Working through this chapter of the study guide will enable you to:

1. Describe the general features of the Moon, along with its special relationship to our planet Earth.
2. Explain the current theories that best explain the origin of the Moon, and trace its extraordinary history up to the present day.
3. Describe and explain the origin of the various surface features of the Moon.
4. Show how the motion of the Moon leads to such diverse phenomenon as monthly phases, solar and lunar eclipses, and the tides in the oceans on Earth.

Discussion

The Moon not only lighted the night sky for ancient people but gave them a periodic mechanism for timing the important events of the year. The modern month is a direct extension of this primitive way of telling time. Since the Moon has played such a large part in our lives for so many years, it is perhaps fitting that it is the first stepping stone in our manned exploration of our solar system. It also helps, of course, that it is the nearest celestial object to Earth, so the Moon is the easiest extraterrestrial destination to reach.
Scientists have studied the Moon with telescopes for centuries, but it was not until we actually traveled through space to visit our nearest neighbor that we got the chance to see the back side of this familiar object. This is due to the exact matching of the orbital and rotational periods of the Moon's motion. These synchronized periods always keep the same side of our celestial neighbor facing us. There were not many surprises about the structure and features of either the front or back sides of the Moon. We already knew a great deal from our telescopic observations and from calculations made using Newton's laws. It is, however, comforting to find that our conclusions were correct and that the direct measurements made by our astronauts on the Moon itself confirmed these theories.
The motion of the Moon is readily apparent to anyone who takes a little time to observe it carefully. Lunar motion is responsible for both solar and lunar eclipses, the changes in the area of lighted surface that is visible from Earth (the lunar phases), and for the periodic surges of water in the oceans of Earth, which constitute the tides. All in all, the Moon still remains a very fascinating object to study or simply to enjoy on clear nights, when it supplies most of the celestial light for our outdoor nocturnal activities.

Section  17.1 General Features

The Moon has a period of rotation of 29.5 days, a diameter of about 3500 km, a mass about 1/81 as great as that of Earth, and a density of 3.3 g/cm³, which is almost the same as that of Earth's mantle. It rotates with the same period as it revolves around Earth, so it always has its same surface facing our planet. The lunar surface is heavily cratered from impacts by large and small objects. Some of these craters have been filled in with lava to form plains, or maria, but most surface features have not changed over billions of years because no water or atmosphere is present to cause the processes of erosion to take place as they do on Earth.
Some craters show bright radial streaks called rays emanating outward from their centers, and others do not. Long, narrow, trench-like valleys called rills extend for hundreds of miles across the Moon's surface. These were probably caused by moonquakes, similar to earthquakes that have caused similar features on our own planet's surface. In addition, tall mountain ranges form circular patterns around the great plains. Although some of these mountains are as high as 20,000 ft, they were probably formed by impact rather than by plate tectonics, which is the principal mountain-building process on Earth. Large vertical and horizontal faults on the Moon's surface are probably the result of shifts in the Moon's outer crustal layers.

Section  17.2 Composition and History of the Moon

The first manned lunar spacecraft was able to gather samples of the lunar crust that have helped us immensely in understanding the history of the Moon. Rocks that were part of these samples have been dated to be between 3.1 and 4.4 billion years old, with none any older or any younger than that ever found. As previously stated, most of the craters of the Moon's surface were formed by impact, but about 1% seem to have been produced in volcanic eruptions that also were responsible for forming the great maria regions over 3 billion years ago. This volcanic activity was probably the result of radioactive processes deep within the Moon's interior that produced enough heat to melt the upper mantle and cause massive eruptions. The Moon's mantle is now so thick that no surface volcanic activity appears to have taken place on the Moon for over 3 billion years.

Section  17.3 Lunar Motions

Although the Moon revolves around Earth every 29.5 solar days, it does not travel in the same plane in which Earth orbits the Sun. The tilt of the Moon's orbital plane is about 5° with respect to Earth's orbital plane. This allows the Moon to appear directly overhead at any latitude on Earth between 28.5° N and 28.5° S, depending on the orientation of the two planes of revolution at that particular time. This 5° tilt is also responsible for the fact that eclipses do not occur regularly twice every month as the Moon circles Earth. We will see in Section 17.5 why solar and lunar eclipses are such rare and fascinating events.
The lunar month as measured with respect to the Sun is 29.5 days. This is called the sidereal month. If the period of lunar revolution is measured with respect to the fixed stars, it turns out to be only 27.5 days. This is known as the synodic month. The fact that the Moon appears to rise in the east and set in the west, when viewed from Earth each day, is a result of the combination of the Moon's motion and the rotation of Earth on its axis. Careful observation shows that the Moon rises approximately 50 minutes later every day, although this figure may vary somewhat with the latitude of the observer on Earth.

