Download The Moon and Eclipses

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The Moon and Eclipses
8 September 2006
Today:
• Motions and phases of the moon
• Eclipses
• Measuring the moon’s size and distance
Motion of the Moon
• On any given day, the moon’s motion is
essentially the same as that of a star (rises in
the east, sets in the west).
• The moon’s position is always near the ecliptic
(the sun’s apparent path among the stars).
• The moon’s motion doesn’t keep up with the
stars or the sun: It completes only 348º of a
circle in 24 hours.
• Falling 12º behind the sun each day, the moon
completes a full circle (with respect to the sun)
once every 30 days (actually 29). One month!
Phases of the Moon
Crescent
Half
Gibbous
Full
Gibbous
Crescent
New
Solar Eclipse
(Moon passes in front of sun)
Sun
Moon
Earth
Lunar Eclipse
(Moon passes through earth’s shadow)
Sun
Earth
Moon
Lunar eclipse photos
Size of the moon
Earth’s
Shadow
Moon’s diameter is about 1/3 that of earth’s shadow,
or about 1/4 that of the earth (Aristarchus).
Distance to the moon
(“Big circle problem”)
1/2 degree
1. Imagine a big circle,
passing through the
moon, centered on
you.
2. 720 moons would fit
around the circle.
3. Each moon is 1/4
earth’s diameter, so
180 earths would fit
around circle.
4. Radius (distance) is
circumference divided
by about 6, or 30
earth diameters
(about 240,000 miles).
Earth and moon to scale
Distance to the sun?
Distance to the sun?
Distance to the sun?
Angle?
Distance to the sun?
Angle?
The angle is indistinguishable from 90º.