Earth, Moon and the Solar System

This note covers the motion of the Earth and the Moon, the causes of day, night, seasons and the Moon's phases, and the structure of the Solar System. It also shows how to calculate orbital speed from the radius and the time for one orbit.

Key ideas

The Earth. The Earth spins on its axis once in about 24 hours. This causes day and night: the side facing the Sun has daytime, and the far side has night. From Earth, the Sun appears to move across the sky because of this spin.

The Earth also orbits the Sun, once in about 365 days (one year). Its orbit is nearly circular. The Earth's axis is tilted at about 23.5° to the line at right angles to its orbit. This tilt stays pointing in the same direction in space as the Earth moves.

• When the northern hemisphere is tilted towards the Sun, it has summer: the Sun is higher in the sky, the days are longer and sunlight is more concentrated.
• Six months later, the northern hemisphere is tilted away, so it has winter. The southern hemisphere has the opposite season at the same time.
• The seasons are caused by the tilt of the axis. They are not caused by the Earth being closer to or further from the Sun.

The Moon. The Moon orbits the Earth in about 27.3 days. It does not give out light of its own. We see it because it reflects sunlight. Half of the Moon is always lit by the Sun, but as it orbits, we see different amounts of the lit half. These are the phases: new Moon (lit side faces away from us), crescent, half, gibbous and full Moon (lit side faces us).

The cycle of phases takes about 29.5 days, a little longer than the orbit, because the Earth is also moving round the Sun.

The Solar System. The Solar System consists of:

• the Sun, a star, which contains almost all the mass of the system;
• eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four nearest are small and rocky. The four outer planets are much larger and made mostly of gas and liquid;
• dwarf planets such as Pluto, and many moons orbiting planets;
• the asteroid belt, a ring of rocky objects between Mars and Jupiter;
• comets, which are balls of ice and dust in very stretched-out orbits, and which develop tails when close to the Sun.

Orbits. The Sun's gravitational pull on each planet provides the force that keeps it in orbit. Planet orbits are almost circular. Comet orbits are long ellipses, and a comet moves fastest when it is closest to the Sun.

• The gravitational field is weaker further from the Sun, so the pull on the outer planets is smaller.
• The outer planets have larger orbits and move more slowly, so they take much longer to complete one orbit. Neptune takes more than 160 Earth years, whereas Mercury takes about 88 days.
• The Moon and artificial satellites orbit in the same way, held by the gravitational pull of the body they orbit.

Orbital speed. In one orbit, an object travels the circumference of its orbit. For a circular orbit of radius r and orbital period T, the speed is the distance divided by the time. The unit of T must be seconds if the speed is in m/s.

Light travel times. Light from the Sun takes about 8 minutes to reach us. We can find the time using time = distance ÷ speed, with the speed of light 3.0 × 10⁸ m/s.

Definitions and formulae

Worked examples

Common errors

• Saying the seasons are caused by the Earth being nearer to the Sun in summer. The cause is the tilt of the axis.
• Using the period in days or years in v = 2πr ÷ T. Convert to seconds when the radius is in metres.
• Saying the Moon makes its own light. It reflects light from the Sun.
• Calling the phases of the Moon the shadow of the Earth. Phases are the different views of the lit half. (The Earth's shadow only causes a lunar eclipse.)
• Forgetting that the circumference is 2πr and using πr² (an area) by mistake.

Exam tip: In orbit calculations, write the period in seconds on its own line before using it. This avoids the common slip of dividing by days.

Check yourself

1. Explain why the northern hemisphere has longer days in June than in December.
   Answer

   In June the northern hemisphere is tilted towards the Sun, so the Sun spends longer above the horizon each day. In December it is tilted away.
2. Mars orbits the Sun at an average radius of 2.3 × 10¹¹ m with a period of 687 days. Find its orbital speed.
   Answer

   T = 687 × 86 400 = 5.94 × 10⁷ s. v = 2π × 2.3 × 10¹¹ ÷ 5.94 × 10⁷ = 2.43 × 10⁴ m/s.
3. The Moon is 3.84 × 10⁸ m from the Earth. How long does light take to travel from the Moon to the Earth?
   Answer

   t = 3.84 × 10⁸ ÷ 3.0 × 10⁸ = 1.28 s.
4. Why does Jupiter take much longer than the Earth to orbit the Sun?
   Answer

   Jupiter's orbit is much larger, so it has further to travel, and the weaker gravitational pull of the Sun at that distance means it moves more slowly.