Earth’s Moon

Origin and Properties of the Moon

The Moon’s Motions

Earth’s Moon

The Moon

The moon revolves around the earth with a period of about 29 ½ days ~ one month!

Facts about the Moon…• We see the moon

changes its appearances and position in the sky with approximately 30-day cycle.

• Unlike the stars, Moon can also be seen during the day.

• We also see that the Moon is always involved in eclipses, and

• Its position seems to be well correlated with the tide of the ocean.

• Understanding the motion of the Moon with respect to the Sun and the Earth will explain these phenomena.

Scientists think the moon formed after a large

object, about the size of a planet, hit Earth.

VOCABULARY

Origin and Properties of the Moon

Earth’s Moon

1. Earth is hit off-center by a planet-sized object.

2. The impact heats and deforms both bodies. Some rocky debris remains in orbit around Earth.

3. The debris ring, made of rock from the outer layer of both objects, gradually coalesces, forming the moon.

crater

micrometeoroid

maria

mascons

rille

ray

regolith

Much of the material broken away from the earth goes into orbit.

The moon reassembles in this orbit - it takes about a month of violent collisions.  Earth is speeded up in rotation as a result of the collision.

This theory is consistent with: 1.) composition of lunar rocks, which is similar to the composition of the crust of the earth; 2.) evidence that the moon had a molten surface for 200 million years; and 3.) the lack of magnetic field for the moon, which together with its low density implies it has very little iron in its core.

Impact theory

Earth’s Moon

The same side of the moon always faces Earth. Dark

areas called maria are great basins and level plains

on the moon. They are younger than the lighter lunar

highlands. Lunar highland rocks are older than mare

rocks.

Mare

HighlandMaria: Extensive dark areas on the moon that represent great basins and level plains.

Most lunar craters were caused by the

impact of meteoroids; rays of shattered

rock and dust were splashed out by the

impacts. Regolith is the loose rock material

covering the moon’s surface. It is formed

as micrometeoroids smash into and

erode the moon’s surface.

Earth’s Moon

Crater: A bowl-shaped depression on the surface of a moon or a planet, usually caused by the impact of a meteorite.

Micrometeoroid: Tiny rock fragment no larger than sand grains that travels

through space.

Lunar rocks have textures similar to Earth rocks but differ in composition.

The moon’s orbit is tilted 5 degrees relative to the plane of Earth’s orbit. The moon’s movement around Earth causes it to rise later each day and to go through phases.

VOCABULARY

The Moon’s MotionsEarth’s Moon

Light from the Sun

Waning Crescent

Waxing Crescent

New Moon

Full Moon

Waning Gibbous

Waxing Gibbous

Last Quarter

First Quarter

Phases of the Moon

phases

waxing

waning

gibbous

umbra

penumbra

lunar eclipse

solar eclipse

Lunar Phases

This causes the “phases” of the Moon Waxing Moon – increasing from day to day Waning Moon – decreasing from day to day

• Sunlight illuminates half of the Moon– it’s a ball and the Sun is

shining on it from one direction

Lunar Phases (cont)

• New Moon– The Moon is on the same part of the sky as the Sun and rises and sets

with the Sun

• Full Moon– The Moon is in the opposite side of the sky as the Sun and rises when

the Sun sets and sets when the Sun rises

New Moon Waxing Crescent Moon Waxing Half Moon Waxing Gibbous Moon Full Moon Waning Gibbous Moon Waning Half Moon Waning Crescent Moon New Moon …

The Phase of the Moon

The phase of the Moon depends on the relative position between the Sun, the Earth, and the Moon…

Moonrise/Moonset The time the Moon rises and sets is correlated to

its phase

Phase of the Moon Rise Zenith Set

New Moon 6 am 6 pm

Waxing Half Moon 6 pm

Full Moon 6 pm 6 am

Waning Half Moon 6 am Noon

New Moon 6 am Noon

Noon

Noon Midnight

Midnight

Midnight

6 pm

Why do we Always See the Same Side of the Moon?

The rotation period of the Moon with respect to the universe is exactly the same as the rotation period of the Moon around the Earth.

Is this a coincidence?

No! It’s due to Tidal Locking

Tidal Locking• A tidally locked body takes just as long

to rotate around its own axis as it does to revolve around its partner. This synchronous rotation causes one hemisphere constantly to face the partner body.

The Moons orbital period is 27.3 days. It also rotates once on its axis in 27.3 days (synchronous rotation) resulting in Tidal Locking.

“Dark Side” of the Moon• The Moon doesn’t have a “dark side”

– Everywhere on the Moon, the Sun rises and sets once per month

• It has a side which faces away from us– During a New Moon, the far side is completely

illuminated

A lunar eclipse occurs when Earth passes between the sun and the moon, and the moon is within Earth’s shadow.

A solar eclipse occurs when the moon passes between the sun and Earth, and the moon’s shadow falls on Earth.

The Moon’s Motions

Total Solar EclipseSun Earth

Moon

SunEarth

Penumbra

Moon

Umbra

lunar eclipse

solar eclipse

Eclipses• Eclipses occur when the Sun, the Earth

and the Moon all lie along a straight line– They must line up in all 3 dimensions + time

– the Moon’s orbit is tilted 5o with respect to

the ecliptic, so there are only two times a year when the paths overlap

Solar and Lunar EclipsesEclipse:

The total or partial obscuring of one celestial body by another…

The obscuration can be either

• One celestial body blocking the view to the other:– Solar eclipse---Moon

blocking Earth’s view to the Sun…

• One celestial body is in the shadow of another:– Lunar eclipse---Moon is

in the shadow of the Earth…

Lunar eclipse image from http://www.mreclipse.com

Lunar Eclipse Umbra: The

darkest part of the shadow cast by the moon or by Earth.

