During the first quarter and third quarter phases of the Moon, how much of the Moon's disk is illuminated?

During the first quarter and third quarter phases of the Moon, how much of the Moon's disk is illuminated?

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

Answer: (D)

This question tests how the Moon’s phase relates to the fraction of its surface that we can see lit. The illuminated portion depends on the angle between the Sun and the Moon as viewed from Earth. When the Moon is a quarter of the way around its orbit from the Sun, that angle is about 90 degrees, so sunlight falls on exactly half of the Moon that we see. The line between light and shadow (the terminator) splits the disk right down the middle. This happens during the first quarter and again during the third quarter, so half of the Moon’s disk is illuminated.

This question tests how the Moon’s phase relates to the fraction of its surface that we can see lit. The illuminated portion depends on the angle between the Sun and the Moon as viewed from Earth. When the Moon is a quarter of the way around its orbit from the Sun, that angle is about 90 degrees, so sunlight falls on exactly half of the Moon that we see. The line between light and shadow (the terminator) splits the disk right down the middle. This happens during the first quarter and again during the third quarter, so half of the Moon’s disk is illuminated.