The Solar System consists of the Sun and everything that orbits it, including 8 planets, moons, asteroids, and comets. The Sun is at the center and provides heat and light. The planets can be terrestrial like Earth or gas giants like Jupiter. Asteroids are small rocky bodies and comets are made of ice and dust with tails. The Earth revolves around the Sun while rotating on its axis, causing day and night and the seasons as its axis is tilted. There are also stars, nebulae, galaxies and constellations of stars in space beyond the Solar System.

2. The Solar system
The 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 the
Solar System. The Sun is an
enormous yellow star which
provides the light and heat
necessary for life on Earth.
The Sun rotates on its own
invisible axis.

3. 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.

4. ASTEROIDS
In the Solar System, there are
thousands of tiny, rocky
celestial bodies called asteroids.
They have different shapes.
COMETS
Comets 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.

5. The earth
THE MOVEMENT OF THE EARTH
The 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.

6. The earth
THE SEASONS
The 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.

7. 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.

8. CONSTELLATIONS
Stars 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

9. 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 belong
to this galaxy.