### Theory:

Celestial bodies:
• Galaxy consists of a billions of stars, planets, and clouds of gases and dust.
• Universe consists of millions of such galaxies.
• A branch of science called Astronomy deals with the study of celestial objects such as the Sun, the stars, the planets, the moon and the phenomena related to it.
Astronomers:
• Aryabhata, a well-known astronomer of ancient India, wrote 'Aryabhatiya', based on astronomy.
• Nicolaus Copernicus proposed a heliocentric theory which states that the Sun was at the centre of the solar system, and the other planets revolved around it. This idea was confirmed by Galileo Galilei when he designed the telescope.
Basic terms:
• Orbit is the fixed path taken by the planet to move around the Sun or by a satellite to move around a planet.
• An imaginary line that passes through the centre of an object in which it rotates or spins is called an axis.
Phases of the Moon:
• The different shapes of the bright portion of the moon when Sun's light falls on it are called as the phases of the moon. They are the new moon, the crescent moon, the gibbous moon, and the full moon.
Stars:
• Stars are celestial objects which emit heat and light on their own. Due to Earth's rotation, the stars appear to move from east to west.
• Pole star, also known as Polaris or North star, is visible only from the northern hemisphere.
• A light-year is defined as the total distance travelled by light in one year.
$1\phantom{\rule{0.147em}{0ex}}\mathit{light}-\mathit{year}=9.4607305×{10}^{12}\phantom{\rule{0.147em}{0ex}}\mathit{kilometres}$

Constellation:
• Constellations are a group of stars with a recognisable shape that resemble the familiar objects known to the people to recognise them easily. There are $$88$$ officially known constellations in the universe.
Types of constellations:
• Ursa Major constellation consists seven stars in the shape of a ladle or a question mark. Out of these seven stars, three of them are in the ladle and four stars in the bowl.
• Orion constellation comprises of seven or eight stars, out of which three stars in the middle is like a belt of the hunter. The other four bright stars form a quadrilateral shape.
• Sirius is the brightest star, which is located close to Orion in the sky.
• Cassiopeia is a constellation clearly visible in the northern sky, like a distorted English alphabet $$W$$ or $$M$$. This resembles the shape of a seated queen.
• Leo, meaning 'Lion', is the oldest constellation that consists of nine bright stars. It can be viewed from the Northern hemisphere and the Southern hemisphere.