Question 1

The direction of motion of the Moon in the sky,viewed from Earth against the background of stars (see Figure 1-22,Comins,Discovering the Essential Universe,6th ed.),is:

Accepted Answer

The Moon moves eastward relative to the background stars due to its orbit around Earth, completing a full cycle approximately every 27.3 days.

Question 2

Why is the period between two successive full Moons NOT equal to the Moon's orbital period,or sidereal month?

Accepted Answer

The period between two successive full Moons is not equal to the Moon's orbital period or sidereal month because the Earth-Moon system is also orbiting around the Sun. This means that during the time it takes the Moon to complete one orbit around Earth (sidereal month), the Earth-Moon system has moved in its orbit around the Sun, causing the Moon to have to travel a little further to reach the full Moon phase. Therefore, the time between two successive full Moons is slightly longer than the Moon's orbital period or sidereal month.

Question 3

The motion of the Moon across the sky,against the background of stars,is approximately:

Accepted Answer

The Moon moves approximately 13° per day against the background of stars due to its orbit around the Earth.

Question 4

If you were on the Moon at the dividing line between dark and light (the terminator)at a particular time,say,sunrise,how long would it be before the dividing line returned to your position?

Accepted Answer

The Moon takes about 29 days to complete one rotation on its axis and one orbit around Earth, so the terminator would return to the same position in approximately 29 days.

Question 5

One synodic month is longer than one sidereal month by about:

Accepted Answer

A synodic month is the time required for the Moon to return to the same phase (e.g. full moon to full moon), while a sidereal month is the time required for the Moon to return to the same position relative to the stars. Because Earth is also moving in its orbit around the Sun during this time, it takes slightly longer for the Moon to return to the same phase than it does for it to return to the same position relative to the stars. This difference amounts to about 2.2 days.

Question 6

Suppose Earth,the Sun,and a planet are aligned along a straight line.The time interval until this alignment occurs again is a:

Accepted Answer

The time interval until the alignment of Earth, the Sun, and a planet occurs again is a synodic period. This is because it is the period of time it takes for the planet to return to the same apparent position relative to the Sun as observed from Earth. The sidereal period is the time it takes for a planet to complete one orbit around the Sun, and the solar period is the 24-hour period of Earth's rotation around its own axis. The tropical period is the time it takes for the Sun to return to the same position relative to the Earth's seasons, and is not relevant to this scenario.

Question 7

The fact that the Earth-Moon system orbits the Sun (covering 30° per month)while the Moon orbits Earth means that,compared with one lunar orbital (sidereal)period,the time between successive full Moons (the synodic month)is:

Accepted Answer

The synodic month is the time between successive full moons, which is approximately 29.5 days. However, during this time, the Earth has also made one orbit around the Sun. Therefore, the Moon needs to travel further in its orbit to return to the same phase (full moon), making the synodic month about 2 days longer than the sidereal (lunar orbital) period.

Question 8

The Moon is seen to keep one face toward Earth at all times.If viewed from a point directly above the plane of the planetary system,how does it have to rotate to maintain this alignment?

Accepted Answer

The Moon must rotate once per month, or once per orbit around Earth, to maintain its alignment with Earth. This is known as synchronous rotation or tidal locking. The same gravitational forces that cause the tides on Earth also cause the Moon to gradually slow down its rotation until it becomes tidally locked with Earth. Therefore, the Moon always shows the same face to us, but it does rotate on its axis once per orbit around Earth.

Question 9

In writing a mystery story,your friend has the fictitious villain planning a robbery and needs to use a statement about the timing of the robbery.Which of the following statements would you recommend to your friend?

Accepted Answer

A full Moon occurs approximately every 29.5 days, so waiting two weeks from a full Moon will bring you to a new Moon, which is when the night is moonless.

Question 10

The length of time for the Moon to move from new Moon to new Moon is known as one synodic month.Compared with one full orbital period with respect to the star background,or one sidereal month,the synodic month is:

Accepted Answer

The synodic month is slightly longer than the sidereal month because during the time it takes the Moon to complete one orbit around the Earth, the Earth has moved in its orbit around the Sun, so the Moon must travel a little farther to get back to the same phase. This adds up to about 2 extra days, making the synodic month about 29.5 days long compared to the sidereal month which is about 27.3 days long.

Question 11

A full Moon will always be at its highest in our sky at about:

Accepted Answer

A full Moon is opposite the Sun in the sky, and therefore rises at sunset and is highest in the sky at midnight.

Question 12

The Moon is visible in the sky in the daytime from most places on Earth:

Accepted Answer

The Moon is visible during the daytime for about half of its cycle, as it orbits Earth and reflects sunlight.

Question 13

The Moon will appear to an observer at mid-latitudes on Earth to rise in the east,in solar time,about (You might attempt to verify this by observation!):

Accepted Answer

The answer of The Moon will appear to an observer...

Question 14

In a mystery story,the culprit made the following statements when describing the murder scene.Which of them was the only TRUE statement?

Accepted Answer

The statement that the full Moon was just rising as the Sun set is true. This is because a full Moon can only rise when the Sun is setting, as the full Moon is always opposite the Sun in the sky. The other statements are false as they describe either a new Moon or a first quarter Moon, neither of which would be visible during the time of night described.

Question 15

How often does the Moon rotate to keep one face pointed toward Earth at all times,as seen in Figure 1-20 in Comins,Discovering the Essential Universe,6th ed.?

Accepted Answer

The Moon rotates on its axis once per month, which matches its orbital period around Earth, resulting in the same side always facing Earth. This is known as synchronous rotation.

Question 16

Which of the following phrases is the only one that truly and meaningfully describes the side of the Moon away from the Sun?

Accepted Answer

The phrase "dark side of the Moon" is incorrect because the far side of the Moon is not always dark, it experiences daylight just like the near side. Similarly, the phrase "anti-Sun side of the Moon" is not meaningful as the Moon is always "anti-Sun" from one particular point of observation. "Region of the Moon's equator not illuminated by the Sun" is also incorrect as the far side of the Moon can be illuminated by the Sun, just not visible from Earth. Therefore, "far side of the Moon" is the most accurate and meaningful phrase to describe the side of the Moon away from the Sun.

Question 17

In which way does the Moon move day by day in the sky,against the background of stars,when viewed from Earth?

Accepted Answer

The Moon appears to move toward the east in the sky, against the background of stars, when viewed from Earth. This is due to the Moon's orbit around Earth, which takes about 27.3 days to complete. As the Moon orbits Earth, it appears to move eastward each day, completing a full circuit of the sky in about 27.3 days.

Question 18

The motion of the Moon across the sky in one hour,as seen against the background of stars,is approximately:

Accepted Answer

The Moon moves approximately its own diameter, about 0.5 degrees, across the sky in one hour due to its orbit around the Earth.

Question 19

How much of the total surface of the Moon is illuminated by the Sun when the Moon is at crescent phase?

Accepted Answer

The answer of How much of the total surface of...

Question 20

The full Moon can be on the horizon:

Accepted Answer

The full Moon can only be on the horizon at sunrise or sunset, when it is opposite the Sun in the sky.

Quiz 1: Discovering the Night Sky

The direction of motion of the Moon in the sky,viewed from Earth against the background of stars (see Figure 1-22,Comins,Discovering the Essential Universe,6th ed.) ,is: