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

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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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

Section 1 of 32Earth’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.

Section 2 of 32Day 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.

Section 3 of 32The 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.

Section 4 of 32The 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.

Section 5 of 32Equinoxes, 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.

Section 6 of 32The Moon and its phases

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.

Light

Light it makes no light of its own and is seen by reflected sunlight; exactly half is always lit.

Day 0

Day 0 new moon: the Moon is between the Earth and the Sun and none of the lit side is seen.

Day 7

Day 7 first quarter; the Moon waxes through a crescent then gibbous shape.

Day 14

Day 14 full moon: the Earth is between the Moon and the Sun and the whole face is lit.

Day 21

Day 21 last quarter; the Moon wanes through gibbous and crescent shapes.

Day 29

Day 29 new moon again and the cycle repeats.

Section 7 of 32Reasoning 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.

Section 8 of 32More from the specification

Section 9 of 32Contents of the Solar System

Contents of the Solar System

Sun

Sun the only star in the system.

Eight planets

Eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The mnemonic "My Very Excellent Mother Just Served Us Noodles" helps recall them.

Other bodies

Other bodies dwarf planets, natural and artificial satellites, asteroids and comets.

Two groups of planets

Two groups of planets the inner rocky planets and the outer gas giants.

Section 10 of 32Dwarf planets and satellites

Dwarf planet

Dwarf planet an object like a planet but much smaller, whose gravity is not strong enough to pull in nearby objects.

Natural satellite

Natural satellite an object that orbits a planet; a moon is a natural satellite.

Artificial satellite

Artificial satellite a man-made object orbiting another body, such as the International Space Station orbiting Earth.

Section 11 of 32Asteroids and comets

Asteroid

Asteroid a small rocky object orbiting the Sun, mostly in the asteroid belt between Mars and Jupiter.

Comet

Comet an object of dust and ice on a highly elliptical orbit, usually in the outer Solar System.

The ice melts as it nears the Sun and forms the comet’s tail.

Section 12 of 32What orbits what

Orbiting body	Body it orbits
Planet	The Sun
Moon	Planet
Comet	The Sun
Asteroid	The Sun
Artificial satellite	The Earth

Smaller bodies orbit larger bodies.

Section 13 of 32Planetary data (Extended)

Planet	Distance from Sun (Earth = 1)	Orbital period (years)	Mean surface temp (°C)	Density (kg/m³)	Diameter (10³ km)	Mass (Earth = 1)	Surface gravity (N/kg)
Mercury	0.39	0.24	350	5429	4.9	0.06	3.7
Venus	0.72	0.60	460	5243	12.1	0.82	8.9
Earth	1	1	20	5514	12.8	1	9.8
Mars	1.5	2	−23	3934	6.8	0.11	3.7
Jupiter	5.2	12	−120	1326	143	320	23.1
Saturn	9.6	30	−180	687	121	95	9.0
Uranus	19	84	−210	1270	51	15	8.7
Neptune	30	160	−220	1638	50	17	11.0

Section 14 of 32Analysing the data

Comparison

Neptune is 30 times further from the Sun than Earth; Jupiter has the mass of 320 Earths.

Trend

Trend the further a planet is from the Sun, the longer its orbital period.

Anomaly

Anomaly temperature falls with distance, except that Venus (460 °C) is hotter than Mercury.

Prediction

Prediction a dwarf planet in the asteroid belt would be around −100 °C,
- but anywhere between about −63 °C and −130 °C, the values for Mars and Jupiter.

Section 15 of 32Inner and outer planets compared

Inner planets

Mercury, Venus, Earth and Mars are small and rocky, and have atmospheres.
- (apart from Mercury)

Outer planets

Jupiter, Saturn, Uranus and Neptune are large and gaseous, made mostly of hydrogen and helium.

Why they differ

Why they differ the accretion model explains this difference;
- see the lesson on formation of the Solar System.

Section 16 of 32More from the specification

1Earth's Axial Tilt and Seasonal Daylight

Earth rotates on its tilted axis once every 24 hours;
when the northern hemisphere is tilted toward the Sun, it receives solar energy at steeper angles and experiences longer daylight hours,
resulting in summer.

2Gravitational Field Strength and Orbits

The Sun's massive gravitational field strength holds planets in elliptical orbits,
where orbital speed v = 2πr / T depends on the orbital radius r and orbital period T.

3Astronomical Distances and the Speed of Light

Because distances across the Solar System and interstellar space are vast, astronomical distances are measured using the finite speed of light (3.0 × 10⁸ m/s), such that sunlight takes over eight minutes to reach Earth.

4Nebula

Cloud of gas and dust in which stars form.

5Gravitational field

Region where mass experiences gravitational force.

6Orbital speed

Speed of an orbiting object, √(GM/r).

7Field strength

G = F_g/m.

8Kinetic energy

K = ½mv², energy of motion.

9Gravitational potential

Work done per unit mass in bringing a test mass from infinity to the point.

