Chapter 3 The Cycles of the Moon Goals: 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 1
Outline 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 (continued) 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 2
The Phases of the Moon (1) The Moon orbits Earth in a sidereal period (period of revolution) of 27.32 days. 27.32 days Moon Earth Fixed direction in space The Phases of the Moon (2) sidereal vs. synodic period Fixed direction in space 29.53 days Moon Earth Earth orbits around Sun => Direction toward Sun changes! The Moon s synodic period (to reach the same position relative to the sun) is 29.53 days (~ 1 month). 3
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) 4
The Phases of the Moon (4) Evening Sky New Moon First Quarter Full Moon The Phases of the Moon (5) Morning Sky Full Moon Third Quarter New Moon 5
The Tides Caused by the difference of the Moon s gravitational attraction on the water on Earth Forces are balanced at the center of the Earth Excess centrifugal force pushes water away from the moon on the far side Excess gravity pulls water towards the moon on the near side 2 tidal maxima 12-hour cycle Spring and Neap Tides Spring tides 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. 6
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. 7
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) 8
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. 9
Small Angle Formula (SLIDESHOW MODE ONLY) Solar Eclipses The sun appears approx. as large in the sky (same angular diameter ~ 0.5 0 ) as the moon. When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse. 10
Solar Eclipses: 2002-2012 Approximately 1 total solar eclipse per year Total Solar Eclipse Chromosphere and Corona Prominences 11
Solar Atmosphere Revealed Diamond Ring Effect 12
Earth and Moon s Orbits Are Slightly Elliptical Perihelion = position closest to the sun Sun Earth Perigee = position closest to Earth Apogee = position furthest away from Earth Moon (Eccentricities greatly exaggerated!) Aphelion = position furthest away from the sun Annular Solar Eclipses When Earth is near perihelion, and the moon is near apogee, we see an annular solar eclipse. Perigee Apogee Perihelion Aphelion The angular sizes of the moon and the sun vary, depending on their distance from Earth. 13
Annular Solar Eclipses (2) Almost total, annular eclipse of May 30, 1984 Conditions for Eclipses (1) The moon s orbit is not quite in the sun s ecliptic. 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. 14
Conditions for Eclipses (2) Eclipses occur in a cyclic pattern. Saros cycle: 18 years, 11 days, 8 hours The Saros Cycle 15
New Terms 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 Discussion Questions 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? 16