The Cycles of the Moon

•The phases of the moon

•The tides

•Lunar eclipses

•Solar eclipses

The Phases of the Moon

From Earth, we see different

portions of the Moon’s surface lit by the sun, causing the

phases of the Moon.

The Phases of the Moon

• The Moon orbits Earth in a sidereal period of

27.32 days.

27.32 days

EarthMoon

Fixed direction in space

Is the moon going to show the same lunar phase after one

sidereal period?

1. Yes.

2. No, it will not have completed a full cycle of phases.

3. No, it will have completed more than a full cycle of phases.

The Phases of the Moon

• The moon’s synodic period (to reach the

same position relative to the sun) is 29.53 days (~ 1 month).

Fixed direction in space

Earth

Moon

Earth orbits around Sun => Direction toward Sun changes!

29.53 days

The moon orbits counterclockwise around Earth (viewed from the North). => It appears to move eastward against

the background of the stars. => The waxing crescent is visible

1. in the morning sky.

2. in the evening sky.

3. the whole night, from sunset to sunrise.

4. only around midnight.

5. never.

The Phases of the Moon

New Moon → First Quarter → Full Moon

Evening Sky

The Phases of the MoonFull Moon → Third Quarter → New Moon

Morning Sky

Waning

The Tides

The tides are caused by the difference of

the Moon’s gravitational attraction on the water on Earth

• Between the near side and the center of

the Earth

• Between the center and the far side of the

Earth

→ 2 tidal maxima

→ 12-hour cycle

On the day of full moon, high tides occur …

1. around noon and 6 p.m.2. around noon and midnight.3. around 6 a.m. and 6 p.m. 4. around 6 p.m. and midnight.5. Impossible to tell. The times of tides are not

correlated with the phases of the moon.

Spring and Neap Tides

The Sun is also producing tidal effects, about half as strong as the Moon.

• Near Full and New Moon, those two effects add up to cause spring tides

• Near first and third quarter, the two effects work at a right angle w.r.t. each other, causing neap tides.

Spring tides

Neap tides

The Tidally Locked Orbit of the Moon

The Earth also exerts tidal forces on the

Moon’s rocky interior.

→ It is rotating with the same period around

its axis as it is orbiting Earth (tidally locked).

→ We always see the same side of the

moon facing Earth.

The Near Side of the Moon

A total lunar eclipse …

1. is a high-performance moon vehicle built by Mitsubishi.

2. occurs when the moon disappears behind the sun.

3. occurs when the moon becomes invisible because it is too close to the sun.

4. occurs when the moon moves through Earth’s shadow.

5. occurs when the moon disappears behind Mars.

Lunar EclipsesThe Earth’s shadow consists of a zone of full shadow, the Umbra, and a zone of partial shadow, the Penumbra.

If the Moon passes through the Umbra, we see a lunar eclipse.

If the entire surface of the Moon enters the Umbra, the lunar eclipse is total.

A Total Lunar Eclipse (I)

A Total Lunar Eclipse (II)

A total lunar eclipse can last up to 1 hour

and 40 min.

During a total eclipse, the moon has a faint, red

glow, reflecting sun light scattered in

the Earth’s atmosphere.

Typically, 1 or 2 lunar

eclipses per year.

Solar Eclipses

The Sun has approx. the same angular diameter of ~ 0.50 as the Moon.

Thus, when the Moon passes in front of the Sun, it can

cover it completely, causing a total solar eclipse.

Total Solar Eclipse

Prominences

Chromosphere and Corona

Diamond Ring Effect

If the sun and the moon have the same angular diameter on the sky, does that

mean that the sun and the moon actually have about the same size?

1. Yes.2. No because the sun is much farther away, but also

much larger than the moon.3. No, because the sun is much farther away, but also

much smaller than the moon.4. No because the moon is much farther away, but also

much larger than the sun.

5. No, because the moon is much farther away, but also much smaller than the sun.

Earth’s and Moon’s orbits are slightly elliptical:

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun

Aphelion = position furthest away from the sun

Perigee = position closest to Earth

Apogee = position furthest away from Earth

What do you expect to see if at the time of a solar eclipse the moon is near apogee, and

the Earth is near perihelion?

1. A regular total solar eclipse.2. No solar eclipse at all.3. A partial solar eclipse with a crescent appearance.4. A partial solar eclipse with a ring-like appearance of

un-occulted parts of the sun.5. A lunar eclipse.

Annular Solar EclipsesThe angular sizes of the

Moon and the Sun vary,

depending on their distance from Earth.

When the Earth is near perihelion, and the Moon is near apogee, we see an annular solar eclipse.

Perigee Apogee Perihelion Aphelion

Almost total, annular eclipse of May 30, 1984

If the moon was orbiting around the Earth exactly in the plane of the

ecliptic, …

1. lunar and solar eclipses would occur once every day.

2. lunar and solar eclipses would occur once a week.

3. lunar and solar eclipses would occur once a month.

4. lunar and solar eclipses would occur once a year.

5. lunar and solar eclipses would never occur.

Conditions for Eclipses (I)

The Moon’s orbit is inclined against the ecliptic by ~ 50.

A solar eclipse can only occur if the Moon passes a node near New Moon.

A lunar eclipse can only occur if the Moon passes a node near Full Moon.

Conditions for Eclipses (II)

Eclipses occur in a cyclic pattern.

→ Saros cycle: 18 years, 11 days, 8 hours

Approximately 1 total solar eclipse per year