Chapter 17

Chapter 17

• Moons and Other Solar System Objects

Sections 17.1-17.2

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Parallax

The Moon

• July 20, 1969 – humans first landed on moon

• What was the first word transmitted from the surface of the moon?

• “Houston …”

• Retroreflector – placed on moon and designed to reflect a laser beam from earth

• Average distance = 384,000 km (240,000 mi)

The Moon

• Second brightest object in the sky

• “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

Composition and History of the Moon

• Since the first manned lunar landing on July 20, 1969, there have 5 other lunar landings – Apollo 12, 14, 15, 16, & 17.

• 379 kg (over 800 lbs) of lunar material have been brought back to earth.

• 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 3.2 billion years old have been found.

Section 17.1

General Features of the Moon

• 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, and 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 them.

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 Earth’s.

• 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 reflected)

Section 17.1

Composition of the Moon

• Core, mantle, crust

• All thought to be solid

• Crust, thicker on one side

• Presently does not have a magnetic field

• Interestingly though, the rocks brought back show some magnetism, indicating that the moon had a slight magnetic field at the time of rock crystallization.

Prominent Features of the Moon

• Other than the very obvious phases of the moon, the moon’s most prominent features include craters, basins, plains, rays, rills, mountains, and faults.

• Crater – Greek for “bowl-shaped”

• Lunar craters are large to small and believed to have been formed by meteorite impacts.

• For individual craters, the volume of the rim and the crater are about equal.

Section 17.1

Craters of the Moon

• Most of the craters were formed between 4.4 and 3.9 billion year ago, due to a time of intense meteorite bombardment.

• After 3.1 billion years ago, the moon cooled down enough that molten rock could no longer get to the surface.

• The moon appears to have been geologically quiet for the past 3.1 billion years or so.

Section 17.1

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Lunar Plains

• Also call maria (Latin for ‘seas’)

• Lunar plains - large, dark, flat areas on the moon believed to be craters formed by meteorite impact that then filled with volcanic lava

• Dark in appearance because these flat plain areas reflect even less light than the average surface area of the moon

• Surface of the moon covered by a layer of loose debris called regolith

• Rock samples examined - similar to volcanic rock on earth

Section 17.1

Plains of the Moon

• Plains were produced by volcanic eruptions and the resulting enormous lava flows.

• Apparently the moon’s interior was hot enough to cause these 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 (where it is easier for the molten material to reach the surface).

Section 17.1

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17 | 13

Craters on the Moon

Section 17.1

Rays

• Rays - streaks that extend outward from some of the craters

• Thought to be pulverized rock that was thrown out when the crater formed

• These rays are brighter (reflect more light) than the crater – (powdered rock reflects more light than regular-sized rock)

• Occasionally the ray systems are also marked by secondary craters formed by flying debris thrown from the primary crater.

Section 17.1

Rills

• Rills - long, narrow trenches or valleys

• Some are straight and some are curved.

• Thought to represent a separation (or crack) caused by moonquakes

• Similar features are known to form because of earthquakes.

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 height.

Section 17.1

Origin of the Moon??

• Must take into account these facts:

• Lunar rocks are similar to the earth’s mantle.

• Oxygen isotope ratios indicate that the earth and the moon were formed at a similar distance from the sun.

• There is no water in lunar rocks.

• There is a deficiency of volatile elements (which were driven off by heat).

• Relative to earth, moon has less iron.

• 3.3 g/cm3 = moon; 5.5 g/cm3 = earth

• The oldest rocks on earth and moon are similar.

Section 17.1

Origin of the Moon

• Most widely accepted theory is the great impact theory.

• A planet-sized object (size of Mars) struck the earth with a glancing blow 4.4 billion years ago, resulting in the ejection of matter into orbit to form the moon.

Lunar Motions

• 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 Earth

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.

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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 larger

• Waning phase – the illuminated portion is getting smaller

• 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

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Phases of the Moon As observed from any latitude north of

28.5oN

The “observer” is looking south;

therefore east is on the left. 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

Eclipses

• 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

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 plane

• 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

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Solar Eclipses

Lunar Eclipse

• Lunar eclipse - occurs when the Earth blocks some or all of the sun’s rays to the moon

• 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

A Lunar Eclipse

Section 17.2

Ocean Tides

• Tides – the regular and alternating rise and fall of the ocean’s surface level

• Although by the 1st century people had noticed the relationship between the passage of the moon and the tides, no one could explain why

• Finally in the 17th century Newton applied his law of universal gravitation to explain the ocean tides by the motions of the Earth, moon, and sun

Section 17.2

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Tidal Bulges The two tidal bulges result in two high tides and two low tides daily

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 bulge

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

Spring Tide During Summer Solstice

• 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 this day

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 momentum • 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