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Motion of Earth and Moon

around the Sun

Physics 113 Goderya

Chapter(s): 2 and 3

Learning Outcome:

The Sun and Its Motions

The Sun’s apparent path on the sky is called the

Ecliptic.

Equivalent: The Ecliptic is the projection of Earth’s orbit

onto the celestial sphere.

Due to Earth’s revolution around the sun, the sun

appears to move through the zodiacal

constellations.

The Rotation of Earth

Earth’s rotation is causing the day/night cycle.

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Precession (1)

The Sun’s gravity is doing the same to Earth.

The resulting “wobbling” of Earth’s axis of rotation around the

vertical w.r.t. the Ecliptic takes about 26,000 years and is

called precession.

At left, gravity is pulling on a slanted top. => Wobbling around the vertical.

Precession (2)

As a result of precession, the celestial north pole follows a circular pattern on the sky,

once every 26,000 years.

It will be closest to Polaris ~ A.D. 2100.

There is nothing

peculiar about Polaris

at all (neither

particularly bright nor

nearby etc.)

~ 12,000 years from

now, it will be close to

Vega in the

constellation Lyra.

The Cause for Seasons

Earth’s axis of rotation is inclined vs. the normal to its

orbital plane by 23.5°, which causes the seasons.

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Seasons

(SLIDESHOW MODE ONLY)

Temperature & Sun Rays

Sun Rays & Incidence Angle

They are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern-

hemisphere) winter than in summer.

Light from the sun

Steep incidence

→ Summer

Shallow incidence

→ Winter

The Seasons are only caused by a varying angle of

incidence of the sun’s rays.

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Daylight Hours

Northern summer = southern winter

Northern winter = southern summer

The Phases of the Moon (1)• The Moon orbits Earth in a sidereal period of

27.32 days.

27.32 days

EarthMoon

Fixed direction in space

The Phases of the Moon (2)

• The Moon’s synodic

period (to reach the

same position relative

to the sun) is 29.53

days (~ 1 month).

Fixed direction in

space

Earth

Moon

Earth orbits around Sun =>

Direction toward Sun changes!

29.53 days

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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.

Lunar EclipsesEarth’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

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Why the Red Glow? 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: 2002-2012

Typically, 1 or 2 lunar

eclipses per year.

Solar Eclipses

The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon.

→ When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.

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Solar Eclipses: 2002-2012

Approximately 1 total solar eclipse per year

Total Solar Eclipse

Prominences

Chromosphere and Corona

Diamond Ring Effect

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Conditions for Eclipses

A solar eclipse can only occur if the moon passes a

node near new moon.

The moon’s orbit is inclined against the ecliptic by ~ 50.

A lunar eclipse can only occur if the moon passes a

node near full moon.

Conditions for Eclipses

Eclipses occur in a cyclic pattern.