# 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.

### Tides

• 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

#### 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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