The Earth and Beyond
12.1 The Solar System
F	The Earth spins on its own axis once every day (24 hours). The half of the Earth which faces the Sun is in daylight; the other half of the Earth is in night.
F	The Earth moves round (orbits) the Sun once each year (just over 365 days).
F	Candidates should be able, when provided with appropriate information, to:
F	* describe the "Earth-centred" model of the Solar System which this model replaced;
F	* explain how observations of the motion of planets provided evidence for the new model.
F	The stars in the night sky stay in fixed patterns (called constellations). The planets which are visible to the naked eye look just like stars.
F	They move very slowly across the constellations.
F	The planets do not give out their own light. Like the Earth, they move in orbits around the Sun. We can see planets because they reflect light from the Sun.
F	Where we see the planets against the background of the stars depends on exactly where they, and the Earth, are in their orbits round the Sun.
F/H	Satellites can be put into orbit around the Earth. They can be used:* to send information between places which are a long way apart on the Earth;
F/H	* to monitor conditions on Earth, including the weather;
F/H	* to observe the Universe without the Earth's atmosphere getting in the way.
F/H	The orbits of the planets are slightly squashed circles (ellipses) with the Sun quite close to the centre.
F/H	The Earth, the Sun, the Moon and all other bodies attract each other with a force called gravity. The greater the distance between the bodies, the smaller the force of gravity between them.
F/H	A smaller body will stay in orbit around a larger one because of the combination of its high speed and the force of gravity between the bodies.
H	Comets have  orbits which are far  from circular. They are  very much closer to the  Sun at some times than at others. This is when they can be seen.
H	To stay in orbit at a particular distance, smaller bodies, including planets and satellites, must move at a particular speed around larger bodies.
H	The further away an orbiting body is the longer it takes to make a complete orbit.
H	Communications satellites are usually put into an orbit high above the equator so that they move around the Earth at exactly the same rate as the Earth spins. This means that they are always in the same position when viewed from Earth (a geostationary orbit.)
H	Monitoring satellites are usually put into a low polar orbit so that the Earth spins beneath them and they can scan the whole Earth each day.
12.2 The Universe
F/H	Our Sun is just one of many millions of stars in a group of stars called a galaxy. The stars in a galaxy are often millions of times further away from each other than the planets in the solar system.
F/H	The universe as a whole is made up of at least a billion galaxies.
F/H	Galaxies are often millions of times further apart than the stars within a galaxy.
F/H	Stars, including the Sun, form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets.
H	Individual stars, including the Sun, do not stay the same for ever.
H	Stars are very massive so that the force of gravity which tends to draw together the matter from which they are made is very strong. The very high temperatures create forces which tend to make them expand.
H	During the main stable period of a star, which may last for billions of years, these forces are balanced. The Sun is at this stage of its life.
H	The star then expands to become a red giant. At a later point in its history it contracts under its own gravity to become a white dwarf.
H	The matter from which the star is made may then be millions of times denser than any matter on Earth.
H	If a red giant is massive enough, it may eventually rapidly contract and then explode (become a supernova) throwing dust and gas into space. A very dense neutron star often remains.
H	During a star's lifetime, nuclei of lighter elements (mainly hydrogen and helium) gradually fuse to produce nuclei of heavier element. These nuclear fusion reactions release the energy which is radiated by stars.
H	Nuclei of the heaviest elements are present in the Sun and atoms of these elements are present in the inner planets of the solar system. This suggests that the solar system was formed from the material produced when earlier stars exploded.
H	Theories of the origin of the Universe have to take into account:
H	* that light from other galaxies is shifted to the red end of the spectrum;
H	* that the further away galaxies are, the bigger this 'red-shift'.
H	The current way of explaining this is:
H	* that other galaxies are moving away from us very quickly;
H	* that the further away from us a galaxy is, the faster it is moving away from us.
H	This suggests that the whole Universe is expanding and that it might have started, billions of years ago, from one place with a huge explosion ('big bang').