Motion of the Moon

Orion

The constellations

Traditionally, a constellation is a grouping of stars in the same part of the sky.

The constellations

In modern astronomy, the constellations are 88 irregular areas that completely cover the sky. Thus, every celestial object lies within the boundaries of a constellation.

Ursa Major

Discussion

Why do you think ancient astronomers invented the constellations and made up stories to go with them?

Canis Major

Constellations of the Zodiac

The ecliptic passes through 12 constellations (actually 13) during the year. This are know as the zodiacal constellations.

Discussion

Which way does the Sun move in the sky?

Discussion

Which way does the Sun move with respect to the stars?

Discussion

In what constellation will the Sun be in one month from the time of this picture?

The Moon’s motion

Diurnal motion – rises in the east crosses the meridian and sets in the west every day.

Monthly motion – moves eastward with respect to stars moving its diameter in about an hour.

The Moon

As we pass through the month, the appearance of the Moon changes each night. We say the Moon goes through a series of phases.

Many people believe incorrectly that the phases of the Moon are caused by the shadow of the Earth on the Moon. What simple observation of the Moon could you make to prove that they are wrong?

Discussion

1st Quarter Moon

The “shadow” on a first or last quarter Moon is a straight line dividing the face of the Moon in half. The Earth is round and will always cause a curved shadow on the Moon.

When the shadow of the Earth does fall on the Moon we call this a lunar eclipse.

The Earth is round

The Moon’s phases are actually cause by the changing position of the Moon with respect to the Sun. The side of the Moon facing the Sun is always fully illuminated (except during a lunar eclipse). But because the angle we view the Moon differs from that of the Sun, we see varying degrees of the Moon’s surface facing us illuminated.

The real reason

New Moon

Occurs when the Moon is in the same part of the sky as the Sun. Thus the Moon is up during the day, and the illuminated side of the Moon faces away from the Earth. The Moon is not visible when it is new.

Waxing crescent

When less than ½ of the Moon’s face is illuminated. It is the western side of the Moon’s face that is illuminated and the Moon is visible in the sky before Sunset. Waxing means increasing.

First Quarter

When exactly ½ of the Moon’s face is illuminated. It is the western side of the Moon’s face that is illuminated and the Moon is up before sunset. Although ½ of the Moon’s face is illuminated it is called a quarter because the Moon is ¼ of the way through it’s cycle and occurs about one week after the new phase. The Moon is 90 degrees away from the Sun in the sky.

Waxing Gibbous

When more than ½ of the Moon’s western face is illuminated.

Full Moon

when the Moon’s face is fully illuminated. At this time the Moon is directly opposite the Sun. Occurs about 2 weeks after the new phase.

Waning Gibbous

when more than ½ of the Moon’s face is illuminated. But now it is the eastern face that is illuminated and waning means decreasing.

Last Quarter

when ½ of the Moon’s face is illuminated. Again it is the eastern half that is illuminated and the Moon is visible after sunrise. Occurs about 3 weeks after the new phase.

Waning Crescent

When less than ½ of the eastern half of the Moon’s face is illuminated.

Discussion

If the Moon is full, approximately what time does the Moon rise, cross the meridian, and set?

Discussion

If the Moon is new, approximately what time does the Moon rise, cross the meridian, and set?

Discussion

If the Moon is in its 1st quarter phase what approximately what time will it rise? Where in the sky will it be at sunset?

What time is it in this drawing?

What time is it in this drawing?

Lunar eclipse

Discussion

What phase must the Moon be in for there to be a lunar eclipse?

Discussion

Why don’t we have a lunar eclipse every time there is a full Moon?

Total Lunar eclipse of July 16, 2000

Discussion

Why do you think the eclipsed Moon appears red?

Discussion

The Moon goes through its phases in 29.53 days. This is called the synodic month, i.e. the month relative to the Sun. The actual orbital period of the Moon is 27.32 days, the sidereal month. Why is there a difference in the synodic and sidereal months?

In the 27.32 days that the Moon has taken to orbit the Earth the Earth has moved in its orbit relative to the Sun. The Earth moves about 1 degree per day so that after 27 days the Moon needs to move through an extra 27 degrees in the sky to get back to the same position relative to the Sun.

Synodic and Sidereal months

Discussion

If the Earth moves faster in its orbit does the length of the sidereal month change?

Discussion

If the Earth moves faster in its orbit, does the synodic month get longer or shorter?

Discussion

The Moon always keeps the same side facing the Earth. Does this mean the Moon does not spin on its axis? Explain?

If the Moon did not spin on its axis we would see the Moon’s entire surface over the month. The Moon therefore does rotate but the rotation period is exactly equal to its orbital period and rotates such that we always see the same face. Thus, the Moon’s rotation period is synchronous with its orbital period.

Synchronous Rotation

Discussion

What is the length of the sidereal and mean solar days on the Moon?

29.5 Earth days, the Sun rises in the east sets in the west 14.75 days later, followed by 14.75 days of darkness.

The Moon’s sidereal day is its actual rotation (and revolution) period of 27.32 days.

Mean solar day on the Moon

The motions of the Planets

Each planet follows the same diurnal motion as the Sun, Moon and stars, rising in the east and setting in the west each day.

Like the Sun and Moon, each planet moves eastward with respect to the stars. This is called direct motion. In addition, all the planets stay close to the ecliptic.

Path of Mars