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Chapter 3

Cycles of the Moon

Guidepost In the previous chapter you studied daily and yearly cycles of the sun. Now you can focus on the next brightest object in the night sky, the moon. The moon moves steadily against the background of stars, changing its appearance and occasionally producing spectacular events called eclipses. This chapter will help you answer four important questions about Earth ’ s natural 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 an understanding of your world and its motion, you will be ready to read the next chapter, in which 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 Moon 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 Moon Earth 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 Waning gibbous Last quarter Full Waxing gibbous Waning crescent Full

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) If the

entire

surface of the moon enters the Umbra, the lunar eclipse is

total

.

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

.

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: 2012-2019

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 Apogee = position furthest away from Earth Perihelion = position closest to the sun Earth Moon Perigee = position closest to Earth

Sun

(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: 2012-2019 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