All natural bodies visible in the sky, outside the Earth's atmosphere, constitute the celestial bodies
, e.g. stars, planets, their moons, comets, asteroids, meteors
, etc. The Moon
is the celestial body closest to us. It is the only natural satellite of the Earth
. It is a non-luminous body and it reflects the sunlight incident on it. Due to its revolution around the Earth, when it is at different positions in its path, the apparent disc of the Moon changes, which gives rise to its phases
When the Moon is positioned between the Sun and the Earth, the illuminated portion of the Moon is away from the Earth, and we are not able to see the Moon. We call this day as the New Moon day. With time, the position of the Moon changes and the illuminated portion of the Moon exposed to the Earth gradually increases. Thus, the size of the apparent disc of the Moon increases gradually from a crescent to a full round when the Earth lies between the Moon and the Sun. We call this day the Full Moon day. The waxing or waning of the disc of the Moon every night at it revolves around the Earth is called phases of the Moon. The duration from one New Moon day to the succeeding New Moon day is called the lunar month.
The Moon is about one-fourth the size of the Earth. The surface of the moon has many craters, which might have been formed by the collision of some heavenly bodies like a meteorites
with the Moon. The Moon has no atmosphere because the gravity of the Moon is too small to hold it. Since there is no atmosphere on the Moon there is no life on it.
The huge distances between the Earth and other celestial bodies are measured in light years. A light year
is the distance covered by light in one year.