Astronomy: Chapter 3

A solar eclipse occurs on Earth when the

Moon casts a shadow on Earth.

Aristarchus measured the Sun-Earth-Moon angle at quarter Moon to be 87° and calculated that the Sun is 20 times as far away as the Moon. Actually, the Sun is almost 390 times as far away as the Moon. Thus, the actual Sun-Earth-Moon angle at quarter Moon must be

89.85°.

During the particular solar eclipse, when the Moon and Sun are precisely in line, the eclipse can be either total (Sun completely covered) or annular (Sun not quite covered) when viewed from the eclipse centerline because

the Moon's distance from Earth varies from eclipse to eclipse.

Earth's shadow falling on the Moon is the reason we see

lunar eclipses.

Eclipses of the Moon can occur only

at full Moon.

Eratosthenes measured the radius of Earth by

noting the different angles of the Sun at midday on the same day of the year at different positions on Earth.

How does the Moon rotate to keep one face pointed toward Earth at all times?

It rotates once per month.

How does the duration of a total solar eclipse compare with the duration of a total lunar eclipse?

The lunar eclipse is always longer.

How fast does the Moon move across our sky against the background of stars?

13° per day

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

one half

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?

one week

If the plane of the Moon's orbit were to be the same as the ecliptic plane, there would be a lunar eclipse

once a month.

If you were standing on the Moon in darkness on the opposite side from Earth at a particular time, which of the following conditions would be true?

You would never see Earth from that position.

In 1 hour, the motion of the Moon across our sky as seen against the background of stars is?

its own diameter or 1/2°.

On a particular day, the Sun is at the vernal equinox and the Moon is at the autumnal equinox. The lunar phase on this particular day is

full.

Suppose Eratosthenes had measured the length of a shadow at Syene and then compared it with the shadow measured not at Alexandria, but at a location half way between Syene and Alexandria—both measurements being made at the summer solstice. What would he have found?

The difference in the lengths of these shadows would have been smaller than that found by comparing Syene and Alexandria.

The difference between an umbral eclipse and a penumbral eclipse is

the distance of the Moon above or below the ecliptic.

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

about 2 days longer.

The phase of the Moon at the time of solar eclipse is

new.

The term “sidereal month” refers to the

time over which the Moon completes one orbit around Earth, relative to the stars.

The term “synodic month” refers to the

time from new moon to new moon.

The time for the Moon to rotate around its own axis, relative to the stars, is about

1 month.

There is about a 5° angle between the orbit of the Moon and the

plane of the ecliptic, or Earth's orbit.

We can occasionally see a total eclipse of the Sun on Earth because

the angular sizes of Sun and Moon, when viewed from Earth, are almost the same.

What is the cause of an annular eclipse?

The Moon's position in its orbit is near apogee, its farthest point from Earth.

What is the declination of the Moon?

a number which changes significantly over a month

What is the right ascension of the Moon?

a number which changes significantly over a month

When the Sun and Moon have the same right ascension, the phase of the Moon is a

new moon.

Which of the following is a necessary condition for lunar or solar eclipses?

The Moon must be close to or crossing the ecliptic plane.

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?

27 1/3 days