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Deep dive · Geography

The motions of the Earth: day, night and the seasons

The ground under your feet never stops moving. The Earth spins on its axis like a top, and all the while it travels round the Sun. The spin gives us day and night; the journey gives us the seasons and the year. Here you will spin the Earth, tilt it and send it round the Sun yourself, and see how it all works.

By the end you will be able to explain

  1. why we have day and night, and why morning reaches Arunachal Pradesh before Gujarat
  2. how the Earth’s tilt makes the seasons, and why distance from the Sun does not
  3. why days are long in June and short in December, and how the poles get six months of daylight
  4. what would happen if the tilt or the direction of spin changed
  5. why February gets a 29th day every fourth year, but not every hundredth

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1Rotation: day and night

The Earth spins on its axis from west to east, and one turn takes about 24 hours. This is called rotation. Sunlight can only fall on one half of the Earth at a time: that half has day, the other half has night. Because the Earth keeps turning, every place moves into the light in its turn and then out into the dark again.

Spin the Earth

Drag the globe with a finger or the mouse, or let it run. The Sun is on the right. Along the upright line through the middle it is sunrise right now, and at the right-hand edge it is noon.

PlaceLocal timeLength of daySpin speed
Jaipur
Delhi
Tokyo
London
New York
Sydney

The picture shows the Earth around 21 June, when the North Pole leans towards the Sun, so the day lengths are for that date too; change the tilt and they change. Local time here goes by the Sun: 12 o’clock is when the Sun stands highest in the sky. Clocks across a country keep one standard time, so a clock can run a little ahead of or behind this.

23½° and 66½°: one tilt, two names

The Earth’s axis does not stand straight. It leans 23½° away from the line perpendicular to its orbital plane, which means it makes an angle of 66½° with the plane itself. The two add up to 90°, so they say the same thing. Turn on “Angles and lines” above to see both. Measured precisely today, the tilt is 23.44°.

The circle of illumination

The circle on the globe that divides day from night is called the circle of illumination. Notice that it does not follow the axis. The axis is tilted, but the edge of the light always runs straight across the Sun’s rays. That is why in June more of the north stays in the light, and days there are long.

15° = 1 hour: India’s standard time

The Earth turns 360° in 24 hours: 15° every hour, 1° every 4 minutes. So the Sun rises earlier in places to the east and later in places to the west.

India stretches from 68°7′E in Gujarat in the west to 97°25′E in Arunachal Pradesh in the east. Those 29° or so make a difference of nearly two hours in Sun time: morning comes to the eastern edge of Arunachal roughly two hours before it reaches the western edge of Gujarat.

Look at Rajasthan alone: Jaipur and Jaisalmer sit at almost the same latitude, but Jaisalmer is about 5° further west. The noon Sun stands highest there about 20 minutes after it does in Jaipur.

If every town kept its own time, trains and offices would be in a muddle. So the whole country keeps one time, set by the 82°30′E meridian, which passes near Mirzapur in Uttar Pradesh. This is Indian Standard Time, 5 hours 30 minutes ahead of Greenwich.

So fast, so why can’t we feel it?

Someone standing on the Equator is carried round at about 1,670 kilometres an hour, and someone in Jaipur at about 1,490. We feel nothing because the speed never changes, and the air, the water and the houses all move along with us. In a train running smoothly at a steady speed, you cannot tell whether it is moving or standing still unless you look out of the window.

The proof is in the sky: the Sun, the Moon and the stars rise in the east and set in the west. In 1851, in Paris, Léon Foucault hung a long pendulum and showed that the direction of its swing slowly turns hour by hour, because the ground beneath it is turning.

A 24-hour day, but a slightly shorter turn

Measured against the stars, the Earth completes one full turn in 23 hours 56 minutes 4 seconds. But in that time it also moves about 1° along its orbit, so it must turn about 4 minutes more to bring the Sun back to the same place in the sky. Our 24-hour clock day is the average of this solar day. The period of one rotation is also called an Earth day.

What if the Earth stopped spinning?

It is often said that half the Earth would then have day for ever and the other half night for ever. That would be true only if the same face of the Earth always looked at the Sun, the way the same face of the Moon always looks at us. If the Earth truly stopped turning, its journey round the Sun would still carry every place slowly from light into dark, and one day would last a whole year: about six months of light and six of night. Either way, one side would bake and the other freeze, and life would be very hard.

Checkpoint

  1. 1In which direction does the Earth spin?

