Cycles in the SkyCycles in the Sky

Essential QuestionsEssential Questions What causes the What causes the

seasons?seasons? Why does the Moon go Why does the Moon go

through phases?through phases? What is a lunar eclipse?What is a lunar eclipse? What is a solar eclipse?What is a solar eclipse?

The SeasonsThe Seasons Many people believe we have cool Many people believe we have cool

winters and warm summers because winters and warm summers because the Earth gets closer and farther the Earth gets closer and farther away from the Sun over the year.away from the Sun over the year.

While the Earth’s orbit While the Earth’s orbit isis elliptical, elliptical, and we do get about 1.7% and we do get about 1.7% closer/farther to the Sun than closer/farther to the Sun than average, this is average, this is notnot the cause of the the cause of the seasons.seasons.

Perihelion & AphelionPerihelion & Aphelion In fact, the Earth is at In fact, the Earth is at perihelionperihelion, ,

its closest point to the Sun, its closest point to the Sun, around January 4around January 4thth when we have when we have winter. (~147,500,000 km)winter. (~147,500,000 km)

The Earth is at The Earth is at aphelionaphelion, its , its farthest point from the Sun, farthest point from the Sun, around July 4around July 4thth during our during our summer. (~152,500,000km)summer. (~152,500,000km)

What causes the seasons What causes the seasons then?then?

The tilt of the The tilt of the Earth’s axis.Earth’s axis.

Throughout the Throughout the year, the Earth’s year, the Earth’s North Pole always North Pole always points towards points towards Polaris.Polaris.

Sunlight hits the Sunlight hits the Earth at different Earth at different angles in the four angles in the four seasons.seasons.

The SeasonsThe Seasons In the summer, the Sun is above In the summer, the Sun is above

the horizon longer because it rises the horizon longer because it rises north of east and sets north of north of east and sets north of west. (see diagram)west. (see diagram)

It also shines more directly down It also shines more directly down on the ground. (think of flashlight on the ground. (think of flashlight shining straight down or at an shining straight down or at an angle)angle)

Both effects cause warmer Both effects cause warmer weather.weather.

SolsticesSolstices The Sun is highest in the sky on the The Sun is highest in the sky on the

Summer Solstice,Summer Solstice, June 21 June 21stst or so. On or so. On this day, the Sun rises the farthest N this day, the Sun rises the farthest N of E and there are the most daylight of E and there are the most daylight hours.hours.

The Sun is lowest in the sky on the The Sun is lowest in the sky on the Winter SolsticeWinter Solstice, December 21, December 21stst or so. or so. On this day, the Sun rises the On this day, the Sun rises the farthest S of E and there are the farthest S of E and there are the least daylight hours.least daylight hours.

EquinoxesEquinoxes On March 21On March 21stst or so, the or so, the Vernal Vernal

(Spring) Equinox(Spring) Equinox occurs. The Sun occurs. The Sun rises due East and we have equal rises due East and we have equal hours of daylight and night.hours of daylight and night.

On September 21On September 21stst or so, the or so, the Autumnal (Fall) EquinoxAutumnal (Fall) Equinox occurs. occurs. The Sun again rises due East and The Sun again rises due East and we have equal hours of day and we have equal hours of day and night.night.

Direct Sunlight on Solstices Direct Sunlight on Solstices and Equinoxesand Equinoxes

On the Equinoxes, the Sun’s rays On the Equinoxes, the Sun’s rays shine straight down on the Earth’s shine straight down on the Earth’s Equator.Equator.

On the Summer Solstice, the Sun On the Summer Solstice, the Sun shines straight down on the ‘Tropic of shines straight down on the ‘Tropic of Cancer’ (23.5° North).Cancer’ (23.5° North).

On the Winter Solstice, the Sun On the Winter Solstice, the Sun shines straight down on the ‘Tropic of shines straight down on the ‘Tropic of Capricorn’ (23.5° South).Capricorn’ (23.5° South).

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.

Solar EclipsesSolar Eclipses

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!

Total Solar EclipsesTotal Solar Eclipses During a During a total solar eclipsetotal solar eclipse, the Moon , the Moon

completely covers the visible surface completely covers the visible surface of the Sun called the of the Sun called the photosherephotoshere..

Once the photosphere is covered, the Once the photosphere is covered, the outer atmosphere of the Sun, called outer atmosphere of the Sun, called the the coronacorona, is visible., is visible.

Sometimes the active lower Sometimes the active lower atmosphere, the atmosphere, the chromospherechromosphere, is , is visible as well.visible as well.

Solar Eclipses cont…Solar Eclipses cont… A A partial solar eclipsepartial solar eclipse occurs occurs

when the Moon does not when the Moon does not completely cover the Sun.completely cover the Sun.

Both total and partial solar Both total and partial solar eclipses take place during the eclipses take place during the New Moon phase.New Moon phase.

There is a third type of solar There is a third type of solar eclipse as well…eclipse as well…

Annular EclipseAnnular Eclipse An An annular eclipseannular eclipse is a solar eclipse is a solar eclipse

in which a ring (or annulus) of the in which a ring (or annulus) of the photosphere is visible around the photosphere is visible around the disk of the Moon.disk of the Moon.

This occurs when the Moon is This occurs when the Moon is slightly farther from the Earth than slightly farther from the Earth than average because of its elliptical average because of its elliptical orbit.orbit.

Apogee and PerigeeApogee and Perigee The Moon’s closest approach to the The Moon’s closest approach to the

Earth in its elliptical orbit is called Earth in its elliptical orbit is called perigeeperigee..

ApogeeApogee is the point in its orbit where is the point in its orbit where it is farthest from the Earth. (annular it is farthest from the Earth. (annular eclipses)eclipses)

Apogee and perigee are also used for Apogee and perigee are also used for other satellites orbiting Earth or other satellites orbiting Earth or other planets.other planets.

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