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Cambridge IGCSE Physics · 0625

Chapter 6: Space physics — Part 1

Topic 6.1.1 · The Earth

Day, night and the year

The Earth is a planet that rotates on its axis (the axis is tilted) once in about 24 hours. That explains the apparent daily motion of the Sun and the cycle of day and night.

The Earth orbits the Sun once in about 365 days. Combined with the tilt of the axis, that explains the seasons (a hemisphere tilted towards the Sun has summer).

Earth rotating on a tilted axis: day and night, and the Sun’s apparent daily path; Earth orbiting the Sun once a year, with tilt explaining the seasons.
Diagram 1: Earth rotating on a tilted axis: day and night, and the Sun’s apparent daily path; Earth orbiting the Sun once a year, with tilt explaining the seasons.

The Moon and orbital speed

The Moon takes about one month to orbit the Earth. That explains the periodic cycle of the Moon’s phases.

Average orbital speed: v = 2πr / T where r is the average radius of the orbit and T is the orbital period (use consistent units, e.g. metres and seconds).

A moon orbits at average radius 4.0 × 108 m with period 2.0 × 106 s. Calculate its average orbital speed.

Worked example: orbital speed is 2 pi r over T equals 1.3 times 10 to the 3 metres per second
v = 2πr / T = 1.3 × 103 m/s. Convert days to seconds if T is given in days.
Moon orbiting Earth in about one month, showing the cycle of phases, plus the average orbital speed formula v = 2πr / T.
Diagram 2: Moon orbiting Earth in about one month, showing the cycle of phases, plus the average orbital speed formula v = 2πr / T.

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