  2. 2Roughly how far apart in Sun time are the eastern edge of Arunachal Pradesh and the western edge of Gujarat?

  3. 3What is the circle of illumination?

  4. 4Which meridian sets Indian Standard Time?

2Revolution and the seasons

While it spins, the Earth also travels round the Sun along a fixed path. This is called revolution, and the path is its orbit. One trip takes 365 days and about 6 hours. All the way round, the axis keeps leaning in the same direction, with the North Pole pointing towards the Pole Star. So for part of the year the Northern Hemisphere leans towards the Sun, and for the other part the Southern Hemisphere does. That is what makes the seasons.

A year’s journey

Tap one of the four special days, or slide the bar to any date in the year. The top picture shows the Earth in its orbit; the one below shows where the Sun’s rays fall straight down on that day.

Not to scale: the Sun and the Earth are really tiny next to the distance between them, and the orbit is almost a circle. Dates and distances are for 2026.

Where do the straight rays fall?

Here we look at the Earth from the side, with the Sun on the left. The tilt stays the same all year, but seen from the side the Earth seems to lean sometimes towards the Sun and sometimes away. The two yellow bands carry the same amount of light: the direct one lands on a small patch, the slanting one spreads over a bigger one.

Direct rays at
Distance from the Sun
Speed in orbit
Northern Hemisphere
Southern Hemisphere
Near the poles

These are astronomical seasons, set only by where the Sun is. India’s own summer, rains and winter also depend heavily on the monsoon, so they do not line up exactly with these.

Exam answers usually give the equinoxes as 21 March and 23 September and the solstices as 21 June and 22 December. The exact moment shifts by a few hours each year. In 2026 the moments fall at these times in IST: 20 March 8:16 pm, 21 June 1:55 pm, 23 September 5:36 am and 22 December 2:20 am.

On that day, in one place

Choose a place. For the date picked above, see how long the day is there, and how long a shadow a straight 1-metre stick casts at noon.

Length of day
Height of the noon Sun
Shadow of a 1 m stick

This count leaves out the bending of light by the air, so a real day is a few minutes longer. Every place between the two tropics gets two days a year when the noon shadow all but disappears. Choose Chennai, slide the date and find them yourself.

Winter is not because the Sun is far away

Many people think summer comes when the Earth is close to the Sun. But the Earth is closest to the Sun in early January (3 January in 2026, about 147.1 million km) and farthest in early July (6 July, about 152.1 million km). So the Sun is at its nearest during our coldest weather! The distance changes by only about 3%. The real cause of the seasons is the tilt: the hemisphere where the rays fall straight and the days are long has summer.

Why straight sunlight heats more

Why does the noon Sun feel hotter than the morning Sun? Shine a torch straight at a wall and you get a small, bright circle; tilt it and the same light spreads into a big, faint patch. Sunlight works the same way. Direct rays put more heat on less ground and pass through less air, while slanting rays spread the same heat over more ground.

The northern summer half is longer

Around July the Earth is farthest from the Sun and moves a little slower in its orbit (about 29.3 km a second), while around January it moves fastest (about 30.3). So from the March equinox to the September equinox, while the Northern Hemisphere leans towards the Sun, about 186 days pass, and only about 179 in the other half.

Six months of day at the poles

At the North Pole the Sun rises around the March equinox and does not set until the September equinox; then comes about six months of night. At the South Pole it is the other way round. India’s research stations in Antarctica, Maitri and Bharati, lie some way from the pole, so their winter night is far shorter than six months. At Maitri the Sun stays below the horizon for about two months, from late May to late July; Bharati lies a little further north, so its night lasts about seven to eight weeks.

The Tropic of Cancer runs through India

The Tropic of Cancer crosses eight Indian states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. In Rajasthan it passes through Banswara and Dungarpur districts in the south. Around 21 June the noon Sun stands straight overhead on this line, and every place in India south of it gets two such days a year.

Makar Sankranti and 22 December

The December solstice falls on 21 or 22 December. That day the Sun’s direct rays are at their southernmost, on the Tropic of Capricorn, and the Northern Hemisphere has its shortest day. Makar Sankranti is celebrated on 14 or 15 January; in the Indian calendar it is the day the Sun enters Makar rashi. The two are counted in different ways, so their dates differ.

Even at the equinox, day runs a little long

At an equinox the direct rays fall on the Equator and neither pole leans towards the Sun, so day and night are about equal all over the Earth. “About”, because the air bends light a little and the whole disc of the Sun takes time to set. That is why even on an equinox, daylight lasts a few minutes over 12 hours.

