We also have many man-made satellites in orbit around the Earth. They are placed in orbit to transmit radio signals around the planet or to study what is happening on the Earth. Other satellites, such as the Hubble telescope, are designed to make observations of the rest of the solar system and beyond.
*Geostationary orbit: A circular orbit in the equatorial plane, any point on which revolves about the Earth in the same direction and with the same period as the Earth's rotation.
*Polar orbit: An orbit for which the angle of inclination is 90o. Note: A satellite in polar orbit will pass over both the north and south geographic poles once per orbit.
Comets
Comets make very eccentric orbits of the Sun, passing close to the Sun (when they can be seen from Earth) and then travelling far beyond Pluto. When comets are close to the Sun they move faster and when they are further away they move more slowly.
The asteroid belt
Between the four inner planets and the outer planets lies the asteroid belt, made up of dust and rocks.
Planetary time
A day
The time taken for a planet to turn on its axis is a day. A day on Earth takes 24 hours, but Mercury takes 58.6 Earth days to make one rotation!
A year
The time taken for a planet to orbit the Sun is its year. The further a planet is from the Sun, the weaker the pull of gravity on it and the bigger its orbit. The further from the Sun a planet is, the longer its year is. A year on Earth takes 365.25 days, but Mercury orbits the Sun in 88 Earth days - it is closer to the Sun than us.
Summary
- The solar system consists of the Sun, nine planets, the asteroid belt and a number of comets.
- The planets and moons are seen by the reflected light of the Sun.
- The planets are (in order, moving away from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
- All the planets except Pluto have almost circular (elliptical) orbits in the same plane. The orbit of Pluto is much more elliptical and at an angle to the plane of the other planets.
- The time taken for a planet to orbit the Sun is its year. The further a planet is from the Sun, the weaker the pull of gravity on it and the greater its orbit. The further from the Sun a planet is, the longer its year.
- The Moon is a natural satellite of the Earth. Some other planets have natural satellites. The time taken for the Moon to make one orbit of the Earth is one lunar month.
- Artificial satellites are put into orbit around the Earth. Those close to the Earth orbit much more quickly than those in higher orbits.
