Illumination and circular movement

This episode brings together ideas about movement with ideas about illumination – so drawing on the work on seeing things. The evidence of our eyes – what we so carefully notice – turns out to need careful interpretation in order to make sense of the observations.

Explaining changing patterns

To make anything move along a circular path it is essential to have a force that acts towards the centre of that path.

For example, to make a rubber bung travel around in a circular path on the end of a string, there must be a force acting towards the centre of the motion. The force acting towards the centre of the motion is called a centripetal force.

The word centripetal is a combination of two Latin words: centrum meaning centre and peto meaning to go in. The centripetal force goes in towards the centre.

It's necessary to have a centripetal force to maintain a circular motion because if there is no resultant force acting on an object (that is all forces acting on the object add to zero), then the object travels with uniform motion in a straight line, or stays at rest.

Day and night

One place where it's hard to imagine movement is that our entire planet is moving. On the simplest description the Earth itself is spinning, as well as orbiting around the Sun. Both are nearly spherical, and Earth is really, really smooth (really smoother than a billiard ball, if scaled down).

Try standing outside (preferably on the beach) at sunrise and imagine the Earth rotating under you so as to gradually reveal the Sun, which remains fixed. Repeat at sunset, again imagining the Earth carrying you with it you so that the Sun sinks below the horizon. It's not easy. This may help: drag up on the clock to move through the day.

The cycle of day and night is caused by the rotation of the Earth as it spins on it axis once every 24 hours. Because of this spinning motion, the Sun appears to move through the sky but in reality it is the Earth that does the moving.

This effect is like being on a train when the train next to you starts moving. Often you think you are moving only to realise, when you look out of the other window, that it is the other train that is pulling away.

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Evidence that it is simpler to suggest that Earth is spinning

A piece of significant evidence comes from long exposure photographs of the night sky. To create these photographs, the camera is pointed at the North Pole Star. All the stars come out as long circular trails as if they are all turning around the Pole Star

The same effect is observed with the camera pointing at the Southern Celestial Pole, as shown.

There are two possible explanations for this effect. Either all the stars are turning around a single point or the Earth on which the camera is fixed is turning.

The second explanation is correct. The axis on which the Earth spins is currently pointing at the Pole Star and so as the Earth rotates all of the stars appear to move on circular paths around that star. This apparent circular motion of the stars can easily be seen on a clear night if you pick out the Pole Star and then follow the progress of the other stars around it as the night progresses.

Explaining day and night

The explanation for day and night depends on the idea that the Earth rotates (spins) on its own axis: the Earth spins around and the axis is at the centre of that spin.

When the UK faces the Sun it's daytime because the UK is illuminated. The UK faces away from the Sun at night and so is in the dark.

The apparent path of the Sun during the day(Exposition)

What we know is driven by what we see

That light travels from the Sun to us enables us to see it. But it also enables us to track its path. Its the light arriving here that enables us to infer so much of what we know about the universe.

Simple direct observations show that the Sun rises in the east and sets in the west and that there is a difference in the height of the midday Sun between mid-summer and mid-winter. This information will (later) help with the explanation of the cause of the seasons. For now, its useful for accounting for day and night.

Day and night

The cycle of day and night is caused by the rotation of the Earth as it spins on it axis once every 24 hours. Because of this spinning motion, the Sun appears to move through the sky but in reality it is the Earth that does the moving.

This effect is like being on a train when the train next to you starts moving. Often you think you are moving only to realise, when you look out of the other window, that it is the other train that is pulling away.

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A sphere with imperfections, reflecting light

The Moon is a sphere and it orbits the Earth. It is quite safe to look at the Moon, even through a telescope, because the light that comes from it is not very intense. The Moon reflects the light of the Sun – it is not luminous.

The Moon goes through phases so it seems to be different shapes at different times. A full cycle takes a month or moonth. That's why from one full Moon to the next is one month.

