Cycles of the MoonCycles of the Moon

Essential QuestionsEssential Questions Why does the Moon go Why does the Moon go

through phases?through phases? What causes a lunar What causes a lunar

eclipse?eclipse? What causes a solar What causes a solar

eclipse?eclipse?

How does the Moon move?How does the Moon move?First, two First, two

definitions:definitions: RotationRotation-spin -spin

about an internal about an internal axis. (ex. Earth’s axis. (ex. Earth’s daily rotation)daily rotation)

RevolutionRevolution-spin -spin about an external about an external axis. (Earth’s axis. (Earth’s yearly revolution)yearly revolution)

The motion of the MoonThe motion of the MoonIf you watch the Moon night after If you watch the Moon night after

night you notice three things:night you notice three things:

1.1. It moves eastward against the It moves eastward against the constellations.constellations.

2.2. The markings on its face don’t The markings on its face don’t change.change.

3.3. Different parts of it are Different parts of it are illuminated each night. (illuminated each night. (PhasesPhases))

Revolution and RotationRevolution and Rotation The first motion, eastward against The first motion, eastward against

the background stars, is explained the background stars, is explained by the Moon’s orbit around the by the Moon’s orbit around the Earth.Earth.

We call that a…We call that a… Revolution! Good job!Revolution! Good job! What about the surface features What about the surface features

not changing? Does the Moon not changing? Does the Moon rotate?rotate?

Modeling the MoonModeling the Moon A volunteer with a textbook to A volunteer with a textbook to

model the Moon please…model the Moon please… A volunteer to play the Earth A volunteer to play the Earth

please…please… ““Moon guy” orbit the Earth Moon guy” orbit the Earth

keeping one side of the book keeping one side of the book facing “Earth guy” all the time.facing “Earth guy” all the time.

Did “Moon guy” rotate? Try it Did “Moon guy” rotate? Try it again and don’t rotate…again and don’t rotate…

Earth’s MoonEarth’s Moon OK, so the Moon OK, so the Moon

revolves around revolves around the Earth the Earth andand rotates on its rotates on its own axis at the own axis at the samesame rate. rate.

What’s wrong What’s wrong with the diagram with the diagram on the left?on the left?

To the Moon…To the Moon… Answer:Answer: The scale! The scale! The Moon is 30 Earth diameters The Moon is 30 Earth diameters

away.away. If the Earth was a basketball, the If the Earth was a basketball, the

Moon would be a baseball, 30 Moon would be a baseball, 30 basketballs away!basketballs away!

The average distance to the Moon The average distance to the Moon is 384,000 km. (Seeds, page 33)is 384,000 km. (Seeds, page 33)

And finally, why And finally, why does the Moon does the Moon have Phases?have Phases?

Those odd phase names:Those odd phase names: NewNew and and FullFull I think you get. I think you get. 11stst and and 33rdrd Quarter Quarter are named are named

because they occur one quarter and because they occur one quarter and three quarters of the way through three quarters of the way through the cycle. (Also we only see a the cycle. (Also we only see a quarter of the Moon’s surface at quarter of the Moon’s surface at those times.)those times.)

Wax—Wax—to gradually increase.to gradually increase. WaneWane—to gradually decrease.—to gradually decrease. GibbousGibbous—Latin for humpbacked.—Latin for humpbacked.

Lunar cycles…Lunar cycles… When and where would you see When and where would you see

each phase of the Moon?each phase of the Moon? How much time for 1 lunar orbit?How much time for 1 lunar orbit? The The sidereal periodsidereal period of the Moon is of the Moon is

about 27 days (27.32 to be exact). about 27 days (27.32 to be exact). Sidereal periodSidereal period is measured relative is measured relative

to background stars and represents to background stars and represents the actual time to orbit Earth once.the actual time to orbit Earth once.

Lunar Cycles cont…Lunar Cycles cont… The The synodic periodsynodic period of the Moon is the of the Moon is the

time to complete a cycle of phases.time to complete a cycle of phases. This takes about 29.5 days—just over This takes about 29.5 days—just over

4 weeks, a “moonth.”4 weeks, a “moonth.” The word The word synodicsynodic comes from the comes from the

Greek words for Greek words for togethertogether and and pathpath Why does the phase cycle take Why does the phase cycle take

longer than one lunar orbit?longer than one lunar orbit?

