Chapter 7: Earth's Moon

The Moon's Characteristics

Moon’s Basic Properties

  • We have already learned the methods to find some of the Moon’s basic properties:
  • The Moon’s distance from Earth is found using the method of parallax (Ch 2) = 384,400 km.
  • The Moon’s diameter is found from its distance and angular diameter.
  • Because the Moon’s mass is only 0.012 MEarth, its gravity is weak and its escape velocity is low (Ch 3).
  • Therefore it cannot hold an atmosphere.
  • Therefore it has no erosion from wind or rain.

Moon’s Surface

  • By eye we can see that the Moon’s has light and dark regions, but not much detail.
  • Galileo’s telescope gave better angular resolution (Ch 5), showing more detail.
  • Hubble Space Telescope resolves angles as small as 0.000014o (= 0.05”) showing features as small as 90 m = 0.09 km.
  • We see that the Moon’s surface is covered with craters.
  • There are also large, smooth, dark regions that are called “maria”, which means “seas” in Latin, but they are NOT oceans.
  • The craters are caused by small objects crashing into (“impacting”) the Moon - this still happens occasionally now.
  • The maria are also caused by impacts, but they were much larger bodies.
  • Impact blasted out a huge crater = basin.
  • The energy of impact melted the rock below the basin.
  • The energy of the impact also cracked the rock deep into the Moon’s interior.
  • Lava flowed into and filled the basin, and then cooled, creating the surface we see today.
  • Because the maria are smoother than the heavily cratered regions around them, they must have formed more recently.
  • Moon rocks collected by the astronauts have been dated (Ch 6), showing that the maria formed about 1.4 billion years ago.
  • The far side of the Moon has mostly craters and very few Maria.
  • Earth must have also suffered just as many large and small impacts as the Moon, but our erosion has erased almost all of that evidence.
  • Therefore, studying the Moon tells us about Earth’s history.

Moon’s Orbit and Motions

  • Observing the Moon’s angular size we see that it varies from 0.492o to 0.553o.
  • Because angular size depends on distance (CH 2), we find the Moon’s distance from Earth varies from 360,000 km to 405,000 km.
  • It’s orbit is elliptical, not circular.
  • Using lasers and special mirrors the astronauts left on the Moon, we can measure the Moon’s distance to about 1 cm.

Moon’s Rotation

  • Observing from Earth we see only one side of the Moon.
  • Many people think this means that the Moon does not rotate - WRONG - we would see both sides if this were true.
  • Instead, the observation shows that he Moon rotates once each orbit - synchronous rotation because the Moon’s rotation is synchronized with its orbit.


  • On Earth’s east coasts the water rises and falls twice every day - tides.
  • Tides are caused by the pull of the Moon’s (and the Sun’s) gravity, but mainly the Moon.
  • Before (Ch 3) we used gravity between the centres of two objects.
  • Now we consider gravity’s pull on different parts of a body - differential gravitational force.
  • The differential force stretches Earth’s body.
  • Earth’s side toward the Moon stretches out slightly, which causes the oceans on that side to flow “down hill” and create a bulge.
  • Earth’s centre is stretched away from the far side, which causes the oceans there to flow down into a bulge on the other side of Earth.
  • Earth’s rotation carries us through two high water bulges.
  • The Sun’s gravity also stretches Earth, creating tides.
  • As the Moon orbits Earth, it can align with the Sun to create larger bulges - spring tides.
  • In a different location the Moon’s gravity can partially cancel the Sun’s, creating smaller bulges - neap tides.

Tidal Breaking of Earth

  • Because Earth rotates, its tidal stretch does not align with the Moon.
  • The Moon’s gravity pulls back on Earth’s bulge, slightly slowing - braking Earth’s rotation.
  • We measure this is making our day longer by 0.002 second every century.
  • Earth’s day may have been only 5 hours when Earth formed 4.5 billion years ago.

Tidal Acceleration of the Moon

  • Because Earth rotates, its tidal stretch does not align with the Moon.
  • Earth’s tidal bulge pulls the Moon forward in its orbit.
  • This speeds up the Moon.
  • The faster Moon moves away from Earth at a rate of about 4 cm/y, which we have confirmed using the laser measurement of the Moon’s distance.

Tides and Eclipses

  • In the distant past, when the Moon was closer to us, it looked bigger and total solar eclipses (Ch 1) lasted longer.
  • In the distant future, when the Moon is farther from us, there will be no total solar eclipses because the Moon will be too small to completely cover the Sun.

Tidal Breaking of the Moon

  • Earth stretches the Moon’s body, just as the Moon stretches Earth.
  • Therefore, Earth has slowed the Moon’s rotation just as the Moon has slowed Earth’s.
  • Because Earth’s gravity is stronger and the Moon’s mass is smaller, we have slowed the Moon’s rotation until it equals its orbit rate = synchronous rotation - not an accident, and it happens elsewhere.

Earth’s Stable Climate

  • The Moon’s gravity may also be important for maintaining Earth’s season.
  • Recall (CH 1) that Earth’s seasons are dues to the tilt of Earth’s spin axis.
  • The Moon’s gravity helps to maintain a constant tilt, causing Earth’s seasons to remain constant, which is good for the development of life on Earth.

Structure of the Moon’s Interior

  • Recall (Ch 6) that Earth’s interior consists of: crust, mantle, outer molten iron core, inner solid iron core.
  • The Moon’s interior has also been studied using its average density and moonquakes recorded by instruments left by astronauts.
  • Moon’s average density = 3.3 gm/cm3
  • Mostly rock with only a little iron.
  • Impacts cause moonquakes that show:
  • Moon’s crust is thicker than Earth’s but:
  • Deeper on the Moon’s far side - 150 km.
  • Thinner on the near side - 65 km, maybe why the maria are mostly on the near side.
  • Thick, solid, rigid mantle \rightarrow no convection.
  • Tiny, solid core \rightarrow no magnetic field.

How Did the Moon Form?

  • Several explanations of the Moon’s formation have been developed and tested.
  • Earth and Moon are “twins”, formed together at the same time.
  • Earth captured the Moon, which formed elsewhere in the solar system.
  • Earth originally spun so fast that it split apart into a smaller Earth and Moon.

All 3 theories - Rejected

Twin Theory

  • If this explanation were correct we would expect the composition of the Earth and Moon to be the same.
  • But we find that the Moon has much less iron than the Earth.
  • Rejected

Capture Theory

  • If this theory were correct we expect the Moon’s composition to be quite different from Earth.
  • If the Moon orbited the Sun, it would have too much orbit energy to be captured by Earth’s gravity.
  • Earth’s gravity is not strong enough to capture something orbiting the sun
  • Rejected

Fission - Splitting Theory

  • If this theory were correct we expect the composition of Earth and Moon to be nearly identical - but they are not.
  • Energy is an indestructible quantity
  • Combining Earth’s spin and the moons spin and orbit would equal the spin of the original body.
  • (we don’t have enough energy to make this particular theory work)
  • Rejected

A star filled sky

Description automatically generated

Giant impact Theory (When astronauts brought back moon rocks, they got this idea)

  • The failure of the other explanations plus analysis of the moon rocks suggested another explanation.
  • After Earth formed it was truck off-centre by another planet about the size of Mars.
  • The core of the collider merged with earth
  • The debris from the collider and from Earth’s surface formed a ring around Earth.

Chapter Summary

  • Moon’s basic properties
  • Moon’s gravity is too weak to hold an atmosphere
  • Surface is covered with craters and maria
  • Rotation is synchronized
  • Tides
  • Orbit stabilitatea seasons
  • Interior structure
  • It was formed through a giant impact

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