10Potential energy

Energy stored by the configuration of a system.

11Hemisphere

Half of the Earth, northern or southern.

12Radius

Distance from the centre to the circular path.

Section 17 of 32Inner and outer planets compared

Feature	Inner planets (Mercury, Venus, Earth, Mars)	Outer planets (Jupiter, Saturn, Uranus, Neptune)
Size	Small	Large
Composition	Rocky, high melting point materials such as iron	Mostly hydrogen and helium gas, also water and methane
Atmosphere	Present except Mercury	Thick gaseous planets
Formed	Hot region near the Sun	Cool region far from the Sun

Section 18 of 32The accretion model

Nebula

Nebula the Sun and planets formed from a cloud of gas and dust about 4.5 billion years ago;
- it contained elements made in an earlier supernova.

Gravitational collapse

Gravitational collapse gravity pulled the matter together;
- as it collapsed it spun faster, became hotter and formed a rotating accretion disc.

Accretion

Accretion attractive gravitational forces made particles join together and grow into larger objects.

Sun and planets

Sun and planets the Sun formed at the centre and the planets from remnants in the disc.

Section 19 of 32Why the inner and outer planets differ

Inner region

Inner region too hot for light elements to be solid, so only materials with high melting points such as metals formed solids.

The nebula had few heavy elements, so the inner planets stayed small and rocky.

Outer region

Outer region cool enough for light molecules like hydrogen, helium, water and methane to be solid or gas that gathered.

The nebula had many light elements, so the outer planets became exceptionally large and gaseous.

Heating of the young Sun

Heating of the young Sun it pushed gaseous matter further out than solid matter.

Section 20 of 32Light travel time

Speed of light

Speed of light light is an electromagnetic wave travelling at 3.0 × 10⁸ m/s.

Equation

Equation time = distance / speed of light.

Examples

Examples light takes 8 minutes from the Sun to Earth, about 5 hours to the outer Solar System and 4 years from the nearest other star.

Meaning

Meaning we see the Sun as it was 8 minutes ago.

Section 21 of 32Gravitational field strength

Depends on

Depends on the mass of the planet and the distance from it.

Mass

Mass a greater planet mass gives a stronger field at its surface;
- g varies from planet to planet with mass and radius.

Distance

Distance g decreases as the distance from the planet increases, but is constant at the surface.

Section 22 of 32Gravity and orbits

Orbit

Orbit the Sun contains more than 99% of the Solar System’s mass, so its gravitational attraction keeps objects in orbit, acting towards its centre.

Further away (Extended)

Further away (Extended) the Sun’s field is weaker, the centripetal force is weaker and the orbital speed falls, so the orbital period is longer.

Data

Mercury orbits at 47.9 km/s with a period of 88 days;
- Earth at 29.8 km/s in 365 days;
- Neptune at 5.4 km/s in 165 years.

Wording

Wording say "gravitational attraction" or "strength of the Sun’s gravitational field", not "the Sun’s gravity".

Section 23 of 32More from the specification

Planetary accretion and icy comets

Planetary accretion and icy comets gravitational coalescing of protoplanetary dust left rocky asteroids and distant Kuiper belt comets in planetary orbits.

Seasons and Axial Tilt

The Earth's rotational axis is tilted at 23.5°;
when the Northern Hemisphere is tilted towards the Sun, it experiences summer with longer daylight hours and concentrated solar energy.

Day and Night Cycle

The diurnal rotation of Earth about its axis once every 24 hours creates the cycle of day and night as different longitudes rotate into and out of sunlight.

Planetary and Comet Orbits

While planets follow nearly circular orbits around the Sun, a comet travels in an elongated, highly elliptical orbit that carries it far out into the solar system.

Comet Kinetic and Gravitational Energy

As a comet approaches perihelion along its elliptical path, gravitational potential energy is converted into kinetic energy, dramatically increasing its orbital speed.

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.

Hemisphere

Half of the Earth, northern or southern.

Comet

An icy body on a highly elliptical orbit around the Sun.

Elliptical orbit

Orbit with constant E and L but varying speed.

Section 24 of 32The orbital speed equation (Extended)

Idea

Idea in one orbit a body travels the circumference of its circular path, 2πr.

Equation

Equation v = 2πr / T.
What the symbols mean: v = speed (m s⁻¹) · ω = angular speed (rad s⁻¹) · r = radius (m) · T = period (s)

v is orbital speed in m/s, r is the orbital radius in m and T is the orbital period in s.

Orbital period

Orbital period the time taken to complete one orbit.

Orbital radius

Orbital radius measured from the centre of the object being orbited to the orbiting object.

Section 25 of 32Method for orbital speed calculations

Find the orbital radius r: if given a height h above the surface, add the radius R of the body (r = R + h).

Convert the radius to metres and the period to seconds.

Substitute into v = 2πr / T.

Round to a sensible number of significant figures and give the unit m/s.

Section 26 of 32Elliptical orbits (Extended)

Ellipse

Ellipse a squashed circle.

