The Earth and the Solar System

Key points

6.1.1 The Earth

• The Earth is a planet. It spins (rotates) on its axis once in about 24 hours. The axis is tilted.
• As the Earth rotates, one half faces the Sun and has day. The other half faces away and has night. This repeats every 24 hours.
• The Earth rotates from west to east. So the Sun appears to rise in the east, move across the sky and set in the west. The Sun is not moving round the Earth. This is its apparent daily motion.
• The Earth orbits (travels round) the Sun once in about 365 days.
• Because the axis is tilted, each hemisphere is tilted towards the Sun for part of the year and away from the Sun for another part.
• When a hemisphere is tilted towards the Sun, it has summer. The Sun is higher in the sky, the sunlight is more concentrated and the days are longer.
• When a hemisphere is tilted away from the Sun, it has winter. The Sun is lower in the sky and the days are shorter.
• The orbit takes about 365 days, so the seasons repeat every year.
• The Maldives is close to the equator. Here the length of the day changes very little during the year.
• The Moon orbits the Earth in about one month.
• The Moon does not make its own light. We see it because it reflects light from the Sun. The half of the Moon facing the Sun is always lit.
• As the Moon orbits the Earth, we see different amounts of its lit half. These shapes are the phases of the Moon, for example new Moon, crescent, half Moon and full Moon.
• The orbit takes about one month, so the cycle of phases repeats about once a month. The Islamic calendar follows this cycle: each month begins with the new crescent Moon.
• Average orbital speed is the distance round the orbit divided by the time for one orbit: v = 2πr / T. Here r is the average radius of the orbit and T is the orbital period (the time for one orbit). Extended

6.1.2 The Solar System

• The Solar System contains:
  • one star, the Sun
  • eight planets that orbit the Sun
  • minor planets that orbit the Sun, including dwarf planets such as Pluto and asteroids in the asteroid belt
  • moons, which orbit planets
  • smaller bodies, including comets and natural satellites.
• The eight planets in order from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• The four planets nearest the Sun (Mercury, Venus, Earth, Mars) are small and rocky.
• The four planets furthest from the Sun (Jupiter, Saturn, Uranus, Neptune) are large and gaseous.
• The accretion model explains this difference. Accretion means small particles being pulled together by gravity to make larger bodies.
• The model depends on gravity. Gravity pulls the material together at every stage.
• The Solar System formed from an interstellar cloud of gas and dust. The cloud contained many elements, not just hydrogen and helium.
• The cloud was rotating. As gravity pulled it inwards, it flattened into a spinning accretion disc. Most of the mass collected at the centre and became the Sun.
• Close to the Sun the disc was very hot. Only rock and metal stayed solid there, so the inner planets are small and rocky.
• Far from the Sun the disc was cold. Large cores of rock and ice formed. Their strong gravity pulled in a lot of hydrogen and helium gas, so the outer planets are large and gaseous.
• The gravitational field strength at the surface of a planet depends on the mass of the planet.
• Extra detail: the surface gravitational field strength also depends on the size (radius) of the planet. So a planet with more mass does not always have a stronger field at its surface. Use the data you are given.
• The gravitational field around a planet gets weaker as the distance from the planet increases.
• Light travels very fast, but the distances in the Solar System are very large. So light takes a measurable time to travel between objects. Use t = d / c.
• The Sun contains most of the mass of the Solar System. This is why the planets orbit the Sun.
• The Sun’s gravitational attraction pulls each planet, comet or other orbiting body towards the Sun all the time. This pull is the force that keeps it moving in its orbit.
• Planets, minor planets and comets move in elliptical orbits (orbits shaped like a stretched circle). Extended
• The Sun is not at the centre of an elliptical orbit. It is only at the centre when the orbit is almost a circle. Extended
• Extra detail: the Sun is at a point called a focus of the ellipse.
• The strength of the Sun’s gravitational field decreases as the distance from the Sun increases. Extended
• So planets further from the Sun have lower orbital speeds. They also have longer orbits, so they take longer to go once round the Sun. Extended
• An object in an elliptical orbit travels fastest when it is closest to the Sun. Extended
• This is explained by conservation of energy. As the object moves closer to the Sun, its gravitational potential energy decreases. Its kinetic energy increases by the same amount, so it speeds up. As it moves away, the opposite happens and it slows down. Extended
• You may be given a table of planetary data: orbital distance, orbital duration (period), density, surface temperature and surface gravitational field strength. Look for patterns between two columns and use numbers from the table to support your answer. Extended
• Patterns to look for: planets further from the Sun take longer to orbit it; rocky inner planets have higher densities than the gaseous outer planets; surface temperature generally falls with distance from the Sun. Always check the table for exceptions. Extended

Common mistakes

• Students write that the Sun moves across the sky because it orbits the Earth. / The mark scheme wants: the Earth rotates on its axis, so the Sun only appears to move.
• Students write that summer happens when the Earth is closer to the Sun. / The mark scheme wants: seasons are caused by the tilt of the Earth’s axis as the Earth orbits the Sun.
• Students write that the phases of the Moon are caused by the Earth’s shadow. / The mark scheme wants: we see different amounts of the Moon’s sunlit half as the Moon orbits the Earth.
• Students write that the outer planets are gaseous because they are cold. / The mark scheme wants the accretion model: near the hot Sun only rocky material stayed solid; further out, large cores pulled in a lot of gas by gravity.
• Students explain why an orbiting body speeds up near the Sun only by saying “gravity is stronger there”. / The mark scheme wants conservation of energy: gravitational potential energy decreases and kinetic energy increases by the same amount. Extended
• Students use T in hours or days in v = 2πr / T and give the answer in m/s. / The mark scheme wants consistent units: r in m and T in s give v in m/s. Extended

Exam tips

• State the times: Earth rotates once in about 24 hours; Earth orbits the Sun in about 365 days; Moon orbits the Earth in about one month.
• Explain day and night, the seasons and the phases using the motion and the time it takes. For example: “The Earth rotates once every 24 hours, so each place faces the Sun and then faces away.”
• Describe the accretion model in order: rotating cloud of gas and dust with many elements → gravity pulls it in → accretion disc → hot near the Sun (rocky planets), cold further out (gaseous planets).
• To remember the order of the planets, make a sentence from the first letters: M, V, E, M, J, S, U, N.
• Calculate: write the equation, substitute with units, give the answer to 2 or 3 significant figures with its unit.
• In planetary data questions, describe a pattern by naming both quantities: “As orbital distance increases, orbital duration increases.” Then quote values from the table. Extended
• A typical 1-mark answer to “What keeps the planets in orbit?”: “The Sun’s gravity pulls each planet towards the Sun.”
• A typical 2-mark answer to “Why does a comet move fastest near the Sun?”: “It loses gravitational potential energy as it moves closer to the Sun (1). This energy becomes kinetic energy, so its speed increases (1).” Extended