The Cycles of the Moon

In the preceding program, 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. Guidepost

The Cycles of the Moon  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 programs will show that much of astronomy hinges on the behavior of light.

 In the next program, 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.

sidereal period synodic period spring tides neap tides umbra penumbra total eclipse (lunar or solar) partial eclipse (lunar or solar) penumbral eclipse small-angle formula path of totality photosphere corona chromosphere prominence diamond ring effect annular eclipse perigee apogee node eclipse season line of nodes eclipse year saros 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? Discussion Questions

I. The Changeable Moon A. The Motion of the Moon B. The Cycle of Phases II. The Tides A. The Cause of the Tides B. Tidal Effects III. Lunar Eclipses A. Earth's Shadow B. Total Lunar Eclipses C. Partial and Penumbral Lunar Eclipses Outline

IV. Solar Eclipses A. The Angular Diameter of the Sun and Moon B. The Moon's Shadow C. Total Solar Eclipses V. Predicting Eclipses A. Conditions for an Eclipse B. The View From Space C. The Saros Cycle Outline (continued)

The Phases of the Moon (1) The Moon orbits Earth in a sidereal period of days 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 days (~ 1 month). The Moon’s synodic period (to reach the same position relative to the sun) is days (~ 1 month). Fixed direction in space Earth Moon Earth orbits around Sun => Direction toward Sun changes! 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.

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 Tides Caused 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 side Forces are balanced at the center of the Earth  12-hour cycle Excess centrifugal force pushes water away from the moon on the far side

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, 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 Eclipses Earth’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: 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 ~ ) 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: Approximately 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 ~ 5 0. 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

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. New b. Waxing Crescent c. Waxing Gibbous d. Waning Gibbous e. Waning Crescent

Quiz Questions 7. During Third Quarter phase, the Moon will rise at about a. Sunrise b. Noon c. Sunset d. Midnight e. 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. New b. First Quarter c. Full d. Third Quarter e. 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. New b. First Quarter c. Full d. Third Quarter e. 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.