Contents: 7 sections
Cambridge IGCSE Co-ordinated Sciences 0654 and Combined Science 0653 · Core and Extended
Syllabus points
- Describe the Earth's rotation and orbit, and explain day and night, the year and the seasons.
- Describe the Moon's orbit and the effect of the Moon on the tides.
- Describe the Solar System and how the orbital speed and temperature of a planet vary with distance from the Sun.
- State that the Sun is a star that releases energy by nuclear fusion, and that energy reaches the Earth by radiation.
- Describe the Milky Way as one galaxy among many, and use the light-year as a unit of distance.
The Earth
The Earth rotates on its axis once in about 24 hours. The half facing the Sun has daytime and the half facing away has night, so day and night are caused by rotation, not by the orbit.
The Earth orbits the Sun once in about 365 days, and that is what defines a year. The orbit is very nearly circular.
The seasons are caused by the tilt of the Earth's axis, which is about 23.5 degrees from the vertical and stays pointing the same way all year. When your hemisphere is tilted towards the Sun, the Sun rises higher in the sky, so the same energy falls on a smaller area of ground and the days are longer. Both effects make it warmer. Six months later the same hemisphere is tilted away and it is winter.
The commonest misconception in the whole topic is that summer happens because the Earth is closer to the Sun. It is not. The orbit is very nearly circular, and in any case the northern and southern hemispheres have opposite seasons at the same moment, which distance from the Sun could never explain. Only the tilt can.
The Moon and the tides
The Moon orbits the Earth in about a month. It is a natural satellite, and it produces no light of its own: we see it because it reflects sunlight. The phases of the Moon are the changing amount of its lit half that we can see from the Earth.
Tides are caused mainly by the Moon. Gravity pulls on the near side of the Earth more strongly than on the far side, and it is that difference across the Earth that raises the oceans into bulges.
The Sun raises tides too, and it is far more massive than the Moon, so it is worth knowing why the Moon wins. Tidal effect depends on how much the pull changes across the width of the Earth, not on how strong the pull is, and the Moon is so much closer that its pull varies far more from one side of the Earth to the other. The Sun's tidal effect is roughly half the Moon's. When the two line up their effects add, giving the especially high spring tides.
Tidal power is therefore the one energy resource on Earth that comes mainly from the Moon. Wind and hydroelectric both trace back to the Sun, which heats the atmosphere unevenly and evaporates the water that later falls as rain, and geothermal energy comes from radioactive decay inside the Earth. Tidal power is also unusually predictable, because the Moon's orbit is known years in advance.
The Solar System
The Sun is at the centre. The planets, in order out from it, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
The first four are small, rocky and dense. The outer four are large gas giants of much lower density. Between Mars and Jupiter lies the asteroid belt, and comets travel in long, highly elliptical orbits.
Everything is held in orbit by the gravitational attraction of the Sun, and that single fact explains the pattern in the table below.
| Going further from the Sun | Change | Why |
|---|---|---|
| Orbital speed | Decreases | The Sun's gravitational pull is weaker further out, so less speed is needed to stay in orbit |
| Time for one orbit | Increases | A longer path travelled more slowly |
| Surface temperature | Decreases | The Sun's radiation is spread over a larger area, so less arrives per square metre |
An object in a stable orbit keeps the same speed but is constantly changing direction, so its velocity is changing all the time. That change of velocity is an acceleration, and gravity is the force producing it, always directed towards the Sun.
The Sun
The Sun is a star, an enormous ball of hydrogen and helium. It releases energy by nuclear fusion, joining hydrogen nuclei into helium in the huge temperature and pressure of its core, with a small loss of mass appearing as energy.
Not fission, which splits heavy nuclei such as uranium, and there is almost nothing heavy in the Sun to split. Not burning either, since a chemical reaction releases millions of times less energy per kilogram and could not have kept the Sun going for billions of years.
Energy reaches the Earth by radiation alone. Conduction needs particles touching one another to pass vibrations along, and convection needs a fluid that can circulate. Space is a vacuum, so neither is available. Electromagnetic radiation needs no medium at all, which is why it is the only mechanism that can cross the gap. What arrives is mostly visible light and infrared.
Distances and galaxies
Space is so large that ordinary units are unusable, so distances are given in light-years.
A light-year is the distance light travels in one year. It is a distance, not a time, and calling it a unit of time is a standard wrong answer.
Light travels at 3.0 × 10⁸ m/s in a vacuum, and distance is speed multiplied by time.
Worked example. Light takes 500 s to travel from the Sun to the Earth. How far away is the Sun?
distance = 3.0 × 10⁸ × 500 = 1.5 × 10¹¹ m
That is 150 million kilometres, which is the figure to sense-check any answer against. Dividing instead of multiplying gives numbers that are far too small, and quoting 3.0 × 10⁸ m as the answer gives how far light travels in one second rather than in 500 s.
The Sun is one star in the Milky Way, a galaxy of billions of stars held together by gravity, and the Milky Way is one galaxy among billions in the Universe. The Solar System is roughly 30 000 light-years from the centre of the Milky Way.
Because light takes time to reach us, looking at a distant object means seeing it as it was when the light left it. Light from a galaxy a million light-years away left it a million years ago, so looking out into space is looking back in time.
Common mistakes
- Saying the seasons are caused by the Earth being closer to the Sun in summer.
- Saying day and night are caused by the Earth's orbit rather than its rotation.
- Saying the Moon gives out its own light.
- Saying the Sun has no effect on tides at all, or that it has the larger effect because it is more massive.
- Saying wind or hydroelectric power comes mainly from the Moon.
- Saying planets further out move faster because they have further to travel.
- Saying an orbiting planet has constant velocity. Its speed is constant; its direction, and so its velocity, is always changing.
- Saying the Sun releases energy by fission, or by burning its fuel.
- Saying heat reaches the Earth by convection or conduction through space.
- Calling a light-year a unit of time.
- Confusing a galaxy with the Solar System, or with the Universe.