Question 1

Which of the following statements about the Moon's position and motion is NOT correct?

Accepted Answer

The Moon does rotate on its axis, which is why it keeps one face toward Earth at all times. This synchronous rotation matches its orbital period around Earth, allowing only one hemisphere to be visible from Earth.

Question 2

As seen by a distant observer who is fixed with respect to the stars, approximately how long does it take for the Moon to rotate once around its own axis?

Accepted Answer

The Moon rotates once around its own axis approximately every month, which is the same amount of time it takes to orbit the Earth. This synchronous rotation causes the same side of the Moon to always face the Earth.

Question 3

The approximate rotation period of the Moon is:

Accepted Answer

The Moon's rotation period is approximately one month, which is why we always see the same side of the Moon from Earth. This synchronous rotation is due to tidal locking with Earth.

Question 4

During a month we see only approximately half of the moon's surface. Which one of the following changes would allow us to see all of the Moon's surface during 1 month?

Accepted Answer

The Moon shows us only one side because it is tidally locked with Earth, meaning its rotation period matches its orbit period around Earth. Reversing the direction of the Moon's rotation about its axis would eventually alter this synchronous rotation, allowing us to see all of its surface over time.

Question 5

The Moon is now in its waxing crescent phase. If you could view the far side of the Moon it would be:

Accepted Answer

The far side of the Moon is in the waning gibbous phase when the near side is in the waxing crescent phase. This is because the phases of the Moon are determined by the relative positions of the Earth, Moon, and Sun, so when one side of the Moon is illuminated less (waxing crescent), the opposite side is illuminated more (waning gibbous).

Question 6

You are standing at the equator of the Moon and a particular star is overhead at that time. How long will it be before that star is approximately overhead again?

Accepted Answer

The Moon rotates on its axis once every 27.3 days (approximately). Therefore, the star will be overhead again in 27 1/3 days.

Question 7

If astronauts landed on the Moon near the center of the visible surface at full moon, how many Earth days would pass before the astronauts would experience darkness on the Moon?

Accepted Answer

The answer of If astronauts landed on the Moon near...

Question 8

If Earth orbited the Sun in half the time it does now, how would the length of time between full moons change?

Accepted Answer

The answer of If Earth orbited the Sun in half...

Question 9

Approximately how long will it take for the dark-light dividing line on the Moon, the terminator, to move through 90° of lunar longitude?

Accepted Answer

The Moon takes about 29.5 days to complete one full rotation and orbit around the Earth. Therefore, the terminator moves through 90° of lunar longitude in approximately 7 days, which is about one-quarter of the lunar cycle.

Question 10

One day from sunrise to sunrise for a space explorer at her lunar base will be how many Earth days long? (A diagram might help.)

Accepted Answer

The lunar day, or the time it takes for the Moon to make one full rotation on its axis, is about 29.5 Earth days long. This means that one day from sunrise to sunrise for a space explorer at her lunar base would also be about 29.5 Earth days long.

Question 11

The fact that the Earth-Moon system orbits the Sun (covering 30° per month) in a clockwise direction viewed from the celestial north pole while the Moon orbits Earth, also in a clockwise direction, means that the time between successive full moons, the synodic month, compared to one lunar orbital (sidereal) period is:

Accepted Answer

The synodic month is about 2 days longer than the sidereal month because as the Moon orbits Earth, both the Earth and Moon are also moving around the Sun, requiring the Moon to travel a bit more than one full orbit to reach the same phase.

Question 12

The Moon rises later each day because each day it has moved farther along its orbit around Earth (except for observers at polar latitudes, for whom the Moon can remain above or below the horizon for 24 hours each day). On average, how much later does it rise each day (you might attempt to verify this by observation)? (Hint: What fraction of a month does a day take up?)

Accepted Answer

The Moon takes about 29.5 days to orbit the Earth, which is roughly one month. Since a day is about 1/29.5 of a month, the Moon rises approximately 1/29.5 of a full circle later each day, which translates to about 50 minutes to 1 hour later each day.

Question 13

How fast does the Moon move across our sky against the background of stars? (Hint: Through what angle does the Moon move around the sky in 1 month?)

Accepted Answer

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

Question 14

In 1 hour, the motion of the Moon across our sky as seen against the background of stars is: (Hint: Through what angle around the sky does the Moon move in 1 month?)

Accepted Answer

The Moon moves about 13° across the sky each day, given it takes about 27.3 days to orbit the Earth. In one hour, this translates to roughly its own diameter, or about 1/2°.

Question 15

Earth and the Moon both move eastward in their orbits. Suppose the Moon moved westward instead, at the same rate as its actual eastward movement. What consequence would this have?

Accepted Answer

The sidereal month is the time it takes for the Moon to return to the same position relative to the stars. This would not be affected by the Moon moving westward instead of eastward, as the stars do not change their position. Therefore, choice B is correct.

The synodic month is the time it takes for the Moon to return to the same phase (e.g. full moon to full moon). This would be affected by the Moon moving westward instead of eastward, as the relative motion between Earth and Moon would be different. However, it is not possible to determine from the given information whether the synodic month would be longer or shorter than it is now, so choices A, C, and D are not correct.

Question 16

The Moon moves eastward in its orbit. It always keeps the same face toward Earth. This means that it:

Accepted Answer

The Moon rotates on its axis in the same direction that it orbits Earth (eastward), which is why it always keeps the same face toward Earth. This synchronous rotation is why we only see one side of the Moon from Earth.

Question 17

Earth rotates eastward on its axis. The Moon moves to the east in its orbit around Earth. The Moon also rises about 1 hour later each night. Suppose the Moon moved westward instead. What consequence would this have?

Accepted Answer

If the Moon moved westward instead, it would be moving in the opposite direction of Earth's rotation, which means it would be moving faster than the Earth's rotation. This would cause the Moon to rise earlier each night, approximately 1 hour earlier.

Question 18

The gravitational pull of the Sun on the Moon causes each one of the changes listed below EXCEPT ONE. Which one is the EXCEPTION?

Accepted Answer

The appearance of the Moon seen from Earth is primarily affected by the relative positions of the Earth, Moon, and Sun, as well as atmospheric conditions. It is not directly related to the gravitational pull of the Sun on the Moon.

Question 19

During a total lunar eclipse (as viewed from Earth) you are standing on the Moon looking back at Earth. What do you see?

Accepted Answer

During a total lunar eclipse, the Earth passes between the Sun and the Moon, casting its shadow on the Moon's surface. Therefore, from the Moon's perspective, the Earth appears as a black disk with only a faint glow around the edges as some sunlight is refracted through Earth's atmosphere.

Question 20

In a period of 1 month, the Moon moves across the sky:

Accepted Answer

The Moon's path across the sky over the course of a month is due to its orbit around Earth, which is tilted relative to both the ecliptic plane and the celestial equator. Therefore, the Moon's path will not be parallel to the horizon, nor will it follow either of these two defined planes.

Quiz 3: Eclipses and the Motion of the Moon