What If? Lab
What if Earth rotated slower?
What if one day lasted a whole week? Slow down Earth's spin in Olly's lab and see how hot and cold it gets!
Slow Earth's spin to one turn a week in our model and a single afternoon lasts days. The ground has time to bake in the long sunshine and to chill in the long dark, so the gap between the hottest and coldest times grows far bigger.
Stretch the rotation period to 168 hours and the solar day becomes about 7.1 Earth days. With the day now long compared with the surface's thermal response time, the model's day-night swing grows from about 7 K to nearly 50 K.
The big question
Today, the Sun rises every morning. Night comes every evening.
What if Earth turned very slowly? What if one day lasted a whole week?
Every 24 hours, Earth spins you through one sunrise, one sunset and one night. Each night is short, so the ground never has long to cool before the Sun comes back.
What if Earth spun much more slowly, so that one turn took a whole week? A single afternoon would last more than a day. Would anything besides the clock change? In this lab you can slow Earth's spin in a model and find out.
Day length is not just a matter of timekeeping. It controls how long each part of the surface is heated without a break and how long it radiates heat away in darkness.
This lab asks how the daily temperature cycle would change if Earth's rotation period were stretched from about 24 hours to a week, using an energy-balance model of one patch of ground on the equator.
Try the experiment
Make a prediction
Olly makes one day last a week. What happens to the hot and cold times?
If one spin took a week, what would happen to the difference between the hottest and coldest times?
With P = 168 h, how does the model's equatorial day-night swing compare with today's 7 K?
- It gets much bigger
- It gets smaller
- It stays the same
| Control | In our world | What if… | Range |
|---|---|---|---|
| One spin takes · How long Earth takes to turn once. Today it is 23 hours 56 minutes; try one week (168 hours). | 23.93 h | 168 h | 4–1000 h |
| Heat storage of the ground · How much heat the ground soaks up for each degree it warms. Dry sand heats and cools quickly; ocean water slowly. | 2 MJ/m²·K | — | 0.1–20 MJ/m²·K |
| Air and oceans · Air traps heat and winds and oceans carry it around. Switch them off for bare rock, like the Moon. | 1 on/off | — | 0–1 on/off |
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Established fact
On the Moon, days are very long and very hot. Nights are very cold.
The Moon's day lasts about 29.5 Earth days, and with no air its surface swings from above 100 °C to below −170 °C.
The Moon's solar day is 708.7 h and its equatorial surface ranges from about 95 K to 390 K (NASA).
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What the model shows
In the model, a week-long day gets very hot. The long night gets cold.
In this model, a week-long day heats damp equatorial ground to about 50 °C and the long night cools it to about 2 °C.
At P = 168 h the model's swing is 48 K (about 2 °C to 50 °C), versus 7 K at P = 23.93 h.
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What the model shows
Water warms up slowly, so it changes less than sand.
In the model, ground that stores more heat, like the ocean, has a much smaller swing than dry sand.
At P = 168 h the model's swing is about 107 K for dry sand and about 10 K for an ocean-like surface: heat capacity sets the response time.
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An idea scientists discuss
Scientists think strong winds would carry warm air to the dark side.
Climate scientists' computer models of slowly spinning Earth-like planets show winds carrying a lot of heat to the night side, so the real swing would probably be smaller than this model's.
In climate simulations of Earth-like planets that turn once a year, day-to-night heat transport keeps night-side surfaces from falling below about 240 K (Merlis and Schneider); the model's fixed heat sharing overstates the swing.
In our world
Today, the Sun warms the ground for about half a day.
Then night comes before the ground gets too hot. By morning, it has cooled a little.
On Earth today, the Sun shines on the equator for about 12 hours, then night lasts about 12 hours. That is not long enough for the ground to get extremely hot or extremely cold.
In the model, damp ground on the equator warms to about 29 °C in the afternoon and cools to about 21 °C by dawn. Air and oceans also help, by trapping heat and carrying it around the world.
With Earth's 23.93-hour rotation, the model's equatorial patch (Earth-like air, damp soil of heat capacity 2 MJ/m²·K) follows a repeating cycle from about 21 °C to 29 °C, a 7 K swing.
The swing is small because 12 hours of sunshine is short compared with the time the surface needs to approach a full-sun balance, and because the air's greenhouse effect and heat sharing hold the temperature near the planet's mean.
Slow it down
In our pretend lab, Olly slows Earth down. Now one day lasts a whole week!
The long, sunny day gets hot. The long night gets cold.
Set one spin to 168 hours, one week. Now sunrise to sunrise takes about 7 Earth days, so the Sun stays up for about three and a half days in a row, then the night lasts just as long.
In the model, the long afternoon heats the ground to about 50 °C, and the long night cools it to about 2 °C, a swing of nearly 50 degrees. Try dry sand instead of damp soil and the swing grows much bigger still.
At P = 168 h the model's solar day is 171.3 h (7.14 days), with about 86 hours of daylight on the equator. The repeating cycle runs from about 2 °C to 50 °C, a swing of 48 K. With dry sand (0.6 MJ/m²·K) it reaches about 107 K; with an ocean surface (10 MJ/m²·K) it is only about 10 K.
