The Earth Notes

Cambridge IGCSE Physics: Revision notes

Key facts

  • The Earth rotates once every about 24 hours, causing day and night.
  • The Earth orbits the Sun once every about 365 days; its tilted axis (23.5°) causes the seasons.
  • The Moon orbits the Earth in about 29.5 days (a lunar month), giving the phases.
  • Orbital speed v=2πrTv = \dfrac{2\pi r}{T}.

Day and night

The Earth turns on its axis once every 24 hours, so the side facing the Sun has day and the side facing away has night.

The Sun's daily path across the sky from east to west is only apparent: it is the Earth rotating west to east. The terminator is the boundary between day and night.

Sunlightterminator
The Earth lit from one side: the side facing the Sun has day, the side facing away has night, and the terminator is the boundary.
  • Earth rotates onceabout 24 hours (day and night)

The apparent motion of the Sun across the sky is caused by...

The seasons

The tilt of the Earth's axis, about 23.5°, makes the Sun's rays more or less direct as the Earth orbits the Sun.

The hemisphere tilted towards the Sun gets rays at a smaller angle of incidence, so energy is more concentrated, and has longer days: summer. Spring and autumn occur when neither hemisphere is tilted towards or away.

axisSunlight23.5°
The Earth's axis is tilted at 23.5°. Here the northern end leans towards the Sun (June): northern summer, southern winter.

June

  • Northern: tilted towards the Sun: summer
  • Southern: tilted away: winter

December

  • Northern: tilted away: winter
  • Southern: tilted towards: summer

When the Northern Hemisphere is tilted towards the Sun it has...

Phases of the Moon

The Moon reflects sunlight, and as it orbits the Earth we see more or less of its lit side.

The Moon orbits the Earth in about 27 to 29 days and the phases repeat every 29.5 days. Waxing means growing; waning means shrinking. New moon: the Moon is between the Earth and Sun; full moon: the Earth is between the Moon and Sun.

  1. 1

    New moon

    dark side facing Earth

  2. 2

    Waxing crescent

    small illuminated crescent

  3. 3

    First quarter

    half illuminated

  4. 4

    Waxing gibbous

    more than half illuminated

  5. 5

    Full moon

    fully illuminated

  6. 6

    Waning gibbous

    more than half illuminated

  7. 7

    Last quarter

    half illuminated

  8. 8

    Waning crescent

    small illuminated crescent

Then back to step 1

The cycle of Moon phases (about 29.5 days)

At a full moon, the Moon is...

Orbital speed

Average orbital speed is the distance for one orbit, 2πr2\pi r, divided by the orbital period TT.

Use rr in metres and TT in seconds. It is an average because we assume a circular orbit at constant speed. Convert days to seconds first.

  • Average orbital speedv=2πrTv = \dfrac{2\pi r}{T}
rvOPQ
A circular orbit of radius r: one orbit covers the circumference 2πr

Worked example: Earth around the Sun

The Earth has orbital radius 1.5 × 10¹¹ m and orbital period 365.25 days. Find its average orbital speed.

Which unit must the period be in to get m/s?

Three timescales

Rotation (24 hours) gives day and night, the Moon's orbit (about 29.5 days) gives phases and the Earth's orbit (about 365 days) gives seasons.

Do not mix these up in answers: match each effect to the correct motion and period.

0100200300Earth rotatesMoon orbits EarthEarth orbits SunMotionTime (days)
Length of each motion in days: the Earth's rotation is 1 day, the Moon's orbit about 29.5 days and the Earth's orbit about 365 days.

Earth's rotation

  • About 24 hours
  • Day and night

Moon's orbit

  • About 27 to 29 days
  • Moon phases

Earth's orbit

  • About 365 days
  • Seasons

Seasons are caused by...

Try an exam question

Explain why it is summer in the Northern Hemisphere in June.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.