• Quiz 4a: It's elementary, Mr. Watson.
• Dark Side of the Moon really means "far side" (cf., "near side").

Eclipse

• Lunar Eclipse: Sun --> Earth --> Moon ("Shadow of the Earth")
• umbra and penumbra


• total, partial, and penumbra lunar eclipse




• Solar Eclipse: Sun --> Moon --> Earth ("Shadow of the Moon")
• total, partial, and annular solar eclipse


• Quiz 4b: Once in a blue moon?



• Why eclipses do not happen every month?



• Moon's orbit tipped by about 5 degrees with respect to Earth's orbital plane
• Eclipse "season": Moon orbit crosses Earth orbital plane only twice a year
• Moon's diameter only about 1/4 of the Earth diameter
• Moon's distance only about 1/100 of the distance to the Sun

Retrograde Motion

• Like the Sun and the Moon, the planets generally appear to move slowly eastward through the zodiac (OK for a geocentric model as well).


• Occasionally, some planets appear to move westward relative to the stars! (a challenge for a geocentric model)


• As shown by the diagram on the right side, this can be explained by the projected locations of planets in a heliocentric solar system model.


• See this Java applet demonstration at UIUC as well.

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Stellar Parallax

• Apparent Shift in the position of a nearby star as we look at it from different places in the Earth's orbit.


• Parallax should not exist in a geocentric model.


• The more distant the star, the smaller the parallax -- a method to derive distances to stars!

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Reading assignment for next lecture: Chapter 3 (p.64-p.82)

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