Penumbra: The area of partial shadow surrounding the darkest part of the shadow of the Earth or moon.

Lunar eclipses can only occur during a full moon. The maximum time a lunar eclipse can last is 3 hours and 40 minutes.

Solar Eclipse Solar eclipses can only occur during a new moon. The maximum time for a total solar eclipse is 7 minutes and 40 seconds.

Solar eclipses are visible in a narrow path a maximum of 167 miles wide (269km.)

Total Solar Eclipse of 21 June 2001 from Zimbabwe

See Richard Monk’swebpage on eclipses:www.williams.edu/astronomy/IAU_eclipses/

Bailey’s Beads

Solar Corona

“Diamond Ring”

Solar Eclipse ForecastSolar eclipses from 2004 to 2030

Knowing the orbit of the Earth and the Moon, we can now calculate the time and path of solar eclipses with great accuracy.

Eclipses: Summary• The parties involved: Sun, Moon, and Earth• Motion of the Moon around Earth:

1. 29 ½ day revolution of the Moon around the Sun2. Tilt (~5º) of the lunar orbit (around the Earth) w.r.t. the Ecliptic

plane (Earth’s orbital plane around the Sun)3. The precession of the lunar orbit w.r.t. Earth-Sun direction

• Solar eclipse happens when the Moon is between the Earth and the Sun.

• The size and distance of the Moon need to be just right for us to see total eclipse.– The changing distance between the Earth and the Moon

explains the occurrences of the total and ring solar eclipses. – The changing distance between the Earth and the Sun, and the

Earth and the Moon explains the difference in the duration of the solar eclipses.

• Lunar eclipse happens when Earth is between the Moon and the Sun.

Eclipse facts

• Lunar eclipses can only occur during a full moon.

• Solar eclipses can only occur during a new moon.

• A Solar eclipse always occurs two weeks before or after a lunar eclipse.

• Eclipses very often occur in threes, alternating lunar, solar and lunar.

Eclipse facts

• The maximum time a lunar eclipse can last is 3 hours and 40 minutes.

• The longest time the Moon can stay in totality is 1 hour 40 minutes.

• The maximum time for a total solar eclipse is 7 minutes and 40 seconds.

• The maximum time for an annular solar eclipse is 12 minutes 24 seconds.

• Lunar eclipses can occur up to 3 times a year.

Eclipse facts

• Solar eclipses can occur at least 2 and no more than 5 times a year.

• Lunar eclipses are visible over an entire hemisphere.

• Solar eclipses are visible in a narrow path a maximum of 167 miles wide (269km.)

• At any geographic position on the Earth, a total solar eclipse occur an average of once every 360 years.

Eclipse facts

• The cycle of eclipses repeats every 18.6 years called the saros.

• The eclipse shadow moves at 2,000 mph at the Earth's poles and 1,000 mph at the Earth's equator.

Uniform water level

Moon

Moon’s orbit

The twice-daily rise and fall of Earth’s oceans—

known as the tide—is a result of gravitational pulls

from the moon and the sun.

VOCABULARY

TidesThe Moving Ocean

Low tide

Low tide

Direct high tide

Indirect high tide

tides

perigee

apogee

Tides reach different levels depending on Earth’s location in relation to the moon and sun.• High tides are higher and low tides are lower when the moon, sun, and Earth are aligned.• High tides are not as high and low tides are not as low as usual when the moon and sun are not in line with Earth.

VOCABULARY

Tides

The Moving Ocean

The twice-daily rise and fall of Earth’s oceans—

known as the tide—is a result of gravitational

pulls from the moon and the sun.

tides

perigee

apogee

Earth

Moon Moon

Spring Tides

The gravitational affects of the moon and sun combine to influence the flow of the oceans on Earth.

New MoonPhase of the moon?

Full Moon

Phase of the moon?

Moon

New Moon

Phase of the moon?

Higher high tides and lower low tides

Earth

Moon

MoonFirst Quarter

Phase of the moon?

Third Quarter

Phase of the moon?

Neap Tides

The gravitational affects of the moon and sun fight each other with their influence on the flow of the oceans on Earth.

Moon

Third Quarter

Phase of the moon?

Lowest high tides and highest low tides

Tides• Perigee: The point

closest to Earth in the moon’s orbit.

• Apogee: The point farthest to Earth in the moon’s orbit.

Tides & Consequences

• This little tidal behavior goes both ways! The Earth rises tides on the Moon.

• The Earth "brakes" the Moon's rotation AND the braking is complete! Synchronous Rotation of the Moon and its orbital period! (27.3 days!)

• Moon also brakes the Earth's Rotation

• Moon's orbit is growing because of all of this interaction.

• We see a permanent "near" and "far side" of the Moon.

• The length of the "day" is increasing! From fossil reef corals of about 4108 years old - daily growth/annual growth 400 days/year ~ 22 hours a day! Day is increasing about 0.001 seconds/century.

Therefore, the day is getting longer, Moon looks smaller, and tides are weakening.

The Lunar Program and Flagstaff

Two members of the prime crew of the Apollo 17 lunar landing mission participate in lunar surface extravehicular activity simulation training at the Kennedy Space Center, Florida.

“Genesis Rock" returned by the Apollo 15 lunar mission

During the Apollo 12 mission in 1969, commander Pete Conrad retrieves equipment from the Surveyor 3 probe, which landed on the moon three years earlier. This picture, taken by astronaut Alan Bean, shows Apollo 12's lunar module in the far distance.

Apollo 17 astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow during third extravehicular activity (EVA-3).

This photo was taken by Apollo 17 commander Eugene Cernan. It is of lunar module pilot Harrison Schmitt standing in front of a large split boulder on the Moon. Apollo 17 was the last of the Apollo Moon landing missions.