The Cycles of the MoonChapter 3

• To understand the phases of the moon

• To understand how the moon affects tides

• To understand lunar and solar eclipses

• To learn some of the history associated with the moon

Goals:

I. The Changeable MoonA. The Motion of the MoonB. The Cycle of Phases

II. The TidesA. The Cause of the TidesB. Tidal Effects

III. Lunar EclipsesA. Earth's ShadowB. Total Lunar EclipsesC. Partial and Penumbral Lunar Eclipses

Outline

IV. Solar EclipsesA. The Angular Diameter of the Sun and MoonB. The Moon's ShadowC. Total Solar Eclipses

V. Predicting EclipsesA. Conditions for an EclipseB. The View From SpaceC. The Saros Cycle

Outline (continued)

The Phases of the Moon (1)• The Moon orbits Earth in a sidereal period (period

of revolution) of 27.32 days.

27.32 days

EarthMoon

Fixed direction in space

The Phases of the Moon (2)

• 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 dayssidereal vs. synodicperiod

The Phases of the Moon (3)From Earth, we see different portions of the Moon’s surface lit by the sun, causing the phases of the Moon.

The Universe: Moon Phases

3 D animation video

Lunar Phases

(SLIDESHOW MODE ONLY)

The Phases of the Moon (4)

New Moon First Quarter Full Moon

Evening Sky

The Phases of the Moon (5)

Full Moon Third Quarter New Moon

Morning Sky

The TidesCaused by the difference of the Moon’s gravitational attraction on the water on Earth

2 tidal maxima

Excess gravity pulls water towards the

moon on the near sideForces are balanced at the center of the Earth

12-hour cycleExcess centrifugal force pushes water away from the moon on the far side

Spring and Neap TidesThe 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, 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.

Synchronous rotation

Acceleration of the Moon’s Orbital Motion

Earth’s tidal bulges are slightly tilted in the direction of Earth’s rotation.

Gravitational force pulls the moon slightly forward along its orbit.

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

If the moon passes through Earth’s full shadow (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 (1)

A Total Lunar Eclipse (2)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 Earth’s atmosphere.

Lunar Eclipses: 2002-2012

Typically, 1 or 2 lunar

eclipses per year.

Small Angle Formula

(SLIDESHOW MODE ONLY)

Solar Eclipses

The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon.

When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.

Solar Eclipses: 2002-2012Approximately 1 total solar eclipse per year

Total Solar Eclipse

Prominences

Chromosphere and Corona

Solar Atmosphere Revealed

Diamond Ring Effect

Earth 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

Annular Solar Eclipses

The angular sizes of the moon and the sun vary, depending on their distance from Earth.

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

Perigee Apogee Perihelion Aphelion

Annular Solar Eclipses (2)

Almost total, annular eclipse of May 30, 1984

Conditions for Eclipses (1)

A solar eclipse can only occur if the moon passes a node near new moon.

The moon’s orbit is not quite in the sun’s ecliptic.

A lunar eclipse can only occur if the moon passes a node near full moon.

Conditions for Eclipses (2)

Saros cycle: 18 years, 11 days, 8 hours

Eclipses occur in a cyclic pattern.

The Saros Cycle

sidereal periodsynodic periodspring tidesneap tidesumbrapenumbratotal eclipse (lunar or solar)

partial eclipse (lunar or solar)

penumbral eclipsesmall-angle formulapath of totalityphotospherecoronachromosphereprominencediamond ring effect 

annular eclipseperigeeapogeenodeeclipse seasonline of nodeseclipse yearsaros cycle

New Terms

1. If the moon were closer to Earth such that it had an orbital period of 24 hours, what would the tides be like?

2. How would eclipses be different if the moon’s orbit were not tipped with respect to the plane of Earth’s orbit?

3. Are there other planets in our solar system from whose surface we could see a lunar eclipse? a total solar eclipse?

4. Can you detect the Saros cycle in Figure 3-18?

Discussion Questions