The Earth-Moon-Sun System SNC1D. The motion of the Moon The Moon revolves around the Earth in a nearly circular orbit.

The Earth-Moon-Sun System

SNC1D

The motion of the Moon

The Moon revolves around the Earth in a nearly circular orbit.

The motion of the Moon

The Moon is illuminated by the Sun: one side is always bright and one dark.

Phases occur because the fraction of the bright side visible to us changes.

Gravity at close range:Tides

The Earth and Moon are (relatively) close.

Gravity’s pull is much stronger on one side of each body than the other, distorting the Earth and the Moon.

Tides

The water moves more than the solid Earth does.There are two high tides and two low tides per day.

Solar vs. Lunar Tides

The Sun is more massive but very far away:

Solar tide = ½ Lunar tide Solar and Lunar tides may

work together or against each other.

Tidal Locking

The Moon’s pull on the tidal bulge means that:– the Earth is slowing down and the day is getting

longer – the Moon is moving out

Tidal Locking

The tide on the Moon due to the massive Earth is larger so the Moon slowed down long ago.

The Moon’s near face is now locked pointing at the Earth: Synchronous Orbit.

Earth vs. Moon

The Earth is a geologically active (volcanoes, earthquakes, etc.) with a hydrosphere and an atmosphere.

The Moon is smaller, cooled faster, and couldn’t hold on to its atmosphere.

Where did the Moon come from?

Moon Formation Theory

ImpactSomething large (Mars-sized)

struck the Earth just after Earth formed, carved off some rocky mantle.

The Moon and Earth have had similar impact histories.

But the Moon’s craters aren’t eroded by wind and water.

Eclipses

If the Sun, Moon, and Earth line up, you can have an eclipse:

Eclipses

If the Sun, Moon, and Earth line up, you can have an eclipse:

• Lunar Eclipse: the Earth prevents sunlight from striking the Moon

• Solar Eclipse: the Moon prevents sunlight from striking the Earth

Lunar Eclipse

Solar Eclipse

Only a small region right behind the Moon gets a Total Solar Eclipse.

Other observers might see a Partial Solar Eclipse.

Eclipses

We don’t see eclipses every month because the Moon’s orbit is inclined at 5o to the Ecliptic.