Section  17.4 Phases

Because of the continuously changing relative positions of the Sun, Earth, and Moon, the lighted portion of the Moon that can be seen from Earth changes with time. These changes are called the phases of the Moon. Half the Moon's surface is always lighted by the Sun, but we cannot always see all of this lighted surface. When the Moon is between the Sun and Earth, the lighted side is turned away from us, and we cannot see any of it. This phase is called the new moon. A new moon will be on the overhead meridian for an observer on Earth at about 12 noon local solar time. As the lunar month progresses, more and more of the lighted portion of the Moon becomes visible from Earth, and the Moon spends about a week in this initial waxing (increasing) crescent phase. Then 7 3/8 days after new phase, the Moon reaches first-quarter phase, which will be on an observer's overhead meridian at about 6 P.M. During the next week, the Moon continues to wax, but now that more than 50% of its lighted surface is visible from Earth, it is said to be in the waxing gibbous phase.
When the Moon is exactly on the opposite side of Earth from the Sun, it is said to be in full phase. Then the entire lighted surface is visible and the full moon will be overhead for an observer on Earth at midnight. From here the Moon progresses through waning (decreasing) gibbous phase to last-quarter phase (overhead at about 6 A.M.) and then through waning crescent phase until, after 29.5 days it is back in new phase again. This cycle is repeated over and over, with the Moon progressing through the same sequence each month. The rising or setting time for any phase can be estimated by remembering when that phase will be overhead and then subtracting 6 hours for moonrise or adding 6 hours for moonset.

Section  17.5 Eclipses

When the Moon blocks the light coming to Earth from the Sun, we experience a solar eclipse. It is a total eclipse if we are in the umbra portion of the Moon's shadow but only a partial eclipse if we are in the penumbra region of the shadow. A solar eclipse can occur only when the Moon is in or very near new phase, so it is always observed around noon local solar time. Astronomers, other scientists, and even interested citizens often travel great distances to be in the umbra region of the Moon's shadow so that they will be in position to observe a total solar eclipse.
If Earth blocks the light from the Sun that normally falls on the surface of the Moon, we will observe a lunar eclipse. Again, a total lunar eclipse can occur only for that portion of the Moon that is in the umbra portion of the shadow cast by Earth. A partial lunar eclipse is seen when all portions of the Moon are in the penumbra region of Earth's shadow. Lunar eclipses that can be seen by an Earth-based observer occur at or very near full moon phase and so will be observed overhead at or about midnight; however, since the surface of the Moon can be seen from all positions on the back side of Earth (with respect to the Sun), a lunar eclipse can be seen by an Earth-based observer at any time from about 6 P.M. to 6 A.M. This makes a lunar eclipse much easier to see from Earth's surface than a solar eclipse.
As stated previously, the orbital plane of the Moon does not coincide with the orbital plane of Earth as it revolves around the Sun, so we do not see a solar eclipse at every new moon phase or a lunar eclipse at every full moon phase. If the orbital planes were the same, we would have the good fortune to see two eclipses every month, but we would probably soon lose interest if such events were to become that commonplace. As it is, eclipses are rare enough to pique the curiosity of most people, and observing them is still exciting and fun for most of us when we are lucky enough to be in the right location at the proper time.

Section  17.6 Ocean Tides
Working through this chapter of the study guide will enable you to:

1. Describe the general features of the Moon, along with its special relationship to our planet Earth.
2. Explain the current theories that best explain the origin of the Moon, and trace its extraordinary history up to the present day.
3. Describe and explain the origin of the various surface features of the Moon.
4. Show how the motion of the Moon leads to such diverse phenomenon as monthly phases, solar and lunar eclipses, and the tides in the oceans on Earth.

