The Moon

The textbook web resources for Chapter 6 are here.

The Moon...

 - is about 1/4 the size of Earth.
 - has about 1/6 of Earth's gravity.
 - has no atmosphere or magnetic field.
 - has no plate tectonics or volcanic activity.

The Lunar Surface...
 - Maria - (Latin for "seas".)  Dark, flat lava plains.  Rock here is made of basalt (iron, magenesium, & titanium silicates) - darker, heavier and older than rock in the highlands.  Maria probably formed from huge impacts, early in the Moon's history, creating deep craters that were flooded with lava.
 - Highlands - Bright areas around maria.  Rock here is mostly anorthosite (calcium & aluminum silicates).  Highlands are pitted with craters, while maria have much fewer craters.
 - Craters - Circular pits caused by the impact of material on the surface.  Larger craters can have high walls where the rock was pushed up by the impact, and even a central peak where some of the material rebounded immediately after the collision.
 - Rays - Long, bright streaks of pulverized rock material that extend out from craters in all directions.  Another result of the impact that formed the crater.
 - Rilles - Narrow canyons on the surface of the Moon.  Some may have been caused by ancient lava flows, others might simply be cracks in the crust.

The Moon's Structure
0. Atmosphere - None.  No volcanic activity to make one, too little gravity to hold onto one.
1. Regolith - Layer of broken-up rock on the surface.  Anywhere from tens to hundreds of meters thick.  (Regolith rock matches the rock type below it.)
2. Crust - Layer of solid silicate rock (like Earth's crust), averaging 100km thick.  Thinner on the side facing the Earth (65km) than on the side facing away from Earth (150km) - possibly from Earth pulling the core slightly towards itself.
3. Mantle - Layer of solid rock, about 1000km thick.  Too cold to undergo convection.
4. Core - Small, with very little iron and nickel.  Is not able to generate a magnetic field.

Moon Movements
The Moon orbits the Earth once every 27.33 days.  It's rotational period is also 27.33 days.  This is called synchronous rotation, and it means that the Moon rotates at the same speed that it revolves around the Earth.  This means that the same side of the Moon always faces the Earth.

The Moon's orbit is tilted about five degrees to the plane of the Earth's orbit around the Sun.  It is also tilted in comparison to Earth's equator.

Lunar Origin
Currently, the most popular theory about the formation of the Moon is the violent birth hyposthesis.  This states that a proto-planet crashed into the Earth about 4.5 billion years ago, and the Moon formed out of the debris that was blasted into space by this impact.

An eclipse happens when the shadow of one astronomical body falls on another.  On Earth, we can observe two types of eclipses:
1. Solar Eclipse
    The Moon passes between the Sun and the Earth, so that it
    appears to block out the Sun.  (The Moon's shadow falls on the
    Earth.)  Totality is when the Moon appears to completely
    cover the Sun.
2. Lunar Eclipse
    The Earth passes between the Sun and the Moon, so that the
    Moon is cast into darkness.  (The Earth's shadow falls on the
We do not get eclipses every month because the Moon's orbit is tilted compared to the Earth's orbit.  We do get eclipses during the eclipse seasons - times during the year (about 6 months apart) when you can have eclipses.  Since the Moon's orbit goes through precession, just like the Earth, eclipse seasons occur about 20 days earlier each year.

The daily changes in height of the ocean, caused mainly by the Moon's gravity, are called the tides.  Tides can also occur in atmospheres and the solid parts of a planet.

The Moon's gravity creates two tidal bulges on the Earth.  The one on the side nearest the Moon is a result of the Moon's gravity pulling the ocean towards itself.  The one on the opposite side of the Earth is more due to inertia, as the Moon swings the Earth around itself.  (In order to understand this, you have to remember that the Earth and the Moon orbit around each other.)

The bulges remain towards and away from the Moon, but the earth rotates underneath the Moon.  So, we (on Earth's surface) rotate into and out of the bulges twice a day - this makes our two high and low tides.

The Sun also has an affect on the tides, but is only about half as strong as the Moon in this regard.  During New Moon and Full Moon phases, the Sun reinforces the Moon and we get exceptionally high high tides and exceptionally low low tides, which are called spring tides (because the ocean is "springing up").  At First and Third Quarters, the Sun and the Moon work against each other, and we get high low tides and low high tides, called neap tides.  (In other words, spring tides are more extreme, neap tides show smaller changes in sea level.)

As there Earth rotates underneath the tidal bulge, there if friction between the crust and the ocean.  The bulge gets pushed slightly ahead of the Moon, and the Earth's rotation get slowed down slightly.  This is called tidal breaking and it increases the length of our day by about 0.002 seconds every 100 years.



Homework from the Text:

  • Read Chapter 6 (pg. 189-212).
  • Review Questions #1-13 on pg. 212.
    • On #8, only the first question.
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