Looking towards the sky on a clear night it is easy to imagine patterns in the stars. 88 patterns or constellations are recognized by astronomers today, the oldest of which are the twelve constellations of the zodiac, first named by Babylonian astronomers about 5000 years ago. Stars exist in three dimensional space, however, and what appears to us as a pattern against a ‘flat’ or dome-like sky is not actually the case. In the constellation Orion, for example, one of the easiest constellations to identify in winter skies over the UK, the stars observed, including Betelgeuse and Rigel, all lie between 70 and 2,300 light years away. Orion observed from the Earth is almost unique to observers on the Earth. Stars are, like galaxies, also in constant motion and the constellations visible from the Earth today will change over time.


Surveying the Solar System

The Solar System is the name astronomers give to our nearest star, the Sun, together with its family of planets, moons, asteroids, comets and meteoroids. The Sun is by far the most important feature of the Solar System, containing about 99.8% of its entire mass. It is the Suns strong gravitational attraction that ‘holds’ the Solar System together and keeps all the planets in elliptical orbits around it. Before a Sun-centred (or heliocentric) Solar System was finally accepted, centuries and more ago people believed that everything they could see was Earth-centred (or geocentric). In addition to the Earth, five other planets, Mercury, Venus, Mars, Jupiter and Saturn, have been known since ancient times. Uranus, Neptune and Pluto were only discovered much more recently. Astronomers generally believe that the Sun and the other objects in the Solar System formed at pretty much the same time, about 4.6 billion years ago.

The solar nebula from which the Solar System evolved contained the gases hydrogen and helium, particles of ice (water, carbon dioxide, methane, ammonia) and dust (silicate minerals, metallic elements such as iron), debris left over from earlier generations of stars that died in our neighbourhood. Perhaps triggered by a shock wave from a nearby supernova explosion, the gas, ice and dust rich solar nebula began to collapse under the influence of gravity. As it collapsed it also began to rotate and flatten out like a disk around a large central mass. Within the central mass, the temperature rose until it reached about 10 million degrees at which point hydrogen started to burn and the Sun was born. Within the hotter, innermost parts of the surrounding disk the four small, terrestrial (or rocky) planets formed (Mercury, Venus, Earth, Mars) and within the cooler, outermost parts of the surrounding disk the four giant, Jovian (or gassy) planets formed (Jupiter, Saturn, Uranus, Neptune). The origin and nature of ‘normally’ the most distant planet from the Sun (Pluto) is still hotly debated. The entire formation of the Solar System is thought to have taken about 100 million years.


The Universe
The Milky Way Galaxy
The Sun
Planets and their moons
Asteroids, comets and meterorites, Exploring the Earth-Sun-Moon System
Day and night
Seasonal change
The Moon's phases and eclipses
Further Reading
Self assessment(1)
Self assessment(2)
Self assessment(3)