Discussion


The Moon not only lighted the night sky for ancient people but gave them a periodic mechanism for timing the important events of the year. The modern month is a direct extension of this primitive way of telling time. Since the Moon has played such a large part in our lives for so many years, it is perhaps fitting that it is the first stepping stone in our manned exploration of our solar system. It also helps, of course, that it is the nearest celestial object to Earth, so the Moon is the easiest extraterrestrial destination to reach.
Scientists have studied the Moon with telescopes for centuries, but it was not until we actually traveled through space to visit our nearest neighbor that we got the chance to see the back side of this familiar object. This is due to the exact matching of the orbital and rotational periods of the Moon's motion. These synchronized periods always keep the same side of our celestial neighbor facing us. There were not many surprises about the structure and features of either the front or back sides of the Moon. We already knew a great deal from our telescopic observations and from calculations made using Newton's laws. It is, however, comforting to find that our conclusions were correct and that the direct measurements made by our astronauts on the Moon itself confirmed these theories.
The motion of the Moon is readily apparent to anyone who takes a little time to observe it carefully. Lunar motion is responsible for both solar and lunar eclipses, the changes in the area of lighted surface that is visible from Earth (the lunar phases), and for the periodic surges of water in the oceans of Earth, which constitute the tides. All in all, the Moon still remains a very fascinating object to study or simply to enjoy on clear nights, when it supplies most of the celestial light for our outdoor nocturnal activities.

Section  17.1 General Features

The Moon has a period of rotation of 29.5 days, a diameter of about 3500 km, a mass about 1/81 as great as that of Earth, and a density of 3.3 g/cm³, which is almost the same as that of Earth's mantle. It rotates with the same period as it revolves around Earth, so it always has its same surface facing our planet. The lunar surface is heavily cratered from impacts by large and small objects. Some of these craters have been filled in with lava to form plains, or maria, but most surface features have not changed over billions of years because no water or atmosphere is present to cause the processes of erosion to take place as they do on Earth.
Some craters show bright radial streaks called rays emanating outward from their centers, and others do not. Long, narrow, trench-like valleys called rills extend for hundreds of miles across the Moon's surface. These were probably caused by moonquakes, similar to earthquakes that have caused similar features on our own planet's surface. In addition, tall mountain ranges form circular patterns around the great plains. Although some of these mountains are as high as 20,000 ft, they were probably formed by impact rather than by plate tectonics, which is the principal mountain-building process on Earth. Large vertical and horizontal faults on the Moon's surface are probably the result of shifts in the Moon's outer crustal layers.

Section  17.2 Composition and History of the Moon

The first manned lunar spacecraft was able to gather samples of the lunar crust that have helped us immensely in understanding the history of the Moon. Rocks that were part of these samples have been dated to be between 3.1 and 4.4 billion years old, with none any older or any younger than that ever found. As previously stated, most of the craters of the Moon's surface were formed by impact, but about 1% seem to have been produced in volcanic eruptions that also were responsible for forming the great maria regions over 3 billion years ago. This volcanic activity was probably the result of radioactive processes deep within the Moon's interior that produced enough heat to melt the upper mantle and cause massive eruptions. The Moon's mantle is now so thick that no surface volcanic activity appears to have taken place on the Moon for over 3 billion years.

Section  17.3 Lunar Motions

Although the Moon revolves around Earth every 29.5 solar days, it does not travel in the same plane in which Earth orbits the Sun. The tilt of the Moon's orbital plane is about 5° with respect to Earth's orbital plane. This allows the Moon to appear directly overhead at any latitude on Earth between 28.5° N and 28.5° S, depending on the orientation of the two planes of revolution at that particular time. This 5° tilt is also responsible for the fact that eclipses do not occur regularly twice every month as the Moon circles Earth. We will see in section 17.5 why solar and lunar eclipses are such rare and fascinating events.
The lunar month as measured with respect to the Sun is 29.5 days. This is called the sidereal month. If the period of lunar revolution is measured with respect to the fixed stars, it turns out to be only 27.5 days. This is known as the synodic month. The fact that the Moon appears to rise in the east and set in the west, when viewed from Earth each day, is a result of the combination of the Moon's motion and the rotation of Earth on its axis. Careful observation shows that the Moon rises approximately 50 minutes later every day, although this figure may vary somewhat with the latitude of the observer on Earth.

