The Earth-Moon system

# The Earth-Moon system - The Earth-Moon system 1 PHYSICAL...

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The Earth-Moon system 1. PHYSICAL PROPERTIES OF THE MOON DISTANCE: 356,000 - 407,000 km (1.2 light-sec) DIAMETER: 3,476 km (0.27 Earth's diameter) MASS: 7.35 x 10 22 kg (0.0123 Earth's mass) DENSITY: 3.34 g/cm 3 (0.61 Earth's density) Q: How do we measure these parameters? Using Parallax (triangulation), Ptolemy observed the Moon from two different spots on the Earth and determined that the distance to the Moon is 27.3 Earth diameters. If the Earth's diameter is 13,000 km, then the distance to the Moon is about 390,000 km. The time delay of radar echos (2.4 light-sec) gives twice the Earth-Moon distance. This image illustrates Ptolemy's method and the radar method Angular size of the Moon is close to 0.5 degrees. Small angle formula yields a value of 3,480 km for Moon's diameter, as shown in the figure below. Kepler's third law applied to satellites orbiting the Moon gives the lunar mass. This law relates the mass of the central object to the period and the distance of the orbiting satellite, as indicated below.

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Kepler's law With the mass and the radius of the Moon, we can compute the density. 2. LUNAR PHASES The changing appearance of the Moon are caused by the relative positions of the Earth, Moon, and Sun. The phases follow the sequence of new Moon, first quarter, full Moon, and last quarter: SYNCHRONOUS ROTATION means that the Moon's rotation period is identical to its orbital period. Hence, we always see the same face of the Moon. This figure shows that the
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## This note was uploaded on 04/02/2008 for the course ASTR 204 taught by Professor Zabludoff during the Spring '08 term at Arizona.

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The Earth-Moon system - The Earth-Moon system 1 PHYSICAL...

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