IMPACT THEORY OF MOON FORMATION

Moon Orbits the Earth – but appears to rise in the east and set in the

west due to earth’s faster rotation

Moon’s Rotation = Moon Revolution

rotation period = 27.3 days revolution period =

27.3 daysMoon shows the

same “face” to the Earth

Velocity: 360°/27.3 days = 13° per day

Daily Movement of the Moon

• Each day the Moon rises in the east and sets in the west as a result of the Earth's rotation.

• It moves about 13° eastwards against the background of stars as a consequence of its revolution around the Earth.

Moon Phases

Half of the moon is always receiving light from the sun.

The “lit” portion that is visible to earth varies as the moon REVOLVES around earth. THIS CAUSES THE PHASES OF THE MOON.

Visible portion of moon growing = waxingVisible portion of moon shrinking = waning

Lunar Phases

How much of the moon we see depends on where the moon is in its orbit:

Appearance:

Appearance:

Appearance:

What we observe from earth

Cycle of Lunar Phases

Takes 29.53 daysThis is because when moon gets back to its

original position in 27.3 days, the earth has moved 1°/day or about 27°.

The moon moving at l3°/day takes about 2 days to catch up with Earth and align with it and the sun in a new moon phase.

Lunar and Solar Eclipses

The Moon orbits the Earth, and travels with the Earth about the Sun. 

Sometimes the Moon gets between the Earth and the Sun, causing solar eclipses, and sometimes the Moon goes into the Earth's shadow, causing lunar eclipses.

Solar Eclipses – can occur only during the new moon phase

Lunar eclipses – can occur only during the full moon phase

The Cause of Eclipses: Shadows

Lunar Eclipses

Solar Eclipses

If Lunar eclipses happen in the full moon phase and solar eclipses happen in the new moon

phase, why don’t they happen every month???

As the Moon orbits the Earth, its orbit is tilted slightly (about 5 degrees) from earth’s orbital plane (ecliptic)

The moon crosses earth’s plane twice during its orbit.  If this crossing happens at the new moon phase, the Moon will be lined up with the Sun and pass in front of it.

This alignment has to be perfect in order for the Moon to completely cover the Sun for a total solar eclipse (the sky will darken just like nighttime, and the stars will be visible) Total solar eclipses last about 7 minutes and occur in a very narrow range across Earth. Full moon phase = total lunar eclipse if perfect.

Solar Eclipses Occur when moon’s orbit intersects ecliptic

during the new moon phase

Total Solar Eclipses only last about 7 minutes in a very narrow range

across Earth

Daily Cycle of Tides

Driven by the gravitational pull of the moon and Sun

Affected by nearness of the moon more than the size of

the Sun

As earth rotates, the tides move around the Earth

alternating between high and low tides approximately

every 6 hours.

Spring Tides – Neap Tides

Tidal range: Difference in height between high tide and low tide

Spring Tides: new and full moon phase – greatestrange

Neap Tides: 1st and 3rd Quarter phases – smallestrange

Moon motion like all celestial bodies except planets which sometime appear to

undergo Retrograde Motion

Retrograde Motion