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Chapter 2

Chapter 2 Continued

Astronomy & Astrophysics
Course Code
Clifford Orwin

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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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