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Our Closest Neighbour - the Moon


First Quatrer Moon - photographed afocally with digital camera and 8 inch reflector

The Moon - Quick Overview

The moon is approximately ¼ the diameter of the Earth. It once had an active geology, but we believe it no longer has a molten core like the Earth. It is covered with craters from ancient meteor impacts, some almost 200 km in diameter. The smooth, dark areas were once thought to be seas, so were called "mare" (plural: maria) -- Latin for sea. These are plains of smooth lava from ancient volcanic activity or supermassive meteor impacts. Most craters and maria are over 3.5 billion years old.

The Moon is 384 000 km from earth -- or roughly 30x the earth's diameter.

The moon revolves around the Earth in such a way that it always keeps the same face towards the Earth. Thus, we always see the same side of the moon. For obvious reasons, we call this the near side of the moon, and the other, the far side of the moon. 

Because of the orbit of the moon around the earth, different parts are illuminated on different days. This produces the phases of the moon. The applet at the link below will help illustrate the phases - but not as clearly as the activity performed in class.

Phases of the Moon

One of the most prominent characteristics of the moon is how it shows phases - from night to night the illuminated portion of the moon changes. The phases are produced by the moon orbiting the earth, so the portion that is lit by the sun changes. Starting with a new moon, which is completely in shadow, it is first visible as a crescent within a couple days later. At about one week old the moon shows as a half-illumionated disk, called first quarter (it is one quarter of a lunar cycle, not one quarter of a disk, so that can be confusing). for the next week it i smore than half lit, and is called "gibbous". A full moon occus about two weeks after new moon, and at this time the disk of the moon is fully illuminated. The moon then goes through gibbous again, third quarter, crescent, and finally new again.

The moon can be observed quite well with simple binoculars, but a telescope can really bring out fabulous detail. The worst time to look at the moon through a scope is at full moon - since there are no shadows. My favourite time to observe the moon is in crescent phase, during the evenings for the week after new moon. along the shadow line (called the "terminator"), there is terrific contrast, and the mountains and craters really stand out. 

This little applet shows dynamically how the phases change over the moons orbit: Moon Phase Applet

There is a curious conundrum with the moon's orbital period. The moon completes one full orbit (360°) in 27.3 days. However, the new moons (or full moons, for that matter) are 29.5 days apart. Why might this be? The answer lies in the fact that the Earth is also orbiting the sun. In 27 days the earth has moved about 27° around its orbit, so the moon has to orbit more than a full 360° for us to see a full cycle from earth.

Origins of the moon

The origins of the moon were a mystery for a long time, and there is still some speculation, though the most widely accepted current view is sometimes known as the "big splat". In this model, during the early formation of the solar system when the Earth's crust was just beginning to cool, our planet was struck a glancing blow by another planetary body about the size of Mars. The resulting impact sprayed massive quantities of material from the surface of the Earth and from the impacting body out into space, some of which remained in orbit. Dirung this time, Earth would have had a ring, much like Saturn! This material then began to clump and aggregate, and became the moon.

 During a period known as the Late Heavy Bombardment, the planets were pummeled by rocky asteroids and comets. Much of the cratering on the moon occured during this stage. While the moon was still young and volcanically active, many of these craters were filled with lava, vreating smooth floors. The dark "maria" are believed to be lava flooded regions, and many of these are clearly round, andcient giant impact craters.

The Earth, too, sustained heavy damage from impacts at this time, but geological change and weather erased these impact scars except for the most recent major impacts.



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