Sep 20/23 - Distance scale and lunar phases

Question 1

Why do we see phases of the moon?

Answer 1

As the Moon orbits Earth, it returns to the same position relative to the Sun in our sky (such as along the Earth–Sun line) about every 29.5 days.

This marks the cycle of lunar phases: the Moon’s appearance in our sky changes as its position relative to the Sun changes

Question 2

What direction does the moon orbit?

Answer 2

Counter clockwise, as seen from the north pole

Question 3

New moon

Answer 3

when we cant see the moon at the time (seeing the shadowy side)

Question 4

Full moon

Answer 4

when the sunny side of the moon is completely illuminated

Question 5

First quarter moon

Answer 5

when the moon is 1/4 of the way through its orbit (light on RIGHT side)

Question 6

Third quarter moon

Answer 6

when the moon is 3/4 of the way through its orbit (light on the LEFT side)

Question 7

Waxing crescent

Answer 7

when only a portion of the sunny side is visible and the light on the moon is INCREASING

Question 8

Waning crescent

Answer 8

when only a portion of the sunny side is visible, and the light on the moon is DECREASING

Question 9

Waxing gibbous

Answer 9

phase before full moon; not fully illuminated, but to increase

Question 10

Waning gibbous

Answer 10

phase after full moon; not fully illuminated, but to decrease

Question 11

When is a full moon at its highest point in the sky?

Answer 11

At midnight

Must drop lower as a person reaches sunrise; at the horizon at this point

A full moon rises when the sun sets, reaches its highest point at midnight, and sets when the sun rises

Question 12

First quarter moon reaches its highest point when?

Answer 12

during sunset

Sets at midnight
Remains below horizon until next noon

Visible during the day

Question 13

When are waning crescents at their highest point in the sky?

Answer 13

Approx 9am when highest in the sky

Question 14

The first step in understanding phases is…

Answer 14

to recognize that sunlight essentially comes at both Earth and the Moon from the same direction

Half of the moon always faces the sun, while the other is away and dark

The amount that is illuminated depends on the Moon’s position in its orbit

Question 15

Although we see the moon’s phases, we do not see its faces (always see the same face of the Moon) - WHY?

Answer 15

This happens because of synchronous rotation: the moon rotates its axis in the same amount of time it takes to orbit earth

Note that the Moon’s synchronous rotation is not a coincidence; it is a consequence of Earth’s gravity affecting the Moon in much the same way that the Moon’s gravity causes tides on Eart

Question 16

What would you see looking down from the moon?

Answer 16

Earth’s daytime side and hence would see a full earth.