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The Cycles of the MoonChapter 3

In the preceding chapter, we saw how the sun dominates our sky and determines the seasons. The moon is not as bright as the sun, but the moon passes through dramatic phases and occasionally participates in eclipses. The sun dominates the daytime sky, but the moon rules the night.

As we try to understand the appearance and motions of the moon in the sky, we discover that what we see is a product of light and shadow. To understand the appearance of the universe, we must understand light. Later chapters will show that much of astronomy hinges on the behavior of light.

In the next chapter, we will see how Renaissance astronomers found a new way to describe the appearance of the sky and the motions of the sun, moon, and planets.

Guidepost

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 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 days

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 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.

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 Phases

(SLIDESHOW MODE ONLY)

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 inclined against the ecliptic by ~ 50.

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

Quiz Questions

1. Which statement below best describes the motion of the Moon relative to the stars?

a. The Moon moves eastward relative to the stars about 1 degree per day.b. The Moon moves westward relative to the stars about 1 degree per day.c. The Moon moves eastward relative to the stars about 13 degrees per day.d. The Moon moves westward relative to the stars about 13 degrees per day.e. The Moon does not move relative to the stars.

Quiz Questions

2. How does the Moon's motion relative to the stars compare to the Sun's motion relative to the stars?

a. The Moon moves slower than the Sun and in the same direction.b. The Moon moves faster than the Sun and in the same direction.c. The Moon moves slower than the Sun and in the opposite direction.d. The Moon moves faster than the Sun and in the opposite direction.e. The Moon does not move relative to the stars.

Quiz Questions

3. How does one cycle of the Moon's motion relative to the stars compare to one cycle of the Moon's motion relative to the Sun?

a. The two cycles take the same amount of time.b. The cycle relative to the stars is shorter than the cycle relative to the Sun.c. The cycle relative to the stars is longer than the cycle relative to the Sun.d. The two cycles vary in length, such that at times the star cycle is shorter and at other times the Sun cycle is shorter.e. The Moon does not move relative to the Sun.

Quiz Questions

4. The time for one cycle of lunar phases is

a. about one day.b. about 24.8 hours.c. about one year.d. the same as the time for one cycle of the Moon relative to the stars.e. the same as the time for one cycle of the Moon relative to the Sun.

Quiz Questions

5. In one cycle of lunar phases each crescent and gibbous phase has a duration of about one

a. hour.b. day.c. week.d. month.e. year.

Quiz Questions

6. If the moon reaches Full phase on May 6, it is at _____ phase on May 17.

a. Newb. Waxing Crescentc. Waxing Gibbousd. Waning Gibbouse. Waning Crescent

Quiz Questions

7. During Third Quarter phase, the Moon will rise at about

a. Sunriseb. Noonc. Sunsetd. Midnighte. There is no way to predict the time that a particular phase rises.

Quiz Questions

8. Which statement below is the best simple description of the tidal force?

a. It is an attractive gravitational force due to the presence of very high-mass celestial bodies.b. It is a centripetal (center attracting) force due to rapid rotation.c. It is a centrifugal (center fleeing) force due to rapid rotation.d. It is a differential gravitational force that acts on the different parts of a body.e. It is a centrifugal force due to orbital motion.

Quiz Questions

9. The ocean tides on Earth are primarily due to the influence of what celestial body?

a. The Sun.b. The Moon.c. Venus.d. Mars.e. Jupiter.

Quiz Questions

10. How many tidal bulges on planet Earth are due to the Moon's gravitational influence?

a. Zero.b. One.c. Two.d. Three.e. Four.

Quiz Questions

11. The most extreme tides (Spring Tides) on planet Earth occur during which phase of the Moon?

a. Newb. First Quarterc. Fulld. Third Quartere. Both a and c above.f. Both b and d above.

Quiz Questions

12. What changes occur in the Earth-Moon system due to tidal forces?

a. Earth's rate of rotation is slowing.b. The same side of the Moon now always faces Earth.c. The Earth-Moon distance is increasing.d. Both a and c above.e. All of the above.

Quiz Questions

13. The least extreme tides (Neap Tides) on planet Earth occur during which moon phase?

a. Newb. First Quarterc. Fulld. Third Quartere. Choices a and c above.f. Choices b and d above.

Quiz Questions

14. Which of the following is true about the umbra of the Moon's shadow?

a. It is very dark, or a total shadow.b. It is semi-dark, or a partial shadow.c. Its "cone" points away from the Sun.d. Its "cone" points toward the Sun.e. Choices a and c above.f. Choices b and d above.

Quiz Questions

15. During a total lunar eclipse the Moon is

a. completely dark.b. a reddish color.c. in Earth's umbra.d. in Earth's penumbra.e. Both b and c above.f. Both a and c above.

Quiz Questions

16. Which type of eclipse may be visible to all the people on one side of Earth?

a. An annular eclipse.b. A total solar eclipse.c. A total lunar eclipse.d. None of the above.e. All of the above.

Quiz Questions

17. What part of the Sun is visible during the "peak" of a total solar eclipse?

a. The photosphere.b. The chromosphere.c. The corona.d. Both b and c above.e. All of the above.

Quiz Questions

18. If the Moon is at one of its nodes it must be

a. at New phase.b. at First Quarter phase.c. at Full phase.d. at Third Quarter phase.e. on the Ecliptic.

Quiz Questions

19. A total lunar eclipse occurs when the Moon is

a. at New phase.b. near the ecliptic.c. at Full phase.d. Both a and b above.e. Both b and c above.

Quiz Questions

20. A total solar eclipse occurs when the Moon is

a. at New phase.b. near the ecliptic.c. at Full phase.d. Both a and b above.e. Both b and c above.

Answers

1. c2. b3. b4. e5. c6. e7. d8. d9. b10. c

11. e12. e13. f14. e15. e16. c17. d18. e19. e20. d