The Moon always keeps the same side facing the Earth – the other side is the dark side of the Moon. We never see the dark side from Earth and the only people who have seen it are the astronauts who have orbited the Moon.

It's a real coincidence that the diameter of the Sun is 400 times the diameter of the Moon but it is also 400 times further away! This extraordinary co-incidence means that the disc of the Sun, as we see it from the Earth, is almost identical in size to the disc of the Moon in the sky. The Moon can therefore just cover the Sun and obscure it completely during a total eclipse.

Phases – not for now

The reason for the shape of the Moon changing periodically, going through its different phases is tricky and widely misunderstood. This is not caused by the Moon being in the shadow of the Earth. It is caused by where the Moon is in relation to the Sun and the Earth and so how you see the Sunlight reflecting off the Moon. It really is best not to try to explain this to primary children. There is more on the phases of the moon in the SPT: Earth in space topic.

The solar system – what's in our locality?

We live on a planet called the Earth that orbits the Sun once every 365 days. The Earth is one of eight known planets, while the Sun is a very ordinary star about half way through its lifetime with another 5000 million years to go. The only reason the Sun does not look like the other stars is because it is much nearer to us. Even so, at 147 million kilometres (93 million miles) away, it still takes about 8 minutes for light to reach us from the Sun. All the planets orbit the Sun in more or less the same plane. This is called the plane of the ecliptic.

The planets are not evenly spaced but are in three groups: the inner planets, Mercury, Venus, the Earth and Mars ; the gas giants, Jupiter and Saturn; the outer planets, Uranus, and Neptune.

Pluto lost it status as a planet in 2006 and analysis of the orbits of comets (see later in this episode) has suggested that there may be another planet, between 1 and 10 times the size of Jupiter. This planet, if it exists, is about three trillion miles out from the Sun and is invisible to telescopes. Expect such claims and counter claims to arise from time to time.

Knowledge about the planets, ideas based on evidence

Much of the information about the planets in the solar system has been determined by observation. The planets Venus, Mars, Jupiter and Saturn can all be seen with the naked eye.

However much better information can be gathered with a telescope and, better still, by satellite and space probes.

The name planets comes from the Greek word planetos, which means wanderer. This is because, unlike the stars whose position relative to each other is fixed, the planets appears to wander across the sky, first going ahead of the fixed stars and then appearing to stop and fall behind. As you can imagine, trying to devise theories to explain why this happened was a major preoccupation for many astronomers for much of history.

Much of our knowledge about planets has come from simple observations with telescopes.

For example, telescopes have revealed the rings of Saturn, the great red spot of Jupiter, the polar ice caps of Mars, the meteorite craters on the Moon and the clouds on Venus.

In the field of astronomy, scientists are limited in the extent to which they can carry out direct experiments on planets (as they have done on Mars and Venus). They are mainly limited to observation, so they must develop theories that are consistent with observational data.

For example:

• The fact that Venus has clouds means that it must have an atmosphere.
• The fact that the Moon has large craters over its surface and no visible signs of water suggests that it does not have an atmosphere. The craters are produced by meteors hitting the surface. In the cases of the Earth and Venus, the meteors are burnt up as a result of friction with the atmosphere. Those that collide with the Moon (and with Mercury, which also has no atmosphere) go flying in with nothing to slow them down.
• The observation that Mars has polar caps, that shrink and grow with the seasons, suggests that it must have an atmosphere in which snow and ice can vaporise and then condense.

In the past 50 years, enormous advances have been made in our knowledge of the planets. The improvements began with radar observations but the big leap forward came with the ability to fly space probes to the planets and their moons or in the case of our own Moon, to visit it.

Most of our recent knowledge about Jupiter and Saturn has come from a succession of space probes sent to them in the 1970s and 1980s.

The outer planets, Uranus and Neptune, have been visited by a single space probe, Voyager 2, but Pluto has not. Consequently we know little about Pluto other than what we can calculate from its period of orbit and observe through telescopes.

