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James T. Shipman
Jerry D. Wilson
Charles A. Higgins, Jr.
Chapter 17
Moons and Smaller
Solar System Bodies
Sections Covered: 1, 2 & 6
Hmwk: M: a-k, m-t & w-y; MC: 1-14 & 24-26; Ex: 2, 4, 6, 10, 12 & 14
Moons and Small Solar System
• Moons are natural satellites
• 176 are known at present
• Many are small (less than 1 km in diameter) and
some are large.
• Some small solar system bodies orbit beyond
the orbit of Neptune – Trans Neptunian Objects
• Other objects are
• Comets
• Asteroids
• Interplanetary dust
The Moon
• July 20, 1969 – humans first landed on moon
• Retroreflector – placed on moon and designed
to reflect a laser beam from Earth
• Rates of Continental Drift on Earth, change in
Earth’s tilt, distance to moon, Gravitational Constant
• Average distance = 384,000 km (240,000 mi)
Section 17.1
The Moon
• Second brightest object in the sky (what is
• “moon” – unknown origin of the word
• Many primitive and modern societies base their
religious ceremonies on the cycles of the moon
(e.g., new and full moons).
• Our month is based on moon’s cycle.
• Human ovarian cycle is also synchronized to the
29.5 day lunar cycle.
Section 17.1
General Features of the Moon
• Fifth-largest moon in solar system
• Largest moon of any of the terrestrial planets
(Mercury, Venus, Earth, and Mars)
• One complete revolution = 29.5 days (approx.)
• The moon rotates on its axis at the same rate as
it revolves around Earth; therefore, we only see
one side of the moon.
• Since the moon rotates every 29.5 days, the sun
would appear to rise/set every 29.5 days.
• The surface features of the moon formed
millions of years ago without erosion to erase
Section 17.1
Characteristics of the Moon
• Nearly spherical, with a diameter of 3476 km
(2160 mi) – approx. ¼ the Earth’s diameter
• Mass of the moon = 1/81 of the Earth
• Average density of 3.3 g/cm3 (Earth is 5.5)
• Surface gravity of the moon is only one-sixth of
• Therefore one’s weight on the moon would only
be one-sixth of that on Earth.
• Average reflectance (albedo) = only 7% (only
7% of the light received from the sun is
Section 17.1
Composition and History of the Moon
• Using radiometric dating techniques
• Mountain rocks – 4.4 to 3.9 billion years old
• Plain rocks – 3.9 to 3.1 billion years old
• No rocks older than 4.4 or younger than 3.1
billion years old have been found.
Section 17.1
Composition of the Moon
• From inside to surface -- Inner core (iron rich
solid), outer core (iron rich semi-liquid), rocky
mantle, crust, regolith (covers the crust with
pulverized dust and rock)
• Crust thickness varies across the surface
• Presently does not have a magnetic field (not
rotating fast enough)
• Interestingly though, the rocks brought back
show some magnetism, indicating that the moon
had a slight magnetic field at the time of rock
Section 17.1
Origin of the Moon
Most widely accepted theory: great impact
• Planet-sized object (size of Mars) struck Earth
with glancing blow 4.4 billion years ago
• Composition of lunar mantle and crust similar to
Earth’s mantle
Section 17.1
Prominent Features of the Moon
• Besides phases of the moon, moon’s most
prominent features include craters, basins,
plains, rays, rills, mountains, and faults.
• Crater – Greek for “bowl-shaped”
• Large to small
• Believed to have been formed by meteorite impacts
• Highlands: cover 85% of lunar surface
• Light colored surface
• Extend to several kilometers above lunar surface
• Form an older lunar crust
Section 17.1
Lunar Plains (maria- Latin for ‘seas’)
• Dark, flat areas believed to be craters formed by
meteorite impact that filled with volcanic lava
• Surface covered by layer of loose debris called
• Rocks are similar to volcanic rock on Earth
• Plains produced by volcanic eruptions and the
resulting enormous lava flows
• Major eruptions: from 3.9-3.1 billion years ago
• Plains are more common on the near side of the
moon, probably due to the thinner crust
Section 17.1
Craters and Basins
Best known features of moon’s surface
• ~30,000 seen using an Earth-based telescope.
