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6.1.1Space Physics

The Earth — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 2 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

6.1.1 The Earth — IGCSE Physics Notes — IGCSE PHYSICS notes: Describe the Earth's rotation, orbit and tilt, and use them to explain day and night, the Sun's apparent motion, the year and the seasons.Rotation — the Earth spins on its axis once in about 24 hours.The axis is an imag…

  • Fixed tilt
  • Daily path
  • Day length
  • Summer in the northern hemisphere
  • Winter in the northern hemisphere
  • Southern hemisphere

Part 1 of 8Earth's two motions

Rotation
  • Rotation the Earth spins on its axis once in about 24 hours.
  • The axis is an imaginary line through the North and South poles.
  • The axis is tilted at about 23.5° from the vertical.
Orbit
  • Orbit the Earth travels once round the Sun in about 365 days (one year).
  • The extra quarter day in each year is why a leap year of 366 days occurs every fourth year.
Fixed tilt
  • Fixed tilt the axis keeps the same tilt and direction in space throughout the orbit.

Part 2 of 8Day and night

Cause
  • Cause day and night are caused by the Earth's rotation on its axis, not by its orbit.
Half of the Earth always faces the Sun and has day.
  • The half facing away from the Sun has night.
  • One full rotation takes about 24 hours, so every place has one day and one night in that time.

Part 3 of 8Apparent motion of the Sun

1Daily path
  • Daily path the Sun appears to rise in the east, reach its highest point at noon and set in the west.
  • This apparent motion is caused by the Earth rotating, not by the Sun moving.
2Day length
  • Day length the number of hours of sunlight at a place, from sunrise to sunset.
  • Near the equator it is about 12 hours all year.
  • North and south of the equator it varies through the year.
3Equinoxes
  • Equinoxes day and night are about equal in length and the Sun rises due east and sets due west.
4Summer in the northern hemisphere
  • Summer in the northern hemisphere the Sun rises in the north-east, sets in the north-west and climbs higher above the horizon.
  • The summer solstice has the longest daylight and the greatest height of the Sun.
5Winter in the northern hemisphere
  • Winter in the northern hemisphere the Sun rises in the south-east, sets in the south-west and stays closer to the horizon.
  • The winter solstice has the shortest daylight.
6Southern hemisphere
  • Southern hemisphere the directions of rising and setting are mirrored in the opposite seasons.

Part 4 of 8Cause of the seasons

1Basis
  • Basis seasons are caused by the Earth's tilted axis together with its orbit round the Sun.
2Hemisphere tilted towards the Sun
  • Hemisphere tilted towards the Sun it receives more solar radiation and has summer.
  • The Sun is higher in the sky and the days are longer.
3Hemisphere tilted away from the Sun
  • Hemisphere tilted away from the Sun it receives less solar radiation and has winter.
  • The Sun is lower in the sky and the days are shorter.
4Opposite seasons
  • Opposite seasons when it is summer in the northern hemisphere it is winter in the southern hemisphere.
5Spring and autumn
  • Spring and autumn both hemispheres receive about the same amount of solar radiation.
6Six months later
  • Six months later the hemispheres tilt in the opposite directions as the Earth reaches the other side of its orbit.

Part 5 of 8Equinoxes and solstices

DateNorthern hemisphereSouthern hemisphereDaylight
20 MarchSpring equinoxAutumn equinoxEqual day and night
21 JuneSummer solsticeWinter solsticeLongest in north, shortest in south
23 SeptemberAutumn equinoxSpring equinoxEqual day and night
21 DecemberWinter solsticeSummer solsticeShortest in north, longest in south

Part 6 of 8Moon's orbit and phases

1
Moon
  • Moon a natural satellite that orbits the Earth about once a month and turns once on its axis in the same time,
    • so the same side always faces the Earth.
2
Light
  • Light the Moon makes no light and is seen by reflecting sunlight.
3
Phases
  • Phases half of the Moon is always lit, but we see different amounts of the lit half as it orbits.
  • New moonthe Moon is between the Earth and the Sun and none of the lit side is visible.
  • Full moonthe Earth is between the Moon and the Sun and the whole face is lit.
  • Between them the lit part grows (waxes) through crescent, first quarter and gibbous, then shrinks (wanes) through last quarter.

Part 7 of 8Orbital speed of the Earth (Extended)

Equation
  • Equation average orbital speed v = 2πr / T for a near-circular orbit.
  • v is the orbital speed in m/s.
  • r is the orbital radius in m, measured from the centre of the Sun.
  • T is the orbital period in s, so one year is about 3.15 × 107 s.
Meaning
  • Meaning in one orbit the Earth travels a distance equal to the circumference 2πr.

Part 8 of 8More from the specification

1
Gravitational orbits of icy comets
  • Gravitational orbits of icy comets highly eccentric elliptical trajectories carry periodic comets from the outer solar system inward around the Sun.
2
Solar system structure
  • Planets orbit the Sun in elliptical paths held by its gravitational field, and the Moon orbits the Earth.
3
Gravitational field strength
  • Stronger for more massive planets and decreasing with distance;
    • it sets the weight of an object on a planet's surface.
4
Orbital speed
  • A planet moves faster when nearer the Sun in its elliptical orbit, because gravitational potential energy converts to kinetic energy.
5
Day, year and seasons
  • Rotation of the Earth about its tilted axis gives day and night, and its orbit with the tilt gives seasons.
6
Moon phases
  • Phases depend on how much of the Moon's sunlit surface is visible from Earth as it orbits.
7
Solar system
  • A star with the objects that orbit it.
8
Gravitational field
  • Region where mass experiences gravitational force.
9
Field strength
  • G = Fg/m.
10
Kinetic energy
  • K = ½mv2, energy of motion.
11
Artificial satellite
  • A man-made object orbiting a body in space.
12
Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
13
Potential energy
  • Energy stored by the configuration of a system.
14
Planet
  • A large body that orbits the Sun and whose gravity clears its orbit.
15
Nebula
  • Cloud of gas and dust in which stars form.
16
Comet
  • An icy body on a highly elliptical orbit around the Sun.

Key terms

TermMeaning
RotationSpinning of the Earth on its axis, once in about 24 hours.
OrbitThe curved path of one body around another, such as the Earth round the Sun in a year.
AxisImaginary line through the poles, tilted at about 23.5° on the Earth.
EquinoxA time of year when day and night are about equal in length.
SolsticeThe day with the longest or shortest hours of daylight in a year.
HemisphereHalf of the Earth, northern or southern.
Natural satelliteA body, such as the Moon, that orbits a planet.

Avoid these mistakes

1
Seasons are caused by the Earth being nearer the Sun in summer — they are caused by the tilt of the axis, because the orbit is almost circular.
2
Day and night are caused by the Earth orbiting the Sun — they are caused by its rotation, and the orbit gives the year.
3
The Sun moves across the sky — the Earth's rotation makes the Sun appear to move.
4
Using T in days or years in v = 2πr/T — convert T to seconds and r to metres first.

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