Checkpoint

  1. 1When is the Earth closest to the Sun?

  2. 2Where do the Sun’s direct rays fall on 21 June?

  3. 3Why is December summer in Australia?

3What if?

Now change the Earth’s rules yourself. Raise or lower the tilt, move the Earth to another point in its orbit, or reverse its spin, and see what happens to day, night and the seasons.

Build your own Earth

Place in orbit

Direction of spin

What would happen

Try these three

  1. Get rid of the seasonsSet the tilt to 0° and see how day and night are shared out.
  2. Bring summer to the southKeep the real tilt of 23½° and move the Earth to its December position, so the direct rays fall on the Tropic of Capricorn.
  3. Make the Sun rise in the westReverse the spin so the Earth turns from east to west.

The real Earth and its neighbours

The tilt itself changes slowly
Today the Earth’s tilt is 23.44°. Over a cycle of about 41,000 years it rocks between 22.1° and 24.5°, and at present it is slowly getting smaller. So the tropics, too, creep very slowly.
Uranus: the planet lying on its side
The axis of Uranus is tilted about 98°, so it spins almost lying down. Its poles take turns to have days and nights many years long.
Venus: spinning the other way
Venus turns the opposite way to most planets, and very slowly. There the Sun rises in the west.

4Leap years

The Earth takes 365 days and about 6 hours to go once round the Sun. A calendar can only count whole days, so each year is given 365 days and the 6 hours are left over. In four years, 6 × 4 = 24 hours, a whole day, has built up. Every fourth year that day is added to February as the 29th, and that year is called a leap year.

Add up the leftover hours

Nothing saved yet. Press “+1 year”.

But the 6 hours are not quite 6

Measured exactly, a year is 365 days 5 hours 48 minutes 45 seconds, about 11 minutes short of 6 hours. Adding a whole day every fourth year adds a little too much, and over centuries that grows into several days. To stay in step, today’s calendar follows three rules:

  1. A year that divides exactly by 4 is a leap year, like 2024 and 2028.
  2. But a century year, one that divides by 100, is not, like 1900 and 2100.
  3. Unless it also divides by 400; then it is a leap year after all, like 2000.

The next 29 February falls in 2028.

Checkpoint

  1. 1Why is a day added every fourth year?

  2. 2Will 2100 be a leap year?

5Final check

Caused by rotation or by revolution?

Tap each statement and put it in the right box.

Checkpoint

  1. 1What angle does the Earth’s axis make with its orbital plane?

  2. 2Around which dates are day and night about equal all over the Earth?

  3. 3Why do direct rays heat more than slanting rays?

  4. 4What happens beyond the Arctic Circle on 21 June?

  5. 5If the Earth’s axis stood straight (0° tilt), what would happen?

  6. 6How long does the Earth take to turn once, measured against the stars?

Glossary

Rotation
The Earth spinning on its axis from west to east. One turn takes about 24 hours and gives day and night.
Revolution
The Earth travelling round the Sun in its orbit. One trip takes 365¼ days; together with the tilt it gives the seasons.
Axis
The imaginary line through the Earth joining the North and South Poles, around which it spins.
Orbit and orbital plane
The Earth’s path round the Sun is its orbit, and the flat surface this path lies in is the orbital plane. The axis makes an angle of 66½° with it.
Circle of illumination
The circle on the globe that separates the half in daylight from the half in night.
Earth day
The period of one rotation: one day and one night.
Equator
The line of 0° latitude, which divides the Earth into the Northern and Southern Hemispheres.
Tropics of Cancer and Capricorn
The lines of latitude at 23½°N and 23½°S. The noon Sun can stand straight overhead only between them.
Arctic and Antarctic Circles
The lines at 66½°N and 66½°S. Beyond them there is at least one day a year with 24 hours of light and one with 24 hours of dark.
June solstice
The day around 21 June when the direct rays fall on the Tropic of Cancer and the Northern Hemisphere has its longest day.
December solstice
The day around 22 December when the direct rays fall on the Tropic of Capricorn and the Northern Hemisphere has its shortest day.
Equinox
The days around 21 March and 23 September when the direct rays fall on the Equator and day and night are about equal everywhere.
Perihelion and aphelion
The points in the orbit closest to the Sun (early January) and farthest from it (early July).
Leap year
A year of 366 days, in which February has 29 days.