when the shape of the moon is spherical, why is it seen as a full circle only once a month?

Answer 1

An excellent question. It is because the half of the moon that is exposed to the Sun and the half of the moon that is visible to us do not always coincide (in fact, they coincide just once a month).

A common misconception is that the moon cycle is cause by the Earth blocking the sun's light, but that is called an eclipse, and is not resposible for the regular lunar cycle.

Answer 2

Starting with the full moon - A day or two before the "full moon" you will see the moon rise just before sunset. At full moon it will rise as the sun sets, and of course after the sun sets as the time goes on. Each night after, notice how the light on the
moon shrinks from full to last quarter and then to nothing (a 2 week time period).

Also during this time you will notice the moon "traveling" closer to the sun.

Two weeks from full moon (day may vary) look for the sliver of the moon just before sunrise.
As the moon moves into NEW moon and you cannot see at all, watch for it to become noticable
as a sliver in the other direction as the sun sets. Follow the light on the moon as it expands to first quarter and then to full. So you can actually see the moon and the sun at the same time virtually most of the month.

When the moon is not full, it is not in the earths shadow, it is causing its own shadow. When it is in the earths shadow, we see a lunar eclipse. Above or below the shadow, a regular full moon.

Answer 3

Actually, it never is seen as quite a full, whole circle. To see it that way you'd have to be directly between it and the Sun, which only happens during lunar eclipses, and at those times Earth's shadow intervenes to block part of the circle. To answer your question, though, different amounts of the Moon's Earthward side* are illuminated by the Sun as the Moon and Earth progress in their orbits. The illuminated part is the most highly visible to us, but often the rest is faintly visible too.

*The Moon always keeps the same side turned toward the Earth.

Answer 4

Because the sun, moon and earth keep changing positions among the 3 of them. As a result, in most positions, the earth's own shadow gets reflected on the moon, which creates the dark space on the surface of the moon (and therefore makes it look like a crescent).

Only in one configuration do we get to see the full moon: when the moon, earth and sun are in an L-shape position.

Answer 5

Because we only see 1/2 of the moon from Earth. The moon is constant in the space--meaning it doens't rotate, so we always see the same side.

Answer 6

Because we ordinarily see only the part illuminated by sunlight. However, just after new moon, when the lit part is a thin crescent, you can see the rest of the moon dimly illuminated by light reflected from the earth.

Answer 7

That is because the sun is partially hidden by the earth except for those times and then you see the sun reflect off the whole side of the moon.
The earth is in the way the rest of the time.

Answer 8

cause we never see the back side of the moon, and the sun doesn't move but the moon revolves around the earth.

Answer 9

Portions of the moon are in the earth's shadow. If you look with a good telescope you can usually see the part in the shadow.

Answer 10

that's the way it's seen from the earth...we only see part of the whole most days