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6.1.1Space Physics

The Earth, Moon and Sun — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 6 of 24 1 quiz

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Lesson summary

6.1.1 The Earth, Moon and Sun — IGCSE Physics Notes — IGCSE PHYSICS notes: Describe the motion of the Earth, Moon and Sun and use it to explain day and night, the seasons, the Moon's phases and eclipses.Rotation — the Earth turns on its axis, a line through the poles, once in abo…

  • Difference
  • Sunrise and sunset
  • Apparent motion
  • Why unnoticed
  • Leaning towards the Sun
  • Leaning away from the Sun

Part 1 of 9Spinning and orbiting

Rotation
  • Rotation the Earth turns on its axis, a line through the poles, once in about 24 hours.
Orbit
  • Orbit the Earth travels once round the Sun in about 365¼ days.
  • The extra quarter day is why a leap day is added every fourth year.
Moon
  • Moon the Earth's only natural satellite orbits the Earth once in about 27.3 days and travels with the Earth round the Sun.
Sun
  • Sun a star far larger than the Earth that holds almost all the mass of the Solar System.
  • It is the source of nearly all the light and thermal energy reaching the Earth.
Difference
  • Difference a rotation is turning about an own axis, whereas an orbit is travelling round another body.

Part 2 of 9Day and night

Lit half
  • Lit half half of the Earth is always lit by the Sun and half is in darkness.
Cause
  • Cause rotation carries a place from the lit half to the dark half and back, giving day and night.
Sunrise and sunset
  • Sunrise and sunset the Sun appears to rise in the east and set in the west because the Earth rotates towards the east.
Apparent motion
  • Apparent motion the daily path of the Sun, Moon and stars across the sky is only apparent and is caused by rotation.

Part 3 of 9Rotation speed at the equator

Angle
  • Angle a place is carried round at 15° each hour (360° ÷ 24 h).
Speed
  • Speed a place on the equator moves at about 460 m/s.
Why unnoticed
  • Why unnoticed everything around us, including the air, moves with us.

Part 4 of 9The seasons

1Tilt
  • Tilt the axis is tilted at about 23.5° and keeps pointing the same way in space while the Earth orbits.
2Leaning towards the Sun
  • Leaning towards the Sun the hemisphere has summer.
  • The Sun climbs high and the days are long.
  • Light strikes the ground steeply, so a beam of energy is spread over a small area and heats it more.
3Leaning away from the Sun
  • Leaning away from the Sun the hemisphere has winter.
  • The Sun stays low and the days are short.
  • The same beam is spread over a larger area, so each square metre receives less energy.
4Opposite seasons
  • Opposite seasons the two hemispheres have opposite seasons at the same time.
5Spring and autumn
  • Spring and autumn both hemispheres receive about the same solar radiation.
6Distance is not the cause
  • Distance is not the cause the Earth is slightly nearer the Sun in early January, during the northern winter, and the difference is only about 3%.

Part 5 of 9Equinoxes and solstices

Equinox
  • Equinox day and night are about equal and the Sun rises due east and sets due west.
Summer solstice
  • Summer solstice the day with the longest hours of daylight, when the Sun is highest above the horizon.
Winter solstice
  • Winter solstice the day with the shortest hours of daylight, when the Sun stays closest to the horizon.
Near the equator
  • Near the equator day length is about 12 hours all year.

Part 6 of 9Phases of the Moon

1Source of light
  • Source of light the Moon only reflects sunlight, and exactly half of it is always lit.
2Cause of phases
  • Cause of phases we see more or less of the lit half as the Moon orbits the Earth.
3New moon (day 0)
  • New moon (day 0) the Moon is between the Earth and the Sun and the lit half faces away, so almost nothing is seen.
4Waxing (days 0 to 14)
  • Waxing (days 0 to 14) a thin crescent grows to first quarter (about day 7), then gibbous.
5Full moon (day 14)
  • Full moon (day 14) the Earth is between the Moon and the Sun and the whole face towards us is lit.
6Waning (days 14 to 29)
  • Waning (days 14 to 29) it shrinks through gibbous and last quarter (about day 21) to a crescent, then new moon again.

Part 7 of 9Length of the lunar cycle

1Phase cycle
  • Phase cycle one complete cycle takes about 29.5 days.
2Orbital period
  • Orbital period the Moon's orbit takes 27.3 days.
  • The phase cycle is longer because the Earth moves on round the Sun during one orbit.
  • The Moon must travel a little further to line up with the Sun again.
3Same face
  • Same face the Moon spins once on its axis in the time it takes to orbit, so the same face points to the Earth.

Part 8 of 9Eclipses

Solar eclipse
  • Solar eclipse the Moon passes directly between the Sun and the Earth and its shadow falls on part of the Earth.
  • People in the darkest part of the shadow see the Sun completely covered.
Lunar eclipse
  • Lunar eclipse the Earth lies between the Sun and the Moon and the Moon moves into the Earth's shadow.
  • The Moon turns dim and often red.
Rarity
  • Rarity they are not monthly because the Moon's orbit is tilted by about 5° to the Earth's orbital plane.
  • Most months the Moon passes just above or below the line joining the Sun and the Earth.

Part 9 of 9More from the specification

More from the specification
Elliptical orbits of periodic comets
  • Elliptical orbits of periodic comets gravitational pull from the Sun governs eccentric orbital paths of distant icy comets through interplanetary space.
Artificial satellite
  • A man-made object orbiting a body in space.
Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
Potential energy
  • Energy stored by the configuration of a system.
Planet
  • A large body that orbits the Sun and whose gravity clears its orbit.
Radius
  • Distance from the centre to the circular path.
Nebula
  • Cloud of gas and dust in which stars form.
Comet
  • An icy body on a highly elliptical orbit around the Sun.
Gravitational field
  • Region where mass experiences gravitational force.
Field strength
  • G = Fg/m.
Kinetic energy
  • K = ½mv2, energy of motion.
Orbital radius
  • Distance from the centre of the central body to the orbiting body.
Elliptical orbit
  • Elliptical orbit the Earth's orbit is slightly elliptical, but this does not cause the seasons;
    • the axial tilt does.

Key terms

TermMeaning
RotationSpinning of a body on its own axis; the Earth takes about 24 hours.
OrbitThe curved path of one body around another under the pull of gravity.
Axis tiltAbout 23.5° for the Earth; the cause of the seasons.
SatelliteA body that orbits a larger one; the Moon is the Earth's natural satellite.
PhasesThe changing lit shape of the Moon seen from Earth over about 29.5 days.
Solar eclipseThe Moon lies between the Sun and the Earth and casts its shadow on the Earth.
Lunar eclipseThe Moon passes into the Earth's shadow.

Avoid these mistakes

1
Seasons are caused by the Earth being closer to the Sun in summer — they are caused by the tilt, which changes day length and how steeply sunlight strikes.
2
Day and night are caused by the Earth orbiting the Sun — they are caused by rotation, and the orbit gives the year.
3
The phases are caused by the Earth's shadow on the Moon — that is a lunar eclipse, and the phases are our changing view of the lit half.
4
The Moon gives out its own light — it only reflects sunlight.

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