Phases of the Moon

The eight phases of the Moon are New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third (Last) Quarter and Waning Crescent. One full cycle takes about 29.5 days.

The Moon doesn't really change shape. The Sun always lights half of it. As the Moon goes around Earth, we see a different part of that lit half, so it looks like a thin crescent, a half Moon, a bulging gibbous Moon or a full circle.

In a solar eclipse the Moon passes between the Sun and Earth and blocks the Sun. It can only happen at New Moon and is seen from a narrow strip of Earth. In a lunar eclipse Earth passes between the Sun and the Moon and its shadow falls on the Moon, often turning it red. It can only happen at Full Moon and is seen from the whole night side of Earth.

Amavasya is the New Moon night, when the Moon's lit side faces away from us and we can't see it. Purnima is the Full Moon night. Diwali falls on an Amavasya and Holi, Guru Purnima and Buddha Purnima fall on a Purnima.

Earth's atmosphere bends some sunlight into its shadow. On the way the air scatters away the blue light, the same reason sunsets look red, so the light that reaches the Moon is red or orange. That's why a total lunar eclipse is called a Blood Moon.

The Moon spins once on its axis in the same time it takes to go once around Earth, about 27.3 days. So the same side always faces us. The far side isn't always dark: it gets sunlight at New Moon, when we can't see the Moon at all.