Chapter 24
24.1 Earth, Sun and Moon. Day and night The most obvious sign of movement
in the Solar System is the cyclical daily change from light to dark. It is not
surprising that our ancestors thought the Sun travelled round the Earth. Each
day we see the apparent movement of the Sun from rising in the east to setting
in the west. We now know that this effect is caused by the Earth spinning on its
axis (the imaginary line between the poles). The side of the Earth facing the Sun
experiences daylight whilst the other side is in darkness. At sunrise at a
particular spot on Earth, the Sun is just visible on the eastern horizon. As the
Earth turns, the spot moves into the full glare of the Sun so the Sun appears
directly overhead at midday. As the Earth continues to turn, the spot moves out
of the direct sunlight until, at sunset, the Sun appears to slip below the western
horizon. Earth's axis 23.5% the Equator Earth's rotation
Years
As well as the daily changes, early civilizations were aware of periodic changes
which happened over a longer time - the difference between seasons. The
Earth orbits the Sun. It takes just over 365 days to complete one orbit. The
seasons occur because of the tilt of the Earth's axis.
Consider a country in the northern hemisphere (the half of the Earth north of
the Equator). In Figure 24.4a, due to the tilt of the Earth, it is tipped away from
the Sun and the energy from the Sun's rays is more spread out, making it
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colder. This means that area receives fewer hours of sunlight. These countries
are experiencing winter. In Figure 24.4c, the northern hemisphere is tipped
towards the Sun, so it receives longer hours of more direct sunlight. These
countries are experiencing summer. Countries at the Equator do not experience
seasons because the Sun's rays always hit them at the same angle. The
seasonal differences are more apparent the further from the Equator you are.
In the far north or south, seasons are so extreme that, in winter, the Sun is
hardly seen and, in summer, it can be sunny at midnight.
Months
The most obvious object in our sky after the Sun is the Moon. The Moon
features in many folk tales. It has often been seen as a mystical object due to
its fainter light and its changing shape. With the benefit of telescopes and
space travel, we know the Moon is a rocky sphere which we only see when it
reflects light from the Sun. The Moon orbits Earth every 27.5 days. Its position
relative to Earth changes the way it appears to us as different parts of it are
illuminated by the Sun. This causes the changes called the phases of the Moon.
The phases of the Moon. As the Moon orbits the Earth, the half of the Moon
that faces the Sun will be lit up by the Sun. As the Moon moves, the shape of
the light part, which can be seen from the Earth, changes. The outer circle of
Moon diagrams shows how the Moon looks to an observer on Earth.
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The Solar System consists of the Sun which is our star, and all the objects
which orbit it. It includes the following:
• There are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus
and Neptune.
• There are minor planets, such as Pluto and Eris. In 2014, the International
Astronomical Union recognised five dwarf planets but it is believed there are
more than 200 in all.
• Moons that orbit planets and dwarf planets.
• Millions of asteroids and meteoroids: these are rocky objects which are
smaller than planets. Most asteroids are found in the asteroid belt between the
orbits of Mars and Jupiter.
• Comets, which are often described as giant snowballs, orbit the Sun in very
irregular orbits. When they are furthest from the Sun, they are frozen balls of
gas, rock and dust. As they get nearer to the Sun they heat up and leave a trail
of dust and gases behind them. (Note: this trail of dust is not the tail of the
comet; the tail always points away from the Sun, so could actually be at 90° to
the motion of the comet.)
The Sun's gravitational pull
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The orbits of the planets are almost circular. To move in a circle an object needs
a force pulling it towards the centre of the circle. Imagine spinning a ball on the
end of a piece of string. The ball will spin in a circle as long as you hold on. Once
you let go, the ball will fly outwards. The force needed to keep the planets
orbiting the Sun comes from the gravitational attraction of the Sun.
