The Solar systemThe Solar System consists of the Sun, eight planets and their moons, comets and asteroids. They are all called celestial bodies. Everything in the Solar System orbits the Sun.

THE SUN: is the largest celestial body. It is much bigger than any planet. It is at the centre of theSolar System. The Sun is anenormous yellow star whichprovides the light and heatnecessary for life on Earth. The Sun rotates on its owninvisible axis.

PLANETS AND SATELLITES: There are eight large celestial bodies called planets. Each planet rotates on its own invisible axis. Each planet also orbits the Sun. The planets can be classified into two groups: terrestrials and gas giants.

o Mercury, Venus, the Earth and Mars are the terrestrial planets. They are small, and mainly made up of rock.

o Jupiter, Saturn, Uranus and Neptune are called gas giants because they are large and made up of gases.

o Natural satellites are smaller celestial bodies, such as moons, which orbit their planets.

ASTEROIDSIn the Solar System, there are thousands of tiny, rocky celestial bodies called asteroids.They have different shapes.

COMETSComets are small celestial bodies made up of ice, dust and rock. They have bright tails which point away from the Sun. We can only see the tail of a comet when it is close to the Sun.

The earthTHE MOVEMENT OF THE EARTHThe Earth revolves around the Sun. Its orbit is an elongated circle, called an ellipse.The Earth takes exactly 365 days and six hours to complete its orbit. Our normal calendar years are only 365 days long.

THE ROTATION OF THE EARTH: The Earth is a sphere. It rotates on its own invisible axis, which passes through the North and South Poles. It takes twenty-four hours to complete this rotation. At any time, one half of the Earth is facing the Sun. In that half, it is day. The other half is facing away from the Sun. It doesn’t receive sunlight. In that other half, it is night.The Earth always rotate in the same direction. Consequently, the Sun always rises in the east and sets in the west.

The earthTHE SEASONSThe Earth’s axis is slightly tilted. At any time, one hemisphere is tilted towards the Sun, and receives more light and heat. It is summer in this hemisphere. The other hemisphere is tilted away from the Sun. It receives less light and heat. It is winter in that hemisphere.

STARS, CONSTELLATIONS AND GALAXIES

STARS: a star is a spherical body which generates light and heat through nuclear reactions. Most stars consist of the gases hydrogen and helium.

Stars have different characteristics:o Colour: depending on their temperature, stars can be red, orange,

yellow, white or blue.o Size: all stars are enormous, but some are larger than others.o Luminosity: this refers to the amount of light from the star which

reaches the Earth.

CONSTELLATIONSStars are divided into eighty-eight groups or constellations to make identification easier. The constellations have the names of characters from classical mythology or the names of animals and objects because of the shapes they form.Some constellations, such as Ursa Major, can be seen all year. However Orion, for example, can only be seen in winter, and Hercules can only be seen in summer.

STARS, CONSTELLATIONS AND GALAXIES

STARS, CONSTELLATIONS AND GALAXIES

NEBULAE:A nebula is a cloud of dust and gas. Some nebulae are illuminated by stars near them. New stars are formed in some nebulae.

GALAXIES: Galaxies are enormous groups of stars, gases and dust. There are three types: elliptical, spiral and irregular.Our Solar System is on the edge of a galaxy called the Milky Way.All the stars which we can see belongto this galaxy.