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The Moon's Motions


Key Terms:

Notes: (25-2)

Just like the Earth revolves around the sun, the moon revolves around the Earth.  As the moon travels around the Earth its travel can be both described by the period of revolution and by the appearance of the moon as it reflects the sun's rays.  

Moon's orbit
Period of revolution - 27.33 days
apparent motion
rises in the east and sets in the west
rises and sets at a different time each day (~50 minutes later)
due to Earth's rotation on its axis
Orbits on a different plane than the Earth
orbit is elliptical

9.4 Tilt of the Moon's orbit

(www.mhhe.com/physsci/astronomy/ fix/student/images/09f04.jpg)

Moon's phases
New Moon - All dark
the moon is between the sun and the Earth
Waxing moon - moon becomes visible
waxing crescent
first quarter
waxing gibbous
Full moon
Waning moon
waning gibbous
last (third) quarter
waning crescent

(www.astro.washington.edu/.../ lecture/pix/moonphase.jpg)

Lunar Eclipses

The lunar eclipse marks the event where the Earth obscures the Sun's rays from reaching the moon.  Every shadow cast by an opaque object has 2 parts, the umbra and the penumbra.   The position of the moon and Earth with regards to these parts determines whether the eclipse will be partial or full.

umbra - complete shadow - all rays of light are obscured
associated with total eclipses
occurs once every year
moon is completely in the umbra - not visible
lasts ~ 2 hours
penumbra - partial shadow - some to most of the rays are obscured
associated with partial eclipses 
dark red or dusky appearance
moon is partially in the umbra and partially in the penumbra
red appearance is due to the bending (diffraction) of the red light around Earth's atmosphere

Lunar Eclipse


(www.opencourse.info/.../04.motion_moon/ lunar_eclipse.gif)



Solar Eclipse

The solar eclipse marks the event where the moon obscures the Sun's rays from reaching the Earth.

Total solar eclipse
Occurs every 30 - 40 years when the moon's umbra covers a portion the earth
due to the moon's offset elliptical orbit
occurs at perigee (closest point from Earth) when the Earth, moon and sun are on the same plane
lasts ~7.5 minutes

Solar eclipse geometry

(www.fest.org.za/images/ eclipse_geometry.jpg)

Annular solar eclipse
occurs once a year when the moon's penumbra covers the Earth
occurs at apogee (furthest point from Earth)

(science.nasa.gov/headlines/images/ eclipse/SEDiagram4c.JPG)


Partial solar eclipse
occurs at perigee
partial umbra and penumbra coverage









Last modified: April 29, 2003