Chapter 2 Continued
DepartmentAstronomy & Astrophysics
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Astronomy Section 2.3 and 2.4 Notes
2.3 The Moon, Out Constant Companion
−Takes moon 271/3 days to orbit Earth.
−Takes 271/3 days for each circuit through constellations.
−Lunar phases: The state determined by the portion of the visible face of the Moon that is
illuminated by sunlight.
−During full moon, we see half the moon that is lit by sunlight.
−During new moon we don’t see the moon because the sun isn’t illuminating the part we
see at all.
−Each cycle of phases from one new moon to the next takes apprx. 291/2 days. Longer
than Earth’s orbital period b/c o’ Earth’s motion around sun during time moon is orbiting
−Moon’s phase is related to time it rises, reaches highest point in sky, and sets.
−Phases from new to full are waxing (increasing).
−Phases from full to new are waning (decreasing.)
−First-quarter/Third-quarter indicates the times when moon is one-quarter or three quarters
finished its monthly cycle.
−Phase just before and after new moon are called crescent (light sliver).
−Phase just before and after full moon are called gibbous (dark sliver).
−We always see same face of the moon b/c moon has synchronous rotation (It’s orbital
period and rotation period are equal).
−Moon’s synch. rot. caused by Earth’s gravity affecting the moon like the moon’s gravity
affects tides on Earth.
−A new moon occurs when the Moon is between the Sun and Earth.
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