• Range in size – microscopic to 100’s km
• Shapes of craters change with diameter.
• 1 km – smooth bowl-shaped interiors
• >1 km – flat interiors
• >>1 km – flat floor with central peak
• Most craters formed 4.4-3.9 billion year agotime of intense meteorite bombardment.
• 3.1 billion years ago: moon cooled down so that
molten rock could no longer get to the surface.
• Moon appears to have been geologically quiet
for the past 3.1 billion years or so.
Section 17.1
Craters on the Moon
Section 17.1
Rays and Rills
• Rays: streaks extending from some craters
• Thought to be pulverized rock that was thrown out
when the crater formed
• Rays are brighter than the crater
• Rills: long, narrow trenches or valleys
• Some are straight and some are curved.
• Thought to represent a separation (or crack) caused
by moonquakes
• These are similar to the features known to form from
Section 17.1
The Straight Wall
• A unique steep slope
on the eastern side of
Mare Nubium
• The wall is 113 km
long by 244 m in
• Rima Birt 1 (upper
left): irregular rill
Section 17.1
Mountain Ranges and Faults
• Mountain ranges: as high as 6100 m (>19,000 ft)
• All mountain ranges appear to be formed in a
circular pattern, bordering the lunar plains.
• Therefore, probably not formed by the same
processes as mountain ranges on Earth.
• Fault: a break or fracture in the surface of the
moon, along which movement has occurred
• Since there is little/no erosion on the surface of
the moon, the difference in elevation caused by
these faults remain for millions of years
Section 17.1
Lunar Motion Effects
• The lunar orbital plane does not coincide with
Earth’s orbital plane.
• Approximately 5o with respect to Earth’s orbital plane.
• Due to this 5o tilt, it is possible for the moon to
be directly overhead at any latitude between
28.5oN and 28.5oS.
• Both the rotation of the Earth on its axis and the
moon’s revolution around the Earth are
counterclockwise (from a N pole perspective).
Section 17.2
Relative Motions of the Moon and
Section 17.2
Two Different Lunar Months
• Sidereal Month – 27.33 days, lunar cycle with
respect to a star other than the Sun
• Synodic Month – 29.5 days, lunar cycle with
respect to the Sun
• From one’s perspective on Earth the synodic month
is one complete month of lunar phases.
• In actuality the moon revolves more than 360o during
a synodic month.
• From Earth it appears that every day the moon
rises in the east and sets in the west.
Section 17.2
Phases of the Moon
• The moon’s periodic change in appearance is its
most outstanding visual feature.
• One-half of the moon is always reflecting light
from the Sun, but only once during that cycle
can an observer on Earth see the entire
illuminated half, called a “full moon.”
• The starting point for the moon’s synodic month
is arbitrarily taken a the “new moon” position.
Section 17.2
New Phase or New Moon
• Occurs when earth,
Sun, and moon are
all in the same plane,
with the moon
positioned between
the Sun and Earth
• At this position, the
dark side of the
moon is fully toward
the Earth (“dark of
the moon”).
Section 17.2
Lunar Phase Vocabulary
• Waxing phase - the illuminated portion is getting
• Waning phase – the illuminated portion is getting
• Crescent moon – less than ½ of the visible
portion of the moon is illuminated
• Gibbous moon – more than ½ of the visible
portion of the moon is illuminated
Section 17.2
Phases of the Moon
As observed from any latitude north of 28.5oN
The “observer” is looking south; therefore east is on the
Section 17.2
Waxing Phases of the Moon
• Waxing crescent phase – appears as a crescent
moon, less than 90o east of sun (7.375 days)
• First-quarter phase – when the moon is exactly
90o east of sun
• Waxing gibbous phase – appears larger than ½
illuminated, but < a full moon (7.375 days)
• Full Moon – 180o
Section 17.2
Waning Phases of the Moon
• Waning gibbous phase – appears < a full moon,
but larger than ½ moon (7.375 days)
• Last-quarter phase – when the moon is exactly
270o east of sun
• Waning crescent phase – appears smaller than
½ illuminated (7.375 days)
• New Moon – 360o
Section 17.2
Phases of the Moon
Section 17.2
Altitude of the Full Moon
• A full moon is always on the opposite side of the
Earth from the Sun.
