SOLUTIONS TO EXERCISE 16

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SOLUTIONS TO EXERCISE 16

(a) If the Sun is setting when the Moon is 60 degrees from the eastern
horizon, then the Moon is 120 degrees away from the Sun tonight.  Since
the diurnal motion of objects in the sky is about 15 degrees per hour,
it will take the Moon 120/15 = 8 more hours to reach the western
horizon and set.  If we estimate sunset to be about 6:00 PM, then the
Moon will set at 2:00 AM tonight.  Thus it's a waxing gibbous phase,
past first quarter and heading toward full moon.  The lit part
of the Moon will be the side closer to the Sun, so it will be the
western side of the Moon that's bright tonight.

(b) At 9:00 PM, the Moon is 45 degrees above the western horizon.  Since
the diurnal motion of objects in the sky is about 15 degrees per hour,
it will take the Moon 3 more hours to reach the western horizon and set.
So the Moon will set at midnight tonight.  Thus it's a first quarter phase.
The lit part of the Moon will be the side closer to the Sun, so it will be
the side near the western horizon.

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