The formation of the planets
Evidence collected by astronomers suggests that the planets were formed at
the same time as the Sun. The Solar System began as a nebula, which is a huge
swirling ball of dust and gas. Most of this gas was hydrogen, but there were
also other elements formed by fusion in other stars, which had exploded at the
end of their life cycle, sending their contents out into the clouds of interstellar
gas. As gravity pulled this mass together, the centre formed a star. You will
learn more detail about this in Chapter 25. The planets formed from the
materials of the nebula which were not pulled into the Sun. The spinning
motion of the dust and gas formed a flat, spinning ring disc known as an
accretion disc. Gravity pulled dust and gas together so they joined to make
rocks which then join to make larger rocks. The process of the dust and gas
being pulled together by gravity is called accretion and it led to the formation of
the inner, rocky planets. The intense heat forced some of the lighter materials
further away and these formed the outer planets - the gas giants. The four
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inner planets, Mercury, Venus, Earth and Mars, are small and rocky. After Mars
there is the asteroid belt. This is made up of left-over pieces of rock. The outer
four planets, Jupiter, Saturn, Uranus and Neptune, are huge balls of gases.
These planets are much bigger than the inner planets.
Distances in the Solar System are almost unimaginably big. The Earth is
approximately 150 million kilometres from the Sun. This is similar to circling the
Earth 4000 times. Distances are often expressed in terms of how long it takes
light to travel; one light-year is the distance travelled by light in a year. The next
nearest star after the Sun is Proxima Centauri, which is 4.2 light-years from
Earth.
Forces
The Sun is at the centre of the Solar System. It is by far the most massive
object in the Solar System and makes up about 99.8% of the mass of the Solar
System. As gravitational attraction depends on mass, the gravitational field
strength of the Sun is far larger than the field of any other object in the Solar
System. The planets, minor planets, asteroids and meteoroids and comets all
orbit the Sun. They are held in orbit by the gravitational attraction of the Sun.
Like other non-contact forces such as magnetism and static electricity,
gravitational attraction decreases with distance. This means that the outer
planets experience less gravitational force from the Sun than the inner planets
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do. Although the planets are small compared to the Sun, they are very massive
objects. Jupiter has a mass of 1.9 x 10^27 kg. The more massive the planet, the
greater the gravitational force experienced by objects at its surface. On Earth
we experience a force of 10 N/kg. On Earth a 60 kg student has a weight of 600
N. On Mercury, where gravity is 4 N/kg, the same student would weigh 240 N.
The gravitational pull of planets is enough to cause moons to orbit them.
Orbits and energy
The orbits of the planets are not completely circular. Their shape is that of a
slightly squashed circle, called an ellipse. The orbits are described as elliptical.
The amount the orbit is squashed is called its eccentricity. Comets have very
eccentric orbits. Comets travel far from the Sun and then return close to it.
planetary orbits the orbit of a comet. Why are orbits elliptical? To explain this,
we need to think about the early swirling mass of the Solar System. Imagine an
object moving past the Sun at high speed, carried along by its own momentum
from the explosive start of the universe. As it passes near the Sun the
gravitational force of the Sun starts to act on the object and to pull it towards
the Sun. This force also causes it to accelerate. This means the mass speeds up
and its kinetic energy carries it slightly further out to the furthest point of the
orbit. The object slows down and is pulled in again towards the Sun.
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The Sun is not quite at the centre of a planet's elliptical orbit. There is a point
close to the centre of an ellipse called the focus. The Sun is at the focus of the
elliptical path of each of the planets. The planet moves closer to, and further
away, from the Sun during each orbit. The Sun's gravity pulls the object in,
speeds it up and then the speed carries it on to the furthest part of the orbit.
The object's orbital speed is therefore greatest when it is nearest to the Sun
and slowest when it is furthest from the Sun.
Comets, which have the most elliptical orbits of any body in the Solar System
accelerate greatly as they approach the Sun and are slung back at high speed to
the far reaches of their orbits.
A planet orbiting in space does not experience any friction or air resistance, so
its energy remains the same throughout its orbit. It has two types of energy:
⚫ kinetic energy
gravitational potential energy.
When it is nearest the Sun, a planet has its minimum gravitational potential
energy and is moving at its fastest so has its maximum kinetic energy. When it
is at its furthest from the Sun, it has maximum gravitational potential and
minimum kinetic energy.
Speeds
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The speed of a planet in orbit round a star is called its orbital speed (v). As the
planets' orbits are almost circular, the distance they travel can be calculated if
we know the average orbital radius, which is the average distance of the planet
from the Sun, or the average radius of the orbit.
To calculate the orbital speed, we assume that the orbits are circular.
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