Planets

Planets have slightly elliptical orbits that are nearly circular, so the Sun is close to the centre.

Comets

Comets have highly elliptical orbits and the Sun is at one of the two foci, not at the centre.

Calculations

Calculations you are not asked to calculate with elliptical orbits;
- assume circular orbits for any speed, period or radius calculation.

Section 27 of 32Speed of a comet

Approaching the Sun

Approaching the Sun the orbital radius decreases and the speed increases because of the Sun’s stronger gravitational pull.

Moving away

Moving away the radius increases and the speed decreases because the pull is weaker.

Stable orbit

Stable orbit it stays stable although the speed and radius change continually.

Section 28 of 32Energy in elliptical orbits

Conservation

Conservation energy is conserved, with gravitational potential energy transferred to kinetic energy and back.

Closer to the Sun

Closer to the Sun less gravitational potential energy and more kinetic energy, so it moves faster.

Further from the Sun

Further from the Sun more gravitational potential energy and less kinetic energy, so it moves slower.

Link

Link kinetic energy = ½mv², so a higher speed means more kinetic energy.

Section 29 of 32Comets and conservation of energy

Total energy

Total energy the sum of kinetic and gravitational potential energy stays constant.

Nearest point

Nearest point kinetic energy is greatest and gravitational potential energy is least.

Furthest point

Furthest point kinetic energy is least and gravitational potential energy is greatest.

Section 30 of 32More from the specification

1Axial Tilt and Seasonal Variations

The Earth's rotational axis is tilted at 23.5° relative to its orbital plane around the Sun,
causing the northern and southern hemispheres to receive differing intensities of solar radiation.

2Daylight Hours and Summer/Winter

During northern hemisphere summer, the North Pole tilts toward the Sun,
producing longer daylight hours and shorter nights;
- during winter, daylight hours decrease as the hemisphere tilts away.

3Sun's Gravitational Field Strength

The gravitational attraction between the Sun and planets depends on solar mass and orbital distance (F = GMm/r²), with gravitational field strength decreasing with the square of distance.

4Orbital Speed and Planetary Periods

A planet's orbital speed is calculated using v = 2πr / T;
- planets at greater distances from the Sun experience weaker gravitational fields and travel slower,
resulting in longer orbital periods.

5Gravitational field

Region where mass experiences gravitational force.

6Field strength

G = F_g/m.

7Artificial satellite

A man-made object orbiting a body in space.

8Distance

Total length of path travelled, a scalar.

9Hemisphere

Half of the Earth, northern or southern.

10Nebula

Cloud of gas and dust in which stars form.

Section 31 of 32Key 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.

Term	Meaning
Dwarf planet	A small planet-like body that cannot pull in nearby objects.
Asteroid	A small rocky body orbiting the Sun.
Comet	A body of dust and ice on a highly elliptical orbit round the Sun.
Satellite	An object that orbits a larger body.
Orbital period	The time taken to complete one orbit.
Gas giant	A large planet made mainly of hydrogen and helium.

Term	Meaning
Nebula	A cloud of gas and dust in space.
Accretion	Particles joining together under gravity to form larger objects.
Accretion disc	The rotating disc from which the Sun and planets formed.
Speed of light	3.0 × 10^{8} m/s.
Gravitational field strength	The force per unit mass in a gravitational field, in N/kg.
Centripetal force	The force towards the centre needed to keep an object in a circular orbit.

Term	Meaning
Orbital speed	The speed at which an object travels along its orbit.
Orbital period	The time taken to complete one orbit.
Ellipse	A squashed circle shape.
Focus	One of the two special points inside an ellipse.
Gravitational potential energy	Energy stored due to position in a gravitational field.
Circumference	The distance around a circle, 2πr.

Section 32 of 32Exam technique

Saying seasons are caused by the Earth’s distance from the Sun — they are caused by the tilt of the axis.

Thinking the Moon makes its own light — it only reflects sunlight.

Confusing rotation (day and night) with orbit (year).

Saying the Moon’s phases are caused by the Earth’s shadow — they depend on the view of its lit half.

Calling the Moon a planet — it is a natural satellite.

Saying comets orbit in circles — their orbits are highly elliptical.

Placing the asteroid belt beyond Jupiter — it lies between Mars and Jupiter.

Saying all planets get colder with distance — Venus is hotter than Mercury, an anomaly.

Saying planets formed from the Sun’s ejected matter — they formed from the leftover nebula.

Forgetting that light has a finite speed — it takes time to cross space.

Writing "the Sun’s gravity" — write the Sun’s gravitational attraction or the strength of its gravitational field.

Thinking g is the same on every planet — it depends on mass and radius.

Using the height above the surface as the orbital radius — add the planet’s radius.

Using minutes instead of seconds, or km instead of m — convert T to s and r to m.

Putting the Sun at the centre of a comet orbit — it is at a focus.

Saying a comet moves at constant speed — it is fastest closest to the Sun.

Last updated: 15 October 2026