This is a thought experiment carried out with a model. Earth's spin cannot be slowed like this, and the model changes only the timing of sunlight.
Why it happens
The Sun keeps shining on the same ground for days. So the ground gets hotter and hotter.
Then the long night gives it lots of time to cool.
Ground warms up slowly, but given enough time it gets very hot. In a week-long day, the Sun shines on the same patch for days without a break, so the ground keeps heating until it is nearly as hot as full sunshine can make it.
Then the long night gives it days to lose that heat into space. Wet ground and water store more heat, so they change temperature more slowly than dry sand.
When the solar day is long compared with the surface's thermal time constant, about C/(4εσT³ + k), the patch nearly reaches radiative balance with the noon Sun and then cools toward a much lower night balance. Higher heat capacity lengthens the time constant and damps the swing.
The model's limits: heat sharing is fixed. On a slowly rotating world, real winds would carry much more heat from the day side to the night side, so the model's hottest and coldest values with air on are too extreme.
Slow spinners
On the Moon, one day lasts about a month. It has no air to keep it cozy.
So its days get very hot, and its nights get very, very cold.
The Moon has very long days. From one sunrise to the next takes about 29 and a half Earth days. With no air to trap or share heat, its surface swings from more than 100 °C in the long day to below −170 °C in the long night.
Venus spins slowly too, but its thick air traps so much heat that its surface is hot enough to melt lead. Scientists expect that a slow Earth's winds would also carry a lot of heat to the night side, softening the swing.
The Moon's solar day is 708.7 hours, and its equatorial surface ranges from about 95 K to 390 K, which the rotation model reproduces with no air and very low heat capacity. Venus has a solar day of 2,802 hours, yet its thick atmosphere traps so much heat that its mean surface temperature is near 464 °C, hotter than any other planet.
Earth's own spin is slowing, by about 0.002 s per century through tidal friction, a tiny change compared with the week-long day in this lab.
Amazing facts
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On Mercury, one day lasts longer than its whole year.
From one sunrise to the next on Mercury takes about 176 Earth days, which is two of Mercury's years.
Mercury's solar day is 4,222.6 hours (about 176 Earth days), two of its 88-day orbits.
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Venus spins very slowly, but it is hot everywhere.
Venus has a day about 117 Earth days long, yet its thick air traps so much heat that it is the hottest planet of all.
Venus's solar day is 2,802 hours, but its dense atmosphere holds its mean surface temperature near 464 °C, hot enough to melt lead.
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The Moon's day lasts about a month.
From one sunrise to the next on the Moon takes about 29 and a half Earth days.
The Moon's solar day equals its synodic month, about 29.5 days (708.7 hours).
Words to know
- rotation
- spinning around a line through the middle, called an axis
- solar day
- the time from one noon to the next, or one sunrise to the next
- temperature swing
- the difference between the hottest and coldest times of day
- heat capacity
- how much heat something needs to warm up by one degree
- atmosphere
- the layer of air around a planet
Discovery challenge
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If a day lasted a week, would afternoons get hotter?
In the model, what happens to the hottest temperature with a week-long day?
At P = 168 h, the model's equatorial maximum is about…
- Hotter
- Colder
- No change
Show the answer
Hotter
Yes! The Sun shines on the ground for days.
It rises to about 50 °C, because the Sun heats the same ground for days.
…50 °C, up from about 29 °C today.
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Which changes temperature slowly: sand or water?
Which surface has the smallest temperature swing on a slow Earth?
At P = 168 h, which surface has the smallest model swing?
- Sand
- Water
- Rock with no air
Show the answer
Water
Water changes slowly. It stores lots of heat.
Ocean water, because it stores much more heat than sand.
Water: its high heat capacity lengthens the response time, giving about 10 K against 107 K for dry sand.
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On the Moon, how long is one day?
About how long is one sunrise-to-sunrise day on the Moon?
The Moon's solar day is about…
- One hour
- One Earth day
- About a month
Show the answer
About a month
About a month!
About 29 and a half Earth days.
…708.7 hours, or about 29.5 Earth days.
Experiments to try
- Watch Try it What If? Lab What if Earth rotated faster? One spin of Earth gives us one day and one night. Spin Earth twice as fast in our model and the days and nights each shrink to about six hours, with less time for the ground to warm up or cool down.
- Watch Try it What If? Lab What if Earth stopped rotating? You might think a world that does not spin would have no day and night. But Earth also travels around the Sun. Stop the spin in our model and the Sun still crosses the sky, just once a year: six months of daylight, then six months of night.
- Watch What If? Lab What if a day lasted 48 hours? Earth spins once about every 24 hours, and almost every living thing has a body clock tuned to that day. Imagine a day twice as long: long, bright days and long, dark nights. What would happen to your sleep, to plants and to animals?
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch Try it What If? Lab What if the whole Earth were covered by ocean? Water soaks up a lot of heat before it gets hot, and gives it back slowly. Turn a patch of damp ground into ocean