PES 105        Fall 2001
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General Astronomy I
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Lecture Notes:

Moon

text: , Chapter 6: Sections 6.1 - 6.4, 6.6 - 6.7

Introduction

The moon is not a planet, but it is another object in the solar system which we know alot about. We will examine it is some detail. Then we will step back and see what predictions we can make from studying Earth and Moon about the rest of the solar system.

Here is a good link for information about the moon: http://seds.lpl.arizona.edu/billa/tnp/luna.html

Here is another good general reference on the moon: http://www.planetscapes.com/solar/eng/moon.htm

Here is a picture of the Moon and Earth taken together by the Gallileo spacecraft: http://www.planetscapes.com/solar/cap/earth/earthmo2.htm

General Properties

  • size
    • about 1/4 the diameter of Earth
  • density
    • 3.3 grams / cm3
    • less than earth => less metals
  • reflection of light (albedo)
    • 7 %
    • does not reflect light well => rocky
    • Why does it look so bright ? . . . . . . . It is so close to us.
  • age: around 4.5 billion years old
    • from radioactive dating of rocks

Motions

Moon always keeps the same side toward the Earth

It does rotate, but rotational period = orbital period = 28 days

Moon rotates once each orbit = "synchronous rotation"

Figure 6.11 shows this motion as well. [Link to Figure 6.11]

Not unusual

caused by gravity and tidal forces

CD: See the animation on the 2nd edition CD in the Chapter 6 gallery, p.1 [Link to animation on web - VERY SLOW download]

Interior

No magnetic field

moon does rotate slowly => probably no liquid in core

(moon is smaller - cools more quickly - all solidified now)

very little evidence of seismic activity

  • few "moonquakes"
  • no volcanoes
  • no continental drift

    Figure 6.9 gives some more detail of this picture. [Link to Figure 6.9]

flow of heat from interior => warm interior

Surface

observe

  • mountains
    • higher than on Earth (no erosion, less surface gravity)
  • craters
    • impacts of meteors
  • canyons
    • from old lava flows - not water

We classify the land as

  • maria (seas)
    • dark color
    • from lava flows
    • 15 % of moon (mainly on the near side)
    • age: 3 billion years
  • highlands
    • light colored
    • heavily cratered
    • 85 % of moon
    • age: 4.5 billion years

    Figure 6.8 shows these features for the near and far sides.

surface ages are 3 - 4.5 billion years => moon has not changed much since formation

mainly silicate rocks (like Earth)

no water in the rocks (but maybe some ice in certain craters ?)

Atmosphere

almost none

weak gravity => everything escapes

observe some hydrogen, helium, argon passing through from outgassing of rocks

no weather, no wind, no rain => no erosion

Moon's surface changes slowly

History of Moon

(best model to date)

We need to explain:

  • moon has little iron
  • moon has no water
  • maria are mainly on near side of moon

terms:

planetisimal = very large rock or collection of rocks (small planet)

protoplanet = almost a final planet

 

Figure 6.13 also shows the formation of the moon. [Link to Figure 6.13]

Earth - Moon system: Tidal forces of gravity

results in

  1. tides of Earth
  2. synchronous rotation of moon

Force of gravity depends on distance.

force is stronger on the near side of Earth and Moon

Figure 6.21 provides a more detailed view of this concept. [Link to Figure 6.21]

Tides on Earth

Solid Earth can not stretch much - but water can.

so we get two high tides each day (not equal in size)

The Earth turns "under" the bulges of water.

CD: See the animation on tides iin the 2nd edition Chapter 6 gallery p. 3 [Link to animation on web - VERY SLOW download]

Synchronous rotation of Moon

Moon also bulges slightly toward Earth

rotation rate gradually slowed until synchronous rotation

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