Day, Night and the Seasons: KS3 Physics Explained

Day and night result from Earth rotating on its axis every 24 hours — whichever side faces the Sun experiences day, the other experiences night. The seasons arise because Earth's axis tilts at 23.5°, so the Northern and Southern hemispheres receive sunlight at different intensities and for different lengths of time as Earth orbits the Sun each year.

What causes day and night?

The Earth rotates (spins) on its own axis — an imaginary line running from the North Pole to the South Pole — once every approximately 24 hours (one solar day). As it rotates:

The side of Earth facing the Sun is lit up — this is day.

The side facing away from the Sun is in shadow — this is night.

The Sun does not move around Earth (as it appears to from the ground). Instead, Earth rotates towards the east, which is why the Sun appears to rise in the east each morning and set in the west each evening.

The boundary between the lit and unlit halves of Earth is called the terminator line. On the ground, you pass through it at sunrise and sunset.

At the equator, day and night are approximately equal in length (12 hours each) all year round, because the equator is always roughly in the middle of Earth's lit face. At higher latitudes (closer to the poles), day length varies much more dramatically with the season.

What causes the seasons?

The seasons are not caused by Earth being closer to or further from the Sun. In fact, Earth is actually slightly closer to the Sun in January (Northern Hemisphere winter) than in July. The real cause is Earth's axial tilt.

Earth's axis is tilted at approximately 23.5° from the vertical (relative to its orbital plane around the Sun). As Earth orbits the Sun over one year, this tilt means:

For part of the year, the Northern Hemisphere is tilted towards the Sun → it receives sunlight at a higher angle, for longer each day → summer in the north.

Six months later, the Northern Hemisphere is tilted away from the Sun → sunlight hits at a lower angle, for fewer hours each day → winter in the north.

The Southern Hemisphere experiences the opposite seasons at the same time.

Why does the angle of sunlight matter?

The angle at which sunlight strikes the surface controls how concentrated the energy is:

When sunlight hits at a high angle (near 90°, directly overhead), the energy is concentrated over a small area → more intense heating → summer.

When sunlight hits at a low angle, the same amount of energy is spread over a larger area → less intense heating → winter.

Imagine shining a torch straight down onto a table versus shining it at a shallow angle — at a shallow angle, the same beam of light covers a much larger, dimmer patch.

Factor | Summer (Northern Hemisphere) | Winter (Northern Hemisphere)
Axial tilt direction | Northern Hemisphere tilted toward Sun | Northern Hemisphere tilted away from Sun
Angle of sunlight | High (more concentrated energy) | Low (energy spread over larger area)
Day length | Longer (more hours of daylight) | Shorter (fewer hours of daylight)
Average temperature | Higher | Lower

What are the solstices and equinoxes?

Event | Approximate date | What happens
Summer solstice (Northern Hemisphere) | 21 June | Longest day; Sun reaches its highest point; North Pole in continuous daylight
Winter solstice (Northern Hemisphere) | 21 December | Shortest day; Sun reaches its lowest point; North Pole in continuous darkness
Spring (vernal) equinox | 20–21 March | Day and night equal length everywhere on Earth
Autumn equinox | 22–23 September | Day and night equal length everywhere on Earth

At the equinoxes, Earth's axis points neither towards nor away from the Sun, so the equator faces the Sun directly and all latitudes get approximately 12 hours of daylight.

What is the difference between Earth's rotation and revolution?

These two terms are often confused:

Rotation — Earth spinning on its own axis. One rotation = 1 day (24 hours). Causes day and night.

Revolution (or orbit) — Earth travelling around the Sun in its elliptical orbit. One revolution = 1 year (~365.25 days). Together with axial tilt, this causes the seasons.

The 0.25 extra days per year is why we add a leap day (29 February) every four years — to keep our calendar aligned with Earth's actual orbital period.

How do different places experience the seasons?

UK (mid-latitudes ~51° N): Distinct four seasons; summer days can exceed 16 hours of daylight; winter days can be under 8 hours.

Equator (~0°): Little seasonal variation; approximately 12 hours daylight all year; always warm.

Arctic/Antarctic Circle (~66.5° N/S): Experience midnight sun in summer (24 hours daylight) and polar night in winter (24 hours darkness).

Frequently asked questions

Why is it hotter in summer if Earth is not closer to the Sun?

It is hotter in summer because Earth's axial tilt means the Northern Hemisphere is tilted towards the Sun, so sunlight strikes at a steeper angle and is more concentrated. Days are also longer, giving more hours of heating. Earth is actually slightly closer to the Sun in January (Northern Hemisphere winter), which proves that distance is not the main factor.

Why do day and night last different lengths at different times of year?

Because Earth's axis is tilted, the amount of each hemisphere that faces the Sun changes throughout the year. In summer, the Northern Hemisphere is tilted toward the Sun, so a greater proportion of the hemisphere is lit for longer — giving longer days. In winter, the Northern Hemisphere is tilted away, giving shorter days and longer nights.

What is the summer solstice?

The summer solstice (around 21 June in the Northern Hemisphere) is the day when the North Pole is tilted most directly towards the Sun. It is the longest day of the year in the Northern Hemisphere — the Sun rises earliest, sets latest, and reaches its highest point in the sky at noon. The South Pole, meanwhile, experiences its winter solstice (shortest day) at the same moment.

Why do the Southern and Northern hemispheres have opposite seasons?

When the Northern Hemisphere is tilted towards the Sun (June), it receives more intense sunlight and experiences summer. At the same time, the Southern Hemisphere is tilted away and experiences winter. Six months later (December), Earth has orbited to the other side of the Sun, so the Southern Hemisphere is now tilted towards the Sun and has summer while the north has winter.

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