why the moon looks like a half circle in a half moon day?

Answer 1

In astronomy, a phase of the Moon is any of the aspects or appearances presented by the Moon as seen from Earth, determined by the portion of the Moon that is visibly illuminated by the Sun. The lunar phases vary cyclically as the Moon orbits the Earth, according to the relative positions of the Earth, the Moon, and the Sun. Since the Moon appears bright only due to the Sun's reflected light, only the half of the Moon facing the Sun is illuminated.
See the moon phases in
http://en.wikipedia.org/wiki/Lunar_phase

Answer 2

You are NOT seeing the shadow of the earth on the moon. That would be a lunar eclipse. The phases of the moon are caused when the moon is between the earth and the sun, causing the other side of the moon to cast a shadow.
This also causes higher than usual high tides (called spring tides, not related to seasonal spring). The increased gravitational pull of the sun and the moon on the same side of the earth pulls water farther than usual. Two or three days a month have higher than usual tides and two or three days have lower than usual tides (called neap tides).
These tides coincide with new and full moons because of the relative positions of the moon, opposite and same side of the earth with respect to the sun.

Answer 3

This is because the earth is directly between the sun and the moon. What you are seeing is the shadow of the earth on the moons surface. If you look carefully at the moon on the half-moon nights you will also see the shadowed part of the moon as well.

However, on half moon days the sun is at right angles to the earth and we are looking at only half of the moons lit surface.

Answer 4

We see only a part of the Moon that the Sun is illuminating!
Depending on the position of Moon - Earth -Sun, we see more or less of the Moon!
When we see only half of the Moon, it mean that the angle between Earth - Moon Sun is 90 degrees!

Answer 5

Only the side illuminated by the sun is bright. It's all in the relative position of the moon and earth to the sun.

Answer 6

When you are looking at the moon in a half-phase, you are not seeing the shadow of the earth on the moon, you are seeing the side of the moon that is Away from the sun's illumination. . .

O (sun)

.................O (earth)

o (moon)

. . .sees part of the moon illuminated by the sun.

Answer 7

Because the sun is only hitting it half way!

Answer 8

the earth's shadow is falling on it...

Sun---->Earth---->You---->Moon.

got it? :)