
aligned so that the Earth blocks the Sun's light from reaching
the Moon. Since the Moon is only visible because it reflects
light from the Sun, the Moon becomes dark during a lunar
eclipse.
Lunar eclipses occur when the Moon is at or near full phase.
This is the time when it is possible for the Earth to come
directly between the Sun and the Moon.
When the Moon enters the Earth's penumbra, it loses some of
its brightness and may even take on a reddish color.
When it enters the Earth's umbra it is very difficult if not
impossible to see.
Since the Moon's orbit is angled at about 5 degrees with
respect to the ecliptic plane, it is normally above or below the
ecliptic plane. Only two times a month is it on the ecliptic
plane. When it is rising above the ecliptic, this point is called
an ascending node. When, in its orbit, it is sinking below the
ecliptic, this is called a descending node. In order for an eclipse
to occur, one of these nodes must correspomd to a new moon
or a full moon.
If it corresponds to a new moon, we have a solar eclipse. If it
corresponds to a full moon we have a lunar eclipse.
Ocean Tides
There are two high tides and two low tides each day. The two
high tides are 12 hours and 25 minutes apart as are the low
tides. This is one half of the Moon's synodic period and is an
indication that high and low tides are caused by the Moon's
gravitational attraction for the oceans on the Earth.
The water on the side of the Earth nearest the Moon is
attracted strongly to form a tidal bulge. On the opposite side of
the Earth, the Earth is actually pulled away from the water
causing another tidal bulge. These bulges correspond to high
tides and the resulting areas of depression correspond to low
tides.
Figure 17.20
Tides on Earth
Copyright © Houghton Mifflin Company
17-2
Although the Moon plays the strongest role in the formation of
tides, the Sun's gravitational attraction is significant. When the
Sun, Earth, and Moon are in a straight line, the Sun's
gravitational attraction is added to that of the Moon causing
the highest high tides and the lowest low tides. These are called
spring tides and occur during full or new moon phases.
When the Sun, Earth, and Moon form a 90 degree angle, the
Sun's gravity pulls water upward in the areas of depression.
This causes the low tides to be higher than normal and the high
tides to be lower than normal. These are called neap tides and
occur during first and last quarter phases of the Moon.
The interaction between the Earth and Moon through tides
causes the Earth's rate of rotation to slow by .002 seconds per
century. Since angular momentum of the system must be
conserved, the Moon moves about 1.3 cm farther from the
Earth each year.
Other Solar System Objects
Other objects in the solar system include asteroids, meteoroids,
comets and interplanetary dust.
Asteroids are small (< 1 meter) to very large(940 km) rocky
objects orbiting the sun between the orbits of Mars and
Jupiter. They occupy a location where a planet should have
formed based on the distance ratios of all the other planetary
orbits.
The largest is called Ceres. The two other largest are Pallas
and Vesta.
Asteroids are believed to be early solar system material that
never collected into a single planet.
Meteoroids
Meteoroids are metallic and stony objects found between the
planets. They can be as small as a grain of sand or as large as a
few hundred meters in diameter.
When they enter the Earth's atmosphere, friction with the air
heats them up so that they give off light. Then they are called
meteors or shooting stars.
If one survives its trip through the atmosphere and its collision
with the Earth, it is called a meteorite. When a meteorite
strikes the ground, it leaves a crater. One of the largest known
is the Barringer Meteorite Crater near Winslow, Arizona.
Comets
The name comet comes from the Latin words meaning long
haired stars. A comet is composed of dust and ice and revolves
around the sun in a highly elliptical orbit.
When it is far away from the Sun it is a dirty ice ball. As it
approaches the Sun, heating from the Sun causes vapors and
dust to stream out from the comet in a direction away from the
sun.
A comet consists of four parts(1) the nucleus, a small rocky
core consisting of stony or metallic material as well as solid
water, ammonia, methane and carbon dioxide, (2) the head or
coma which surrounds the nucleus in a spherical fashion, (3)
the tail which is composed of ionized molecules, dust, or a
combination which can be millions of kilometers in length, and
(4) a spherical cloud of hydrogen surrounding the coma.
Halley's Comet is one of the most famous. It orbits the Sun
once every 76 years. We last saw it in 1986.
Two regions in the solar system are thought to be the source of
comets. The Oort Cloud consists of matter located about
50,000 AU's from the Sun. The Kuiper Belt is located between
Pluto's orbit(39 AU) and the Oort Cloud.
Interplanetary Dust
Interplanetary dust consists of very small, solid particles called
micrometeoroids. A faint glow called zodiacal light can be seen
along the zodiac (ecliptic) just after sunset on a very dark
night.
This faint glow is due to sunlight being reflected by
interplanetary dust.
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