Why is the first quarter moon phase called that?

Answer 1

At "half Moon" we see half of the Moon's hemisphere that is facing Earth.

But "half Moon" isn't what astronomers call that phase of the Moon.

It is called a "quarter Moon" phase.

That's because the Moon has completed one quarter (at First Quarter phase) or three quarters (at Third Quarter phase) of an orbit around the Earth since new Moon.

(Another possible reason is that we see a quarter of the total surface of the Moon at these phases.)

Answer 2

OK. Buckle your seat belt for a few moments. The phases of the moon have to do with how much of the moon is being illuminated by the sun; contrary to a popular misconception, the shadowed part of the moon during normal phases has nothing to do with sunlight being obstructed by Earth.

If the illuminated part of the moon is in the process of getting bigger, then when we see half of the moon's face illuminated like half a pie, that is called First Quarter. I know, we see 'half', so what's up with First Quarter? This phase tells us that the moon is one quarter of the way through its orbit [synodic orbit] around Earth, using New Moon as a starting point. If the illuminated part of the moon is getting smaller, then when we see half of the moon's face illuminated that is called Third Quarter. The moon is three quarters of the way along its orbit around Earth.

In the north, the shadow moves across the face of the moon from right to left, so it is always easy to tell whether the moon is in first or third quarter. In the south, the shadow moves from left to right.

Another way to think about the 'Quarter' thing: When we view First Quarter, we are seeing one quarter of the moon's surface illuminated. There is another quarter that is illuminated, but we never see that part of the moon from earth.

Answer 3

A half moon and quarter moon shouldn't look the same. When you refer to a half moon, you'll be able to see about 50% of the moon, causing it to look like a larger crescent. If you're seeing a quarter moon, it will look like a smaller crescent that is about 25% of the moon. You usually see a half moon at the first or third quarter point of the Moon's orbit around Earth. So a half moon can be called a first quarter moon or a third quarter moon, and this can lead to the confusion between a half moon looking like a quarter moon.

Answer 4

The reason it is called first quarter is because it is the first stage to take place.