The Earth and the Solar System

The spinning Earth: day, night and the apparent motion of the Sun

The Earth is a planet that spins on its own axis, the imaginary line running through the poles. One complete spin is called a rotation and it takes approximately 24 hours, which is why we call that length of time a day. The axis is not upright: it is tilted at about 23.5 degrees from the vertical, and the direction of the tilt stays fixed in space as the Earth travels around the Sun.

At any moment only half of the Earth faces the Sun. The half that faces the Sun is lit and has day; the half that faces away is in shadow and has night. Because the Earth keeps turning, every place on the surface is carried from the lit side into the shadow and back again, so day and night follow one another in a regular cycle once every 24 hours.

The rotation also explains the apparent daily motion of the Sun. The Sun does not really travel across the sky; the Earth turns underneath it. As the Earth turns from west to east, the Sun appears to rise in the east, climb to its highest point around midday and set in the west. Time-lapse photographs of the sky show the Sun and the stars tracing smooth arcs from east to west, and that is an illusion produced by your own rotation.

Notice which fact does which job. The rotation explains day and night and the daily motion of the Sun; the tilt of the axis is not needed for either of those, but it becomes essential for the seasons in the next section.

• Rotation period of the Earth: approximately 24 hours.
• The half of the Earth facing the Sun has day; the half facing away has night.
• The Sun appears to move east to west because the Earth turns west to east.
• The axis is tilted, but the tilt is not what causes day and night.

The Earth's orbit and the periodic nature of the seasons

The Earth also travels around the Sun in a nearly circular path called an orbit. One complete orbit takes approximately 365 days, which is one year, and this time is the Earth's orbital period.

The seasons are not caused by the Earth moving closer to or further from the Sun. They are caused by the tilted axis. Because the tilt keeps pointing the same way in space, for part of the year the northern half of the Earth leans towards the Sun while the southern half leans away; six months later the situation is exactly reversed.

When your hemisphere leans towards the Sun, the Sun's rays strike the ground more directly, so a given amount of energy is concentrated on a smaller area, and the days are longer because the Sun is above the horizon for more hours. Both effects raise the temperature, and you have summer. When your hemisphere leans away, the rays arrive at a shallower angle and are spread over a larger area, and the days are shorter, so the temperature falls and you have winter.

This is why the seasons are periodic: the pattern repeats once every orbit, with a period of about 365 days. It also explains why the two hemispheres always have opposite seasons at the same moment - when it is summer in the north it is winter in the south.

The Moon, its orbit and the cycle of phases

The Moon is a natural satellite of the Earth, which means a body that orbits a planet. It takes approximately one month (about 27.3 days) to complete one orbit of the Earth, and it takes the same time to rotate once on its own axis, so the same face of the Moon always points towards us.

The Moon is not a source of light. It is lit only by the Sun, so at any moment half of the Moon is lit and half is in darkness, exactly as half of the Earth is lit at any moment. What changes during the month is how much of the lit half we can see from the Earth, and that depends on where the Moon is in its orbit.

When the Moon is between the Earth and the Sun, its lit half faces away from us and we cannot see it at all: this is a new moon. When the Earth is between the Sun and the Moon, the whole lit half faces us: this is a full moon. In between we see a thin crescent, then a half moon (called the first quarter), then a gibbous shape, and after the full moon the sequence runs in reverse back to new moon. The whole cycle takes about one month, which is why the phases are periodic.

Do not confuse a phase with an eclipse. A phase is simply the changing fraction of the Moon's lit half that faces us, and it happens every month for every observer. An eclipse is a rare alignment in which one body blocks the light of the Sun from another, and it does not happen every month because the Moon's orbit is tilted relative to the Earth's orbit.

• The Moon orbits the Earth in approximately one month.
• The Moon makes no light of its own; it is lit by the Sun.
• New moon: the lit half faces away from the Earth. Full moon: the lit half faces the Earth.
• The cycle of phases repeats every orbit, so it is periodic with a period of about one month.

Average orbital speed and the equation v = 2 x pi x r / T

An object in a circular orbit travels at a steady speed. Its average orbital speed is the distance travelled in one complete orbit divided by the time taken. The orbit is a circle of radius r, so its circumference is 2 x pi x r, and the time for one complete orbit is the orbital period T. That gives the equation you must recall and use:

v = 2 x pi x r / T

Here v is the average orbital speed in metres per second (m/s), r is the average radius of the orbit in metres (m) and T is the orbital period in seconds (s). Notice that T must be in seconds. Convert the period before you substitute, or the answer will be out by a factor of 24, or 60, or 3600.

Worked example. The Earth orbits the Sun at an average radius of 1.5 x 1011 m and takes 365 days. Find the average orbital speed. Convert the period first: T = 365 x 24 x 60 x 60 = 3.15 x 107 s. Then substitute: v = 2 x pi x r / T = (2 x 3.14 x 1.5 x 1011) / (3.15 x 107) = (9.42 x 1011) / (3.15 x 107) = 3.0 x 104 m/s. The Earth therefore travels at about 30 000 m/s, which is 30 km/s.

The same equation works for any circular orbit: a satellite around the Earth, the Moon around the Earth, or a planet around the Sun. Because r appears on the top of the fraction, a larger orbit would give a larger speed if the period stayed the same - but in the Solar System the period increases faster than the radius, so the planets furthest from the Sun actually move most slowly. You will see that pattern again in the data on the outer planets.

