Why the Moon Has Phases

Distinguish changing views of the Moon's lit half from eclipses.

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The Sun illuminates roughly half the Moon. As the Moon orbits Earth, we see different amounts of that sunlit half. Ordinary phases are not caused by Earth's shadow.

Worked example

At full Moon, the face toward Earth is largely sunlit. At first quarter, we see about half of the visible disk illuminated, even though about half the whole Moon is sunlit.

1. What provides the light we see reflected from the Moon? Streetlights on Earth The Sun A fire covering the Moon

2. What mainly changes during the monthly phases? The Moon grows new rock each week The Sun turns off monthly Our view of the Moon's sunlit half

3. Are ordinary phases caused by Earth's shadow covering the Moon? No, Earth's shadow is involved in lunar eclipses instead Yes, every crescent is an eclipse Yes, clouds create every phase

4. At first quarter, about how much of the visible disk is lit? One quarter Half None

5. Does a crescent mean most of the Moon has disappeared? Yes, the missing rock regrows Yes, the Moon becomes a flat slice No, most of the visible disk is simply not sunlit from our view

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