• Thus when the Sun is at its lowest position
(winter solstice), the moon will be at its highest.
• Due to the 5o difference in the Earth’s and
moon’s orbital planes, the moon will be situated
directly over the 28.5o N latitude line on the
winter solstice.
Section 17.2
• The Sun provides the light for our solar system
• Planets and moons within the solar system cast
shadows that extend away from the Sun
• The size and shape of the shadow cast depends on
the object’s size, shape, and distance from the Sun
• The Earth and the Moon cast conical shadows, as
viewed from space
• Eclipse – the darkening of the light of one
celestial body by another
Section 17.2
• The shadows cast by the Earth and the Moon
cast two different degrees of darkness
• Umbra – the darkest and smallest region
• A total eclipse occurs within the umbra region
• Penumbra – the semidark region
• A partial eclipse occurs within the penumbra region
Section 17.2
Solar Eclipse
• Solar eclipse – occurs when the moon blocks
some or all of the Sun’s rays from an observer
on Earth
• A solar eclipse occurs when the moon is at or
near new phase and is in or near the ecliptic
• When the moon lies between the Sun and Earth
in nearly a straight line, the moon’s shadow will
fall on the Earth
Section 17.2
Positions of the Sun, Moon, and
Earth During a Total Solar Eclipse
The umbra and penumbra are,
respectively, the dark and
semidark shadows cast on the
Earth by the Moon
Section 17.2
Types of Solar Eclipses
• The length of the Moon’s shadow varies with the
moon’s distance from the Sun
• In some cases the Moon’s umbra does not reach all
the way to Earth
• If the umbra does not reach Earth, an observer on
Earth would see the Moon’s disk projected against
the Sun and a bright ring or annulus outside the dark
Moon – this is called an annular eclipse
• The maximum diameter of the umbra on Earth is
about 270 km
• Total eclipse when the umbra reaches Earth
Section 17.2
Solar Eclipses
Solar corona during a total solar eclipse
An annular eclipse of the Sun.
Umbra of the Moon’s shadow does not
reach all the way to the Earth
Section 17.2
Lunar Eclipse
• Lunar eclipse - occurs when the Earth blocks
some or all of the Sun’s rays to the Moon
• A lunar eclipse occurs when the Moon is at or
near full phase and is in or near the ecliptic
• When the Earth lies between the Sun and Moon
in nearly a straight line, the Earth’s shadow will
conceal the face of the Moon
• A total lunar eclipse can last for more than 1.5 hours,
and partial eclipses may last for over 3 hours
Section 17.2
A Lunar Eclipse
Section 17.2
A Lunar Eclipse
Section 17.2
Ocean Tides
• Tides – the regular and alternating rise and fall
of the ocean’s surface level
• Due to the gravitational attraction that the Sun
and Moon exert on Earth.
• The Moon, because it is closer, has a greater
effect on the tides.
• Actually it is the difference in the gravitational
attraction at various parts of Earth that causes
the tides.
