Chapter 3 Cycles of the Moon

Guidepost In the previous chapter, you studied the cycle of day and night and the cycle of the seasons. Now you are ready to study the brightest object in the night sky. The moon moves rapidly against the background of stars, changing its shape and occasionally producing strange events called eclipses. This chapter will help you answer four essential questions about Earth’s satellite: Why does the moon go through phases? What causes a lunar eclipse? What causes a solar eclipse? How can eclipses be predicted?

Guidepost (continued) Understanding the phases of the moon and eclipses will exercise your imagination, and help you answer an important question about how science works: How do we know? How do scientists get from raw data to an understanding of nature? Once you have a 21st-century understanding of your world and its motion, you will be ready to read the next chapter, where you will see how Renaissance astronomers analyzed what they saw in the sky, used their imagination, and came to a revolutionary conclusion – that Earth is a planet.

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 (月相) II. Lunar Eclipses A. Earth's Shadow B. Total Lunar Eclipses C. Partial and Penumbral Lunar Eclipses (月全食 與 月偏食)

Outline (continued) III. Solar Eclipses III. Solar Eclipses A. The Angular Diameter of the Sun and Moon B. The Moon's Shadow C. Features of Solar Eclipses D. Observing an Eclipse 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) The waxing phases of the moon can be seen primarily in the evening sky: New Moon  First Quarter  Full Moon Evening Sky From night to night, the moon appears to move from west to east (right to left) - opposite to the motion within one night!

The Phases of the Moon (5) The waning phases of the moon can be seen primarily in the morning sky: Full Moon  Third Quarter  New Moon Morning Sky Waning gibbous

The Tidally-Locked Orbit of the Moon The Earth 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 (半影), and a zone of full shadow, the Umbra (本影).

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) Motion of the moon against the background of stars Motion of the moon against the horizon

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 ≈ 0.5o = 30’ The sun appears approx. as large in the sky (same angular diameter ≈ 0.50) as the moon.

Solar Eclipses (II)  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 (色球層 與 日冕) Chromosphere and Corona (色球層 與 日冕) Prominences (日珥)

Diamond Ring Effect

Earth and Moon’s Orbits Are Slightly Elliptical 遠地點 Apogee = position furthest away from Earth 近日點 Perihelion = position closest to the sun Earth Moon 近地點 Perigee = position closest to Earth Sun 遠日點Aphelion = position furthest away from the sun (Eccentricities greatly exaggerated!) Eccentricity : 離心率

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

Observing Solar Eclipses Never look directly into the sun, in particular not with binoculars or a telescope without a special filter! A safe way to observe a solar eclipse is to use special eclipse shades or a projection technique as shown here.

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