Earth And Universe

Earth and the Universe
The Universe
All the bodies in the sky and the space between them form the universe.
It began 15 billion years ago after a large explosion.
Distances are huge in the universe. For example , the light emitted by the star nearest our solar system takes four years to reach our planet.

Celestial Bodies
There are many different bodies in the sky. These include stars,like the Sun, planets, like Earth, and satellites, like the Moon. They are called celestial bodies. There are two types of celestial bodies: luminous celestial bodies and nonluminous celestial bodies.
Stars are luminous celestial bodies. This means they emit their own light. Although they look like small points of light, they are in fact very big. They look small because they are very far awayfrom us.

Planets, satellites and comets are nonluminous celestial bodies. This means they do not emit their own light.
Planets orbit stars. Earth is a planet.
Satellites orbit planets. The Moon is Earth´s only satellite.
Comets are small celestial bodies that orbit the Sun. Comets are visible from Earth periodically.

Constellations
Groups of stars are called constellations. Two commonlyseen constellations are Centaurus and the Big Dipper. They got these names because one looks like a centaur and the other like a dipper, or ladel.


Galaxies.
Celestial bodies are not isolated. They form groups called galaxies. There are many millions of galaxies in the universe.
Earth is in the Milky Way galaxy.
Galaxies have different shapes. For example, some are spiral, others ellipticaland others have an irregular shape.


The Solar System
The sun and all the celestial bodies that orbit form the solar system. The solar system contains eight planets and their satellites, and a large number or comets.
Our solar system is part of the Milky Way galaxy


The Sun.
The Sun is a star. Like all stars, it gives off heat and light. It appears yellow and is made up of gases. TheSun is very big. It is bigger than all the planets in the solar system put together. It is more tha 100 times bigger than Earth.
However, the Sun is not particulary large for a star. It looks bigger than other stars because it is nearer to Earth.
Planets and Satellites.
Eight planets revolve around the Sun . The path these planets take around the Sun is called their orbit. In increasingdistance from the Sun, the planets are Mercury, Venus , Earth, Mars, Jupiter, Saturn, Uranus and Neptune. They are all very different.




The Movements of Earth
Earth is constantly moving: Like all the planets in the solar system, Earth orbits the Sun. It also rotates constantly.
Rotation.
Earth rotates on an imaginary axis. This axis crosses Earth from the north pole to the south pole. Thismovement is called rotation.
Earth takes 24 hours to make one complete rotation .These are the 24 hours in a day.
Rotation produces the sequence of day and night.
During rotation, one half or Earth always faces the Sun. In that half it is day. The other half faces away from the Sun. In that half it is night. As Earth rotates, the half where it is day gradually turns away from the Sun and nightsfalls. The half where it is night gradually turns toward the Sun and day breaks.

Revolution
Earth also orbits the Sun. This movement is called revolution. Earth takes about 365 days to complete one revolution around the Sun. Theses are the 365 days in a year. Because it takes slightly more than 365 days for Earth to orbit the Sun, we have to add the additional time every four years.Therefore, every fourth year has an extra day. These are leap years.
Earth tilts during its revolution. That is why the Sun´s rays do not reach all the places on Earth at the same angle. The Sun´s rays fall more vertically in tropical regions and so these regions are hotter. The Sun´s rays fall more diagonally at the poles, which is why the poles are colder.

The Seasons.
Earth´s movement around...