What the Solar System contains

The Solar System is the Sun together with everything that orbits it. The Sun is the only star in the Solar System and it contains more than 99% of its mass, which is why its gravity holds all the other bodies in orbit around it.

There are eight planets. In order from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Learn them as a sequence, and be ready to place a planet from a description of its orbital distance or its density.

Between the orbits of Mars and Jupiter lies the asteroid belt, a broad ring of rocky bodies called asteroids. These are minor planets: bodies that orbit the Sun directly but are too small to be classed as planets. Beyond Neptune there are more minor planets, including the dwarf planets such as Pluto, which was reclassified from a planet to a dwarf planet in 2006. Minor planets orbit the Sun, never a planet.

Most of the planets have moons, which are natural satellites orbiting the planet rather than the Sun. The Earth has one moon, Mars has two, and Jupiter and Saturn have dozens each. Further out, in long thin orbits, travel the comets: small bodies of ice, dust and rock. When a comet comes close to the Sun the ice vaporises and forms a glowing tail that always points away from the Sun, because the solar wind and radiation push it outwards.

The Solar System is enormous, so distances are measured in huge numbers of metres. Light travels at 3.0 x 108 m/s, and because nothing can travel faster, the time light takes to cross a distance tells you how far away the object is. The Sun is 1.5 x 1011 m from the Earth, so light takes t = d / v = (1.5 x 1011) / (3.0 x 108) = 500 s, which is 8 minutes and 20 seconds. If the Sun went out, we would not know for over eight minutes.

• One star: the Sun, which holds more than 99% of the mass of the Solar System.
• Eight planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Minor planets orbiting the Sun: dwarf planets such as Pluto, and the asteroids of the asteroid belt between Mars and Jupiter.
• Moons (natural satellites) orbiting the planets, not the Sun.
• Smaller bodies, including comets, which travel in long elliptical orbits.

How the Solar System formed: the accretion model

The accretion model explains how the Solar System formed about 4.6 billion years ago. It began with a vast cloud of interstellar gas and dust left over from earlier stars, so it contained many elements - hydrogen and helium in the greatest amounts, together with the heavier elements such as silicon, iron and oxygen that would eventually form rocks.

A disturbance, perhaps a shock wave from a nearby supernova, made the cloud begin to collapse under its own gravity. As the material fell inwards the cloud started to rotate, and because a spinning cloud speeds up as it shrinks (just as a skater spins faster when they pull their arms in), it flattened into a spinning accretion disc with a dense, hot centre.

The centre became hot and dense enough for nuclear fusion to begin, and the Sun was born. Out in the disc, grains of dust and gas collided and stuck together, growing by accretion from grains to pebbles to rocks to protoplanets. Gravity then swept up the remaining material, leaving the planets and moons we see today.

The model also explains the pattern in the planets. Close to the Sun the disc was too hot for hydrogen, helium and ices to condense, so only materials with high melting points - rock and metal - could form solids. The four inner planets are therefore rocky, small and dense, and they formed close together. Further out the disc was cold enough for gases and ices to condense, so the four outer planets grew much larger and kept thick atmospheres of hydrogen and helium, making them gaseous, large and of low density.

Gravity, gravitational field strength and elliptical orbits

Every object with mass has a gravitational field around it. The gravitational field strength, g, is the gravitational force per unit mass, measured in newtons per kilogram (N/kg). At the Earth's surface g is about 9.8 N/kg, so each kilogram is pulled downwards with a force of 9.8 N. The field strength at the surface of a planet depends on the mass of the planet: a more massive planet pulls harder on each kilogram, so g is larger. Jupiter is far more massive than the Earth, so its surface field strength is about 24 N/kg.

The field is strongest at the surface and becomes weaker as the distance from the planet increases, because the same pull is spread over a larger sphere. High above the Earth the field is weak, and at the distance of the Moon it is weaker still. The field never reaches zero, so the Moon is always pulled by the Earth - it stays in orbit rather than falling in because it is also moving sideways at the right speed.

The force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun. Because the Sun contains most of the mass of the Solar System, its gravitational pull on a planet is far greater than the pull of any other planet, so the planets orbit the Sun. In the same way, the force that keeps a satellite in orbit around the Earth is the gravitational attraction of the Earth.

The orbits are not perfect circles. Planets, minor planets and comets all have elliptical orbits, and the Sun is not at the centre of the ellipse - it sits at one focus, off to one side. That is why the distance from a planet to the Sun changes during its orbit, and why a comet's distance changes enormously.

An object in an elliptical orbit travels faster when it is closer to the Sun and slower when it is further away. This follows from the conservation of energy. As the object falls towards the Sun its gravitational potential energy decreases, and that energy becomes kinetic energy, so the object speeds up. As it climbs away again its kinetic energy is converted back into gravitational potential energy, so it slows down. The total energy stays the same throughout the orbit.

The same idea explains the data on the planets. The strength of the Sun's gravitational field decreases as the distance from the Sun increases, so the planets furthest from the Sun are held by a weaker force and have slower orbital speeds. Mercury, the closest planet, completes an orbit in 88 days, while Neptune, the furthest, takes about 165 years. So both the orbital speed and the orbital period are set by the distance from the Sun.