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07_MoonOne - GEL36 SOLAR SYSTEM Lecture 7 The Moon part...

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1 GEL36 SOLAR SYSTEM Lecture 7: “The Moon” – part 1 (Ch. 21) Goal of the Lecture: To discuss the characteristics of the Earth’s moon by focusing on a few very important concepts that explain several features of the moon. These fundamental concepts will be returned to throughout the course when we discuss the rest of the planets and their moons. Size of the moon . . . . Diameter of Earth is ~ 4x that of the moon. Volume of Earth is ~ 50x the moon’s. Mass of Earth is ~ 80x the moon’s, making the gravity on Earth about 6x that on the moon. Moon is not that much smaller than Mercury, and, as you’ll see, not that much different because of their similar small size. Moon is about 30 Earth diameters away - not too far. Motion of the moon . . . Moon has prograde orbit around Earth and prograde rotation direction. And its orbit is tipped ~5° from the ecliptic. The rotation of the moon and its orbit around the Earth are locked so that we always see the same face of the Moon - the “near side” . The hemisphere that we don’t see from Earth is called the “far side” (there is no “dark side of the moon” since the moon rotates and both sides of the moon receive equal amounts of sunlight) The reason that we always see the same face of the moon is a phenomenon called tidal coupling - 1:1 relationship between the moon’s spin and its orbit. For every one orbit around the Earth, the moon spins once on it’s axis. - you know how the moon exerts a gravitational pull on the Earth that is expressed as “the tides”, the pull of the oceans toward the moon. But the Earth also exerts a “tidal pull” on the moon - the rock comprising the moon actually flexes just a bit, and as a result the moon’s rotation has slowed. It used to spin much faster, but the gravitational pull of the much more massive Earth has slowed it down so that it now rotates to keep one side permanently facing the Earth. It is “tidally locked” to the Earth. - most of the moons in the solar system are tidally locked to their planets.
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2 Moon’s atmosphere . . . Moon is the second brightest body in the sky, after the sun, of course. And you realize that the light we see from the moon is reflected light. - the moon's brightness is something of an illusion since the proximity of the reflected light affects its brightness. The moon itself is as dark as asphalt. We can see that the moon has no air from the sharpness of surface features near the sunset line between night and day (called the terminator ) - an atmosphere would scatter light and soften shadows. - without an atmosphere the surface of the moon is also completely silent since there
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07_MoonOne - GEL36 SOLAR SYSTEM Lecture 7 The Moon part...

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