Section 17.2
Tidal Bulges
The two tidal bulges result in
two high tides and two low tides daily
Section 17.2
Ocean Tides
• One of the tidal bulges is caused by the Moon’s
gravitational attraction on the side of the Earth
nearest to the Moon
• Another tidal bulge is located on the side of the
Earth farthest away from the Moon
• This distant tidal bulge is caused by the force of
gravity pulling the Earth away from the water on
the distant side, forming a second and opposite
Section 17.2
Spring Tide
• Spring tide occurs when the Sun, Moon, and
Earth are all positioned in nearly a straight line
• In this situation, the gravitational forces of the
Sun and Moon combine to produce higher high
tides and lower low tides
• The variations between high and low tides are
the greatest during a spring tide
• Spring tides occur twice during each lunar
month; at new and full moons
Section 17.2
Neap Tide
• Neap tide occurs when the Sun and the Moon
are at angles of 90o with respect to the Earth
• In this situation, the gravitational forces of the
Sun and Moon tend to cancel and produce lower
high tides and higher low tides
• The variations between high and low tides are at
a minimum during the neap tide
• Neap tides occur twice during each lunar month;
at first and last quarter
Section 17.2
Spring and Neap Tides
Section 17.2
Tidal Height Varies
• The height of the tide also varies with latitude
• The two tidal bulges are the highest at the
latitude of the overhead position of the moon
and on the other side of the Earth opposite the
moon’s position
• The maximum height of the tidal bulge is at
23.5oN and 23.5oS, due to the overhead position
of the moon and the Sun on the summer solstice
Section 17.2
Tidal Effects on the Earth/Moon System
• The regular tidal cycle retards the Earth’s
rotation at a rate of about 0.002 s per century
• Since angular momentum must be conserved,
this decrease in Earth’s angular momentum
results in an increase in the moon’s angular
• The moon’s orbit is increasing about 1.3 cm/y
• If we went back 1 billion years, the solar day
would have been 5.6 h shorter, and the moon
would have been 13,000 km closer to Earth
Section 17.2
Small Solar System Bodies
Meteoroids, Meteors, and Meteorites
Interplanetary Dust
Section 17.6
• Asteroids - over 20,000 named or numbered
objects that orbit the Sun in a belt between Mars
and Jupiter
• This belt contains millions of asteroids
• About 100,000 are bright enough to be photographed
by Earth-based telescopes
• Sometimes referred to as minor planets
Section 17.6
Asteroids or Minor Planets
Section 17.6
Origin of Asteroid Belt
• Sometimes attributed to the breakup of a former
• Estimates of the total mass of the asteroids is much
less than the mass of an ordinary planet.
• A more plausible explanation is that the debris
never formed into a planet
• The enormous gravitational force Jupiter may have
stirred up nearby debris, preventing it from
condensing into a planet.
Section 17.6
Meteoroids, Meteors, and Meteorites
• Meteoroid – interplanetary metallic and stony
objects that range in size from sub-millimeter to
100’s of meters.
• Meteor – a meteoroid that enters the Earth’s
atmosphere and becomes luminous – a
“shooting star.”
• Meteorite – a meteor that survives the flight
through the Earth’s atmosphere and strikes the
Section 17.6
Barringer Meteorite Crater, Arizona
• Meteorite
impact is
estimated to
have been
50,000 years
ago, when a
struck near
Winslow, AZ.
Section 17.6
• Relatively small objects that are composed of
dust and ice, and revolve around the Sun in a
highly elliptical orbit
Section 17.6
Parts of a Comet
• A comet consists of four parts
• Nucleus – typically a few kilometers in diameter and
composed of rocky or metallic material
• Coma (head) – surrounds the nucleus. Several
hundred kilometers in diameter. Formed from the
nucleus as it approaches 5AU of the Sun.
• Tail – long and voluminous composed of ionied
molecules, dust, or a combination. Can be millions of
kilometers in length.
• Hydrogen cloud – surrounds coma and thought to be
composed of dissociation of water molecules from
the nucleus.
Section 17.6
The Principal Parts of a Comet
Section 17.6
• Note how the comet’s tail is affected by and is
directed away from the Sun, and does not
indicate the comet’s direction of movement.
Section 17.6
Micrometeorids or Interplanetary Dust
• A tremendous amount of interplanetary dust can
be found throughout the Solar System.
• Two celestial phenomena can be observed and
photographed, showing that dust particles do
• Zodiacal light—a faint band of light along the zodiac.
• Gegenshein “counterglow”—due to sunlight reflected
from dust particles
Section 17.6