Question

What occurs 15 days after a full moon day
a) Half moon day
b) Crescent moon day
c) New moon day
d) None of these

Answer 1

Hint: The moon revolves around the earth at uniform speed. It takes around 30 days for the moon to complete its one cycle around the earth. When the sun and the moon are on the same side of the Earth all in a line, it is called the new moon day. Normally we consider this as day 0 of the moons cycle. When the sun and the moon are on the different sides of the Earth all in a line it is called the Full moon day. Hence from this data we can calculate the after fifteen day from the full moon day what phase of the moon one can see.

Complete answer:

In the above diagram we can see some of the phases of the moon. The moon takes around 30 days to orbit the Earth. It also moves with uniform speed along its orbit. The new moon day is the phase of the moon when the moon is not visible to an observer on Earth. This is considered as the initial or the first day of the moon's phase. After fifteen days or so we see the entire moon and hence we call it a full moon day. As we now know that the moon goes in orbit at a uniform speed we can conclude that after fifteen days from the full moon day we will again have the new moon day.

Hence the correct answer of the above question is option c.

Note:
In the above figure we can see that when the moon makes an angle of 90 degrees with the sun we call it as the first quarter of the moon. It takes around six to seven days for the moon to reach this phase. This is called as the quarter phase as the moon completes a quarter of its cycle.