What is the universe?

On a clear night, look up at the sky. You may see the Moon, thousands of twinkling stars and perhaps a bright planet. All of these, and everything else that exists, from the tiniest speck of dust to the biggest star, make up the universe. The universe is so huge that light, the fastest thing we know, takes billions of years to cross it.

Galaxies and stars

Stars are not scattered evenly. They gather in enormous families called galaxies. Our galaxy is the Milky Way, with billions of stars, and on a very dark night it looks like a faint, milky band across the sky. A star is a giant ball of hot, glowing gases that makes its own light and heat. Stars look tiny only because they are unimaginably far away. People long ago joined bright stars into patterns called constellations, such as Saptarishi (the Great Bear) and Orion.

Our Sun and the solar system

The Sun is our very own star. It is a medium-sized star, but it is so close to us compared with other stars that it looks big and bright. The Sun and everything that travels around it make up the solar system. This includes eight planets, their moons, dwarf planets like Pluto, and countless asteroids and comets.

Our home, the Earth

The Earth does two things at the same time. It spins on an imaginary line called its axis once every 24 hours. This is called rotation, and it gives us day and night: the side facing the Sun has day, while the other side has night. The Earth also travels around the Sun in a path called an orbit, taking about 365 days, or one year. This is called revolution, and together with the Earth's tilt, it gives us seasons.

The Moon

The Moon is the Earth's natural satellite. It has no light of its own; it shines because it reflects sunlight. As the Moon goes round the Earth about once a month, we see different amounts of its sunlit side, so it seems to change shape from a thin crescent to a full Moon and back. These shapes are called the phases of the Moon. The Moon has no air or water, and its surface is covered with craters made by falling space rocks.

Other space travellers

Besides planets and moons, the solar system has smaller wanderers. Rocky asteroids circle the Sun, mostly between Mars and Jupiter. Icy comets grow long glowing tails when they come near the Sun. Tiny space rocks that burn up in our air make bright streaks we call shooting stars.

Exploring space

Scientists study space with telescopes, send rockets and satellites into space, and train astronauts to live there. India's space agency, ISRO, has sent Chandrayaan missions to the Moon, Mangalyaan to Mars and Aditya-L1 to study the Sun. In 1984, Rakesh Sharma became the first Indian to travel to space. Satellites around the Earth help us forecast weather, watch TV, use phones and find our way with maps.

This Our Universe worksheet for Class 4 turns these ideas into a space adventure on WitKnowLearn with an online quiz, online activities and interactive games. Flip the cards, test models in the Earth and Moon Lab, play the games, do the space labs, take the challenges and then build a constellation viewer in the Virtual Lab.