Miscellaneous announcements…

• Pick up graded homework

• Congratulations to our first winner of the Monty Python Galaxy Song Challenge!

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)

SunMoon Earth

Lunar Eclipse(Moon passes through earth’s shadow)

SunMoonEarth

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.