The EarthMoon system
1. PHYSICAL PROPERTIES OF THE MOON
•
DISTANCE: 356,000  407,000 km (1.2 lightsec)
•
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 lightsec) gives twice the EarthMoon 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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View Full DocumentKepler'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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 Spring '08
 ZABLUDOFF
 Astronomy

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