Which of the following is NOT a phase of the moon?

• A. New moon
• B. Waning crescent
• C. Blue moon
• D. First quarter

Answer: (C)

The correct answer is the option that refers to a "Blue moon," which is not one of the standard phases of the moon. The phases of the moon are regularly occurring stages characterized by the appearance of the moon as observed from Earth as it orbits around our planet.

The new moon occurs when the moon is between the Earth and the sun, making it nearly invisible because the illuminated side is facing away from us. The first quarter moon is when the moon has completed about a quarter of its orbit and half of it is illuminated from our perspective. The waning crescent describes a phase when the moon is decreasing in illumination after a full moon, appearing as a small sliver of light.

In contrast, a "Blue moon" refers to an event that occurs when there are two full moons in a single calendar month, which is a relatively rare phenomenon. This term is more associated with a timing occurrence rather than a specific phase of the moon. Hence, it does not fit into the standard classification of the moon's phases.