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 spectacular 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 scientific imagination, and help you enjoy the sight of the moon crossing the sky. Once you have a 21 st -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 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 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. Waxing crescent 1 st quarter Waxing gibbous Full Full Waning gibbous Last quarter Waning crescent
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
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, 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 entire surface of the moon enters the Umbra, the lunar eclipse is total. If the moon passes through Earth s full shadow (Umbra), we see a lunar eclipse.
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.
Lunar Eclipses: 2010-2017 There are typically 1 or 2 lunar eclipses per year.
Solar Eclipses 0.5 o = 30 The sun appears approx. as large in the sky (same angular diameter 0.5 0 ) 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.
Total Solar Eclipse Chromosphere and Corona Prominences
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 The angular sizes of the moon and the sun vary, depending on their distance from Earth. Perigee Apogee Perihelion Aphelion When Earth is near perihelion and the moon is near apogee, we see an annular solar eclipse.
Annular Solar Eclipses (2) Almost total, annular eclipse of May 30, 1984
Diamond Ring Effect
Solar Eclipses: 2010-2019 There are on average about 1 2 total solar eclipses per year
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 ~ 5 0. 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