6.1Space Physics
Earth and The Solar System — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
15 min read 32 parts Lesson 1 of 24 Unit quiz
Lesson summary
6.1 Earth and The Solar System (Part 1) — IGCSE PHYSICS notes: Explain day and night, the Sun’s apparent motion, the seasons and the phases of the Moon.Rotation — the Earth spins on its axis once every 24 hours.Orbit — it orbits the Sun once every 365 days.Tilted axis — the axis …
- Apparent motion of the Sun
- Length of day
- Northern summer
- Northern winter
- Spring and autumn
- Misconception
Part 1 of 8Earth’s motion
Rotation
- Rotation the Earth spins on its axis once every 24 hours.
Orbit
- Orbit it orbits the Sun once every 365 days.
Tilted axis
- Tilted axis the axis runs through the North and South poles and is tilted about 23.5° from the vertical.
Part 2 of 8Day and night
Day and night
Cause
- Cause the rotation of the Earth makes one half of the surface face the Sun (day) while the other half faces away (night).
Apparent motion of the Sun
- Apparent motion of the Sun the Sun appears to rise in the east, reach its highest point at noon and set in the west, because of the Earth’s rotation.
Length of day
- Length of day about 12 hours near the equator and variable north and south of it.
Part 3 of 8The Sun’s apparent path through the year
1Equinoxes
- Equinoxes day and night are about equal and the Sun rises due east and sets due west.
2Summer
- Summer the Sun rises in the north-east and sets in the north-west in the northern hemisphere; it is higher in the sky, reaching its greatest height on the summer solstice, the longest day.
- (south-east and south-west in the southern)
3Winter
- Winter the Sun rises in the south-east and sets in the south-west in the northern hemisphere;
- it is lower, reaching its lowest height on the winter solstice, the shortest day.
Part 4 of 8The seasons
Cause
- Cause the tilt of the Earth’s axis together with its yearly orbit round the Sun.
- (which stays fixed)
Northern summer
- Northern summer the northern hemisphere tilts towards the Sun and receives more solar radiation, while the southern hemisphere has winter.
Northern winter
- Northern winter the northern hemisphere tilts away and receives less radiation, while the southern hemisphere has summer.
Spring and autumn
- Spring and autumn both hemispheres receive about the same amount of solar radiation.
Misconception
- Misconception the Earth is not closer to the Sun in summer;
- the orbit is only slightly elliptical and this does not cause the seasons.
Part 5 of 8Equinoxes, solstices and daylight hours (northern hemisphere)
| Date | Event | Daylight |
|---|---|---|
| 20 March | Spring equinox | Equal day and night; daylight increasing |
| 21 June | Summer solstice | Longest daylight |
| 23 September | Autumn equinox | Equal day and night; daylight decreasing |
| 21 December | Winter solstice | Shortest daylight |
- The southern hemisphere has the opposite season on each date.
Part 6 of 8The Moon and its phases
1
Orbit
- Orbit the Moon is a natural satellite orbiting the Earth about every 28 days and spins on its axis in the same time,
- so the same side always faces Earth.
2
Light
- Light it makes no light of its own and is seen by reflected sunlight; exactly half is always lit.
3
Day 0
- Day 0 new moon: the Moon is between the Earth and the Sun and none of the lit side is seen.
4
Day 7
- Day 7 first quarter; the Moon waxes through a crescent then gibbous shape.
5
Day 14
- Day 14 full moon: the Earth is between the Moon and the Sun and the whole face is lit.
6
Day 21
- Day 21 last quarter; the Moon wanes through gibbous and crescent shapes.
7
Day 29
- Day 29 new moon again and the cycle repeats.
Part 7 of 8Reasoning about the seasons and the Moon
1Equator
- Equator has roughly 12 hours of daylight all year because the tilt makes little difference there.
2Reverse seasons
- Reverse seasons if it is summer in Australia it is winter in Europe, which shows the tilt and not the distance causes the seasons.
3Moon phases
- Moon phases the same half of the Moon is always lit by the Sun, but from Earth we see different fractions of it as the Moon orbits.
4Calendar link
- Calendar link a month is based on the roughly 28 days the Moon takes to go through its phases.
Part 8 of 8More from the specification
More from the specification
Gravitational capture of eccentric comets
- Gravitational capture of eccentric comets gravitational attraction from the Sun maintains highly elliptical orbits of icy comets originating in the Oort cloud.
Gravitational field strength and orbital speed
- The Sun's intense gravitational field governs planetary motion,
- where orbital speed decreases as orbital radius from the Sun increases (v = 2πr / T).
Gravitational potential energy and orbital dynamics
- An orbiting object or planet possesses kinetic energy and gravitational potential energy;
- total mechanical energy remains constant along an unperturbed orbit.
Highly elliptical orbits of a comet
- A comet travels on an eccentric elliptical orbit;
- as it nears the Sun, gravitational field strength accelerates it to maximum speed while converting potential energy into kinetic energy.
Planetary characteristics and surface gravity
- Gravitational field strength (g) on the surface of a planet depends directly on its planetary mass and inversely on the square of its radius.
Gravitational field strength and orbital speed
- The Sun's intense gravitational field governs planetary motion,
- where orbital speed decreases as orbital radius from the Sun increases (v = 2πr / T).
Gravitational potential energy and orbital dynamics
- An orbiting object or planet possesses kinetic energy and gravitational potential energy;
- total mechanical energy remains constant along an unperturbed orbit.
Highly elliptical orbits of a comet
- A comet travels on an eccentric elliptical orbit;
- as it nears the Sun, gravitational field strength accelerates it to maximum speed while converting potential energy into kinetic energy.
Planetary characteristics and surface gravity
- Gravitational field strength (g) on the surface of a planet depends directly on its planetary mass and inversely on the square of its radius.
Comet
- An icy body on a highly elliptical orbit around the Sun.
Gravitational field
- Region where mass experiences gravitational force.
Orbital speed
- Speed of an orbiting object, √(GM/r).
Field strength
- G = Fg/m.
Kinetic energy
- K = ½mv2, energy of motion.
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.
Period
- Time for one revolution.
Nebula
- Cloud of gas and dust in which stars form.
Key terms
| Term | Meaning |
|---|---|
| Axis | The line through the North and South poles about which the Earth spins. |
| Equinox | A day with about equal day and night. |
| Solstice | The day with the longest or shortest daylight hours. |
| Natural satellite | An object that orbits a planet, such as the Moon. |
| Waxing | Appearing to grow brighter as more of the lit side is seen. |
| Waning | Appearing to grow dimmer as less of the lit side is seen. |
Avoid these mistakes
1
Saying seasons are caused by the Earth’s distance from the Sun — they are caused by the tilt of the axis.
2
Thinking the Moon makes its own light — it only reflects sunlight.
3
Confusing rotation (day and night) with orbit (year).
4
Saying the Moon’s phases are caused by the Earth’s shadow — they depend on the view of its lit half.
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