Question

The phases of the moon are caused by _____________.
A. The rotation of the earth
B. The rotation of the moon
C. The revolution of the earth
D. The revolution of the moon

Answer 1

Hint: The moon is tidally locked into a synchronous orbit of the Earth. The duration of the rotation and revolution of the moon is the same 27 day or 27.3 days.

Complete answer: The moon, as it appears in the night sky, undergoes phases, changing from a full moon to a crescent, half or gibbous moon or even no moon at all - a new moon. These phases are caused by the changing angle at which the illuminated face of the moon can be observed from Earth. Such angles occur because the moon is moving around the Earth, but not rotating on its own axis. Therefore, whenever a full moon cannot be seen, it means that a portion of the far side is illuminated but we cannot see the far side from the Earth. After 27.3 days the moon, having completed one revolution of Earth, returns to its original position and the cycle begins again. The cycle begins with a new moon when the moon lies between the Earth and the sun and therefore the far side is lit up, consequently, no moon can be seen from earth.
Now let us examine the options:
Option A - The phases of the moon do not depend on the rotation of the earth but rather it depends on the revolution of the moon around the earth, but not on its axis.
Option B - The phases of the moon totally depends on the rotation of the moon around the earth. The phases depend on the alignment of the sun and the moon.
Option C - This is a myth that the phases of the moon depend on the revolution of the earth. That’s not the case, it totally depends on the rotation of the moon on its axis and also its alignment with the Sun.
Option D - The moon does revolve around the earth once 27.3 days but the phases of the moon totally depend on the rotation of the moon. It also depends on the alignment of the moon with the Sun.

Thus, the correct answer is option (B).

Note: The moon does not produce its own light, but can be seen because it reflects light. For instance when the moon is in crescent shape; the bright crescent is lit by the sun whilst the darker part is lit by the ‘Earthshine’ or sunlight reflected from the earth.