Section  17.4 Phases

Because of the continuously changing relative positions of the Sun, Earth, and Moon, the lighted portion of the Moon that can be seen from Earth changes with time. These changes are called the phases of the Moon. Half the Moon's surface is always lighted by the Sun, but we cannot always see all of this lighted surface. When the Moon is between the Sun and Earth, the lighted side is turned away from us, and we cannot see any of it. This phase is called the new moon. A new moon will be on the overhead meridian for an observer on Earth at about 12 noon local solar time. As the lunar month progresses, more and more of the lighted portion of the Moon becomes visible from Earth, and the Moon spends about a week in this initial waxing (increasing) crescent phase. Then 7 3/8 days after new phase, the Moon reaches first-quarter phase, which will be on an observer's overhead meridian at about 6 P.M. During the next week, the Moon continues to wax, but now that more than 50% of its lighted surface is visible from Earth, it is said to be in the waxing gibbous phase.
When the Moon is exactly on the opposite side of Earth from the Sun, it is said to be in full phase. Then the entire lighted surface is visible and the full moon will be overhead for an observer on Earth at midnight. From here the Moon progresses through waning (decreasing) gibbous phase to last-quarter phase (overhead at about 6 A.M.) and then through waning crescent phase until, after 29.5 days it is back in new phase again. This cycle is repeated over and over, with the Moon progressing through the same sequence each month. The rising or setting time for any phase can be estimated by remembering when that phase will be overhead and then subtracting 6 hours for moonrise or adding 6 hours for moonset.

Section  17.5 Eclipses

When the Moon blocks the light coming to Earth from the Sun, we experience a solar eclipse. It is a total eclipse if we are in the umbra portion of the Moon's shadow but only a partial eclipse if we are in the penumbra region of the shadow. A solar eclipse can occur only when the Moon is in or very near new phase, so it is always observed around noon local solar time. Astronomers, other scientists, and even interested citizens often travel great distances to be in the umbra region of the Moon's shadow so that they will be in position to observe a total solar eclipse.
If Earth blocks the light from the Sun that normally falls on the surface of the Moon, we will observe a lunar eclipse. Again, a total lunar eclipse can occur only for that portion of the Moon that is in the umbra portion of the shadow cast by Earth. A partial lunar eclipse is seen when all portions of the Moon are in the penumbra region of Earth's shadow. Lunar eclipses that can be seen by an Earth-based observer occur at or very near full moon phase and so will be observed overhead at or about midnight; however, since the surface of the Moon can be seen from all positions on the back side of Earth (with respect to the Sun), a lunar eclipse can be seen by an Earth-based observer at any time from about 6 P.M. to 6 A.M. This makes a lunar eclipse much easier to see from Earth's surface than a solar eclipse.
As stated previously, the orbital plane of the Moon does not coincide with the orbital plane of Earth as it revolves around the Sun, so we do not see a solar eclipse at every new moon phase or a lunar eclipse at every full moon phase. If the orbital planes were the same, we would have the good fortune to see two eclipses every month, but we would probably soon lose interest if such events were to become that commonplace. As it is, eclipses are rare enough to pique the curiosity of most people, and observing them is still exciting and fun for most of us when we are lucky enough to be in the right location at the proper time.

Section  17.6 Ocean Tides

The gravitational pull of the Moon causes the water in the oceans and large lakes on Earth to pile up against the shore twice a day. These occurrences are known as high tides. The water is also pulled away from each shore twice a day to produce low tides. The gravitational pull of the Sun also affects how much the tides vary each day, as does the distance between the Moon and Earth. Correct orientation of Earth, Moon, and Sun in a nearly straight line results in unusually high and low tides known as spring tides. When the Moon and Sun are at 90° with respect to each other, the tidal effects are greatly reduced and neap tides result.
The gravitational pull of the Moon causes the water in the oceans and large lakes on Earth to pile up against the shore twice a day. These occurrences are known as high tides. The water is also pulled away from each shore twice a day to produce low tides. The gravitational pull of the Sun also affects how much the tides vary each day, as does the distance between the Moon and Earth. Correct orientation of Earth, Moon, and Sun in a nearly straight line results in unusually high and low tides known as spring tides. When the Moon and Sun are at 90° with respect to each other, the tidal effects are greatly reduced and neap tides result.

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