Getting something into orbit

Perhaps the first question to think about is how they get into orbit in the first place. Try a thought experiment that was first suggested by Sir Isaac Newton himself.

Imagine a mountain on the Earth's surface that is so big that its summit sticks out above the Earth's atmosphere (it would need to be about ten times as high as Mount Everest). Supposing you climb to the top of this mountain and throw a cricket ball horizontally outwards. The ball is pulled by gravity so that it falls to the ground along a curved path.

Let's assume you now try a lot harder and the ball travels much farther outwards before it hits the ground.

You now summon up all of your strength and manage to throw the ball so fast that it flies outwards and as it falls, its path follows the curvature of the Earth. The ball follows this falling path right round the Earth. In fact, you need to duck as it comes by after completing one orbit! You have managed to throw the ball into orbit around the Earth so that it is now an Earth satellite.

Keeping Planets, satellites and moons in orbit

Exactly the same combination of falling and moving sideways works for anything in orbit. The planet, moon, or satellite falls towards the things that it's orbiting. That's the effect of gravity. But it also travels forwards, at just the right speed, so that it the sideways movement compensates for the movement caused by the falling. So the orbiting thing stays the same distance away from what it's orbiting around.

Thinks about trying to achieve this balance artificially, which is what we do every time we launch an artificial satellite. You can imagine this in two steps: one, use a rocket to get a satellite to the planned height; two, fire some thrusters to set it going it sideways.

If the satellite is thrown sideways too slowly it will fall to Earth because the pull of gravity is too great. If the satellite is thrown sideways too fast it will escape from the Earth's orbit because the gravitational pull is not sufficient to provide the required centripetal force. With the correct launch speed the satellite continues in its falling orbit around the Earth.

You have to set the horizontal speed of the satellite so that the gravitational pull of the Earth tugs it round on its orbital path.

When talking about planets and moons with children it is quite likely that someone will pose the (very good) question:

Abi: Miss, what keeps the moon going?

The short answer to the question is:

Teacher: Nothing keeps it going, it keeps going itself.

As the satellite is launched from the carrier rocket, a rocket thrust acts to throw it out in the desired direction at the prescribed speed. The crucial point to understand here is that the satellite speeds up only for as long as the rocket thrust is acting. Once the rocket motor is switched off the satellite continues at the final speed achieved, neither speeding up nor slowing down, and the gravitational pull of the Earth continuously tugs the satellite in and along its orbital path. In this sense, the satellite just keeps going itself.

If the satellite was moving through empty space it would stay in its orbit forever, there being no forces acting to speed it up or to slow it down. In reality low orbit Earth satellites are not travelling through empty space and so experience a resistive force or drag due to the thin atmosphere which they encounter. In such circumstances, occasional rocket thrusts are needed to maintain the motion of the satellite, otherwise it will fall to Earth.

There are several ideas about how natural satellites in the universe (these could be moons, planets, stars, or even galaxies) got to start moving sideways as well as falling. These rely on matter that's already swirling coming together to make up the orbiting systems. And once stuff is swirling, or spinning, it's rather hard to stop, as you'll know if you have ever tried ice skating. IN any case, it's as well that there are orbits, with the fine balance between falling and moving sideways, otherwise we would not have a home, as Earth would not be.

Newton challenged what counted as natural motion

Before Newton, people used to think that circular motion was natural and needed no explanation. Newton's great insight was to realise that motion in a straight line is the natural form of unforced motion and that it is circular motion that requires an explanation.

This led Newton to ponder on the nature of the force that keeps the Moon in orbit around the Earth. The incident that is supposed to have suggested the answer to him was the apple falling on his head. If gravity can act between the Earth and the apple, why should it not also act between the Earth and the Moon? Newton's subsequent calculations led him to the idea that gravity is a universal force acting between all objects.