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Chapter 3 Cycles of the Moon

Guidepost 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.

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

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

The Phases of the Moon (1) The Moon orbits Earth in a sidereal period of 27.32 days. 27.32 days Moon Earth Fixed direction in space

The Phases of the Moon (2) Fixed direction in space 29.53 days Earth Moon Earth orbits around Sun => Direction toward Sun changes! The Moon’s synodic period (to reach the same position relative to the sun, i.e., the same lunar phase) is 29.53 days (~ 1 month).

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) Evening Sky New Moon  First Quarter  Full Moon

The Phases of the Moon (5) Morning Sky Full Moon  Third Quarter  New Moon

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.

Lunar Eclipses (1) If a light source is extended (like a large light bulb), any object casts a shadow that consists of a zone of full shadow, the Penumbra, and a zone of full shadow, the Umbra.

Lunar Eclipses (2) Also 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.

Typically, 1 or 2 lunar eclipses per year. Typically, 1 or 2 lunar eclipses per year.

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: 2006-2016 Approximately 1 total solar eclipse per year

Total Solar Eclipse Chromosphere and Corona Prominences

Diamond Ring Effect

Earth and Moon’s Orbits Are Slightly Elliptical Apogee = position furthest away from Earth Earth Perihelion = position closest to the sun Moon Perigee = position closest to Earth Sun Aphelion = position furthest away from the sun (Eccentricities greatly exaggerated!)

Annular Solar Eclipses When Earth is near perihelion, and the moon is near apogee, we see an annular solar eclipse. The angular sizes of the moon and the sun vary, depending on their distance from Earth. Perigee Apogee Perihelion Aphelion

Annular Solar Eclipses (2) Almost total, annular eclipse of May 30, 1984

Conditions for Eclipses (1) 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 (2) Eclipses occur in a cyclic pattern.  Saros cycle: 18 years, 11 days, 8 hours

The Saros Cycle