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The Moon - Introduction

Its hard to imagine the history of Earth without our Moon. For as long as man walked this Earth, the Moon served as "lesser light for the night" and faithful time-piece. The phases of the Moon were used to help guide the Harvest, or help determine the time of the river floods.

How our Moon came to existence is still under speculation. There are several possible scenarios:

  • Fission Theory - the Earth spinning so fast during early formation that a piece broke off forming the Moon
  • Capture Theory - the Moon formed elsewhere passed close to Earth and was captured
  • Co-Creation Theory - the Earth and Moon formed and evolved together
  • Collisional Ejection Theory - a large piece impacted the Earth and broke off pieces of the Earth and formed the Moon

The current champion of the Collisional Ejection Theory: Solar System formation was a volatile environment and collisions were common place. If the Moon and Earth formed together, the orbit of the Moon would be more circular and liberation (the "wobble" of the Moon) would not be a factor. Also, Moon rock does contain some "non-Earth" type rock while also having some "Earth-like" features.

The Moon - A Quick Summary: (More information can be found on the Moon Fact Sheet)

Average Distance from Earth: 384,400 km
Eccentricity of Orbit: 0.0549
Average Orbital Speed: 3680 km/h
Orbital Period: 29.531 days
Rotational Period: 27.322 days
Inclination of Equator to Orbit 6.68
Diameter: 3,476 km
Mass: 7.349 x 1022 kg
Average Density: 3344 kg/m3
Escape Speed: 2.4 km/s
Albedo: 0.11
Maximum Surface Temperature: 130 C
Minimum Surface Temperature: -180 C
Average Surface Temperature: N/A
Atmospheric Composition None

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Phases and Time:

The Moon has played a vital role in the formation of our Calendar. The word "month" comes from a root word "moon" or "moonth," the time it takes the Moon to go from New Moon to New Moon. A close look at the history of the Calendar can be found at the NASA/Goddard Space Flight Center.

There are so many excellent diagrams for Lunar Phases that I decided to use one instead of making my own (In the meantime, there is a nice Java applet available):

This image (Image Credit) shows the position of the Moon and its associated phase. For a cool application, NASA has a What Will the Moon Look Like Tonight? tool. Additional information can be found at the Earth and Moon Viewer.

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Basic Structure:

The Moon is composed of two major features:

  • Highlands
  • Maria

The Highlands are like our own mountain ranges and are formed from igneous rock called Anorthosite. These rocks date back to about 4 billion years old. The Maria are similar to our oceans, but were not formed by liquid. The smooth appearance of the Maria is the result of mare basalt rock, formed by past lava flows - dating back to about 3.5 billion years.

The formation of the Moon ended pretty early on as indicated by the dating of the rock. The three basic methods of Lunar formation are:

  • Differentiation - heavier elements sink
  • Cratering - impacts by meteoroids
  • Basin Flooding - impacts by large meteoroids result in lava flows

This very nice lunar image (Image Copyright 1999 by Calvin J. Hamilton) shows the basic internal structures of the Moon. While there is no global magnetic field, the Moon is though to have a small Iron core followed by a very thick mantle and a thin crust.

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