Every day at dawn the Sun rises over the eastern horizon, and climbs into the morning sky, bringing light to our world. At noon the Sun reaches its highest point in the sky. Then in the afternoon it begins to descend.
In the evening at sunset the Sun disappears over the western horizon. Darkness falls, and the sky will remain dark until dawn next day.
The Sun, then, travels from east to west around the Earth. But this is not what really happens.
In fact the Sun remains in the same spot in space. Its movement across the sky is caused by the Earth spinning round in space.
The Earth spins round on its axis once every 24 hours, travelling towards the east. On the Earth, we don't think we're moving; we think that the Sun is moving instead, in the opposite direction, towards the west. The Earth's period of rotation, the time it takes takes to spin, or rotate, once on its axis, is one of our basic units of time: the day.
At night, if you remain stargazing for any length of time, you can see the stars travelling in arcing paths across the heavens from east to west. This too happens because the Earth is spinning on its axis.
The Earth has another motion in space. It travels in an endless path, or orbit, that carries it round and round the Sun. It takes 365 and a quarter days to travel round the Sun once. This is its period of revolution round the Sun, a period we call a year.
The Earth does not spin upright in space as it travels around the Sun. That is, its axis is not perpendicular to the plane of its orbit. Its axis is tilted at an angle of 23 and a half degrees. The Earth's axis is always tilted in the same direction in space.
This means that, as the Earth travels each year around the Sun, the axis is tilted alternately towards, then away from the Sun. Naturally, when the northern hemisphere is tilted towards the Sun, the southern hemisphere is tilted away; and vice versa.
The tilt of the Earth's axis has a marked effect on the climate in most parts of the world, creating what we call the seasons. There are four main seasons: summer, autumn, winter and spring. The temperature varies in each season, depending how much a place is tilted towards or away from the Sun.
It is midsummer in a place when it is tilted most towards the Sun. It is midwinter when the place is tilted most away from the Sun. In the northern hemisphere mid-summer is on June 21st and midwinter is on December 21st. In the southern hemisphere midsummer is on December 21st and midwinter on June 21st.
Between summer and winter is autumn, marked by a point in the Earth's orbit when the axis tilts neither towards nor away from the Sun. And between winter and summer is spring, marked again by a point at which the Earth's axis tilts neither towards nor away from the the Sun. These points in the Earth's orbit are known as the equinoxes because then the lengths of day and night are equal throughout the world. In the northern hemisphere the autumn equinox occurs on September 23rd and the spring equinox on March 21st each year. Again, the seasons are reversed in the southern hemisphere.
The Earth is not the only large body to circle around the Sun in space. There are another eight bodies that do so - the planets. They all orbit at different distances from the Sun. In order, going outwards from the Sun, they are Mercury, Venus, (Earth), Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The planets circle the Sun and travel with it through space. They form the major part of the Sun's family, or solar system.
The Sun is quite a different kind of body from the planets. It is our local star, which generates enormous energy in its interior. We see it shine as it radiates heat and light into space. We also see the planets shine in the night sky. But they shine only because they reflect sunlight. They produce no light themselves.
The planets are all of different sizes. The diagram shows them drawn to scale. The Earth is quite a small planet. It is dwarfed by the four giant planets Jupiter, Saturn , Uranus and Neptune.
In turn the giant planets are dwarfed by the Sun itself. Its diameter is nearly 1,400,000 km. This is almost ten times the diameter of Jupiter and more than 100 times the diameter of the Earth.
Mercury, Venus and Mars are rocky planets like the Earth, and are often called the terrestrial, or Earth-like planets. The four giant planets are quite different, having deep atmospheres above deep oceans of liquid gas, mainly hydrogen.
The four inner planets lie relatively close together, but the remaining ones lie very far apart. This diagram shows the orbits of these inner ones roughly to scale. Mercury orbits at an average distance from the Sun of about 58 million km, Mars at an average distance of about 228 million km. The Earth orbits at a distance of about 150 million km, a distance astronomers call an astronomical unit (AU).
This diagram concentrates on the orbits of the outer planets. The orbit of Mars is shown to scale in the middle. Pluto is the planet that travels farthest from the Sun, more than 7,000 million km away at times. It lies about 40 times as far away as the Earth (or 40 AU).
Note that all the planets circle the Sun in the same direction as the Earth. Viewed from the 'north' of the solar system, the planets circle the Sun in an anticlockwise direction.
Also like the Earth, the planets spin on their axis. Again viewed from the 'north ' of the solar system, all except Venus spin anticlockwise, like the Earth. Venus has a slow clockwise, or retrograde rotation.
Most of the planets orbit in much the same plane as the Earth. The two exceptions are Mercury, whose orbit is angled at about 7 degrees; and Pluto, whose orbit is angled ten degrees more.
We say that the planets circle the Sun, but this is not strictly true. They do not travel in circles around the Sun but in ellipses. The German astronomer Johannes Kepler first stated this in the early 1600s as his first law of planetary motion.
The Sun is at one focus of the elliptical orbit. And the distance of planet to the Sun varies throughout the orbit. The point at which it is closest to the Sun is called the perihelion, and the farthest is called the aphelion.
The orbits of most of the planets deviate little from a circle. The orbit of the Earth deviates from a circle by less than two percent. Mercury and Pluto, however, have highly elliptical, or eccentric orbits.
For example, at perihelion, Pluto lies less than 4,500 million km from the Sun, and edges inside the orbit of Neptune. (It lies inside Neptune's orbit at present.) But at aphelion, 120 years later, it lies over 7,300 million km out.
Like the Earth, most planets spin on an axis that is tilted in relation to the plane of their orbit around the Sun. The exception is Mercury, which doesn't tilt at all; and Venus and Saturn also tilt hardly at all.
Mars has a tilt nearly identical with that of Earth. This means that the planet therefore experiences seasons, as in turn its northern, then its southern hemisphere tilt towards, then away from the Sun. This brings about seasonal changes in temperature in the Martian climate. The tilt in the axes of more distant planets has little effect on their climate because of the weakness of the sunlight at such distances.
The planet with the most pronounced axial tilt is Uranus. Its axis is tilted over at over 97 degrees - more than a right-angle! This means that it spins more or less on its side, with its axis nearly parallel to its orbital plane.
With the exception of Mercury and Venus, the planets travel through space with one or more companions. The Earth has one such companion, the Moon. The Moon is the Earth's natural satellite. Mars has two satellites, or moons.
Between them, the giant planets Jupiter, Saturn, Uranus and Neptune, have at least 61 moons. Saturn alone has at least 22! These planets are indeed themselves miniature solar systems. Most of the moons circle their planets in anticlockwise orbits. But a few have a clockwise, or retrograde orbit.
The planets and their moons are the main bodies in the solar system, but there are other bodies besides. Between the orbits of Mars and Jupiter, a ring of rocky bodies, large and small, circles the Sun. It makes up what we call the asteroid belt.
Other members of the solar system pay fleeting visits to Earth's skies. They are the ice-covered rocky bodies we know as comets. From time to time they travel in towards the Sun. As they get nearer, the Sun heats them up and makes them release clouds of dust and gas.
The clouds reflect sunlight, making the comets visible. The pressure of the solar wind - the stream of particles given out by the Sun - forces the gas and dust into a 'tail', which always points away from the Sun.
Swarms of other rocky matter travel in the space between the planets, much of it the debris shed by passing comets. Tiny rocky particles shower down on the Earth all the time, burning up as they plunge into the air. At night we see them as the fiery streaks we call meteors, which we popularly call shooting or falling stars. Large chunks of rock create spectacular fireballs, and may reach the ground intact as meteorites.
