During the first and third Quarter Moon phases, how is the Moon illuminated?

During the first and third Quarter Moon phases, how is the Moon illuminated?

• A. One-quarter illuminated
• B. Three-quarters illuminated
• C. Half illuminated
• D. Full illuminated

Answer: (C)

When the Moon is at first or third quarter, the Sun–Moon–Earth geometry makes the visible Moon appear half lit. The Sun’s light falls on exactly half of the Moon that we can see from Earth, with the other half in shadow. This is why both quarter phases show a 50% illuminated Moon. (In the northern hemisphere, the illuminated half appears on the right during the first quarter and on the left during the last quarter.)

When the Moon is at first or third quarter, the Sun–Moon–Earth geometry makes the visible Moon appear half lit. The Sun’s light falls on exactly half of the Moon that we can see from Earth, with the other half in shadow. This is why both quarter phases show a 50% illuminated Moon. (In the northern hemisphere, the illuminated half appears on the right during the first quarter and on the left during the last quarter.)