Sidereal vs. SynodicSidereal vs. Synodic As the Moon orbits As the Moon orbits

the Earth, the the Earth, the Earth orbits the Earth orbits the Sun. Sun.

The Moon must The Moon must then revolve more then revolve more than 360° to reach than 360° to reach the same the same alignment for New alignment for New Moon.Moon.

Extra 29° takes an Extra 29° takes an extra 2 days.extra 2 days.

EclipsesEclipses We will discuss various types of We will discuss various types of

eclipses now.eclipses now. What does the word What does the word eclipseeclipse mean? mean? The word The word eclipse eclipse means…means…

to obscure or hide. to obscure or hide. For example: “Politicians’ failures For example: “Politicians’ failures

often eclipse their often eclipse their accomplishments.”accomplishments.”

Lunar EclipsesLunar Eclipses A lunar eclipse A lunar eclipse

occurs at Full occurs at Full Moon when the Moon when the Moon moves Moon moves through the through the shadow of Earth.shadow of Earth.

Earth’s shadow Earth’s shadow consists of two consists of two parts:parts:

The Umbra and The Umbra and PenumbraPenumbra

The The umbraumbra is the region of total shadow. is the region of total shadow. The The penumbrapenumbra is the region of partial is the region of partial

shadow. shadow. Seeds, page 39Seeds, page 39

Lunar EclipsesLunar Eclipses During a During a total lunar eclipsetotal lunar eclipse, the Moon , the Moon

passes completely into Earth’s passes completely into Earth’s umbralumbral shadow.shadow.

It often appears to have a reddish cast It often appears to have a reddish cast because of sunlight refracting around because of sunlight refracting around the Earth.the Earth.

Lunar Eclipses cont.Lunar Eclipses cont. A A partial lunar eclipsepartial lunar eclipse occurs when occurs when

only part of the Moon enters the only part of the Moon enters the umbral umbral shadow.shadow.

A A penumbral lunar eclipsepenumbral lunar eclipse occurs occurs when the Moon enters only the when the Moon enters only the penumbralpenumbral shadow of the Earth. shadow of the Earth. They aren’t very impressive and are They aren’t very impressive and are often not even noticed by the public.often not even noticed by the public.

We are lucky…We are lucky…

Earthlings can see a phenomenon Earthlings can see a phenomenon that is not visible on most planets. that is not visible on most planets.

Our Moon is large enough and close Our Moon is large enough and close enough to completely eclipse the enough to completely eclipse the Sun.Sun.

The Sun may be 400 times larger The Sun may be 400 times larger than the Moon, but the Moon is about than the Moon, but the Moon is about 400 times closer!400 times closer!

Your turn…Your turn…Your turn…Your turn… Now you need to Now you need to

complete some of complete some of the slides yourself. the slides yourself.

See the notes See the notes section at the section at the bottom of the bottom of the slides for slides for instructions on instructions on what information to what information to find and add to find and add to each slide.each slide.

Now you need to Now you need to complete some of complete some of the slides yourself. the slides yourself.

See the notes See the notes section at the section at the bottom of the bottom of the slides for slides for instructions on instructions on what information to what information to find and add to find and add to each slide.each slide.

Solar EclipsesSolar Eclipses

Total Solar EclipsesTotal Solar Eclipses

A Total Solar EclipseA Total Solar Eclipse

Partial Solar EclipsePartial Solar Eclipse

Annular EclipseAnnular Eclipse

Apogee and PerigeeApogee and Perigee

Eclipse GeometryEclipse Geometry If lunar eclipses If lunar eclipses

happen at Full happen at Full Moon and solar Moon and solar eclipses happen eclipses happen at New Moon…at New Moon…

Why don’t we Why don’t we have eclipses have eclipses every New and every New and Full Moon?Full Moon?

The Moon’s orbit is tipped The Moon’s orbit is tipped about 5° with the ecliptic.about 5° with the ecliptic.

NodesNodes For an eclipse to occur, the Moon For an eclipse to occur, the Moon

must reach new or full phase just must reach new or full phase just as its orbital path intersects the as its orbital path intersects the ecliptic.ecliptic.

Such a point is called a Such a point is called a nodenode.. It is more likely the Moon will be It is more likely the Moon will be

just north or south of this node and just north or south of this node and an eclipse will not occur.an eclipse will not occur.

See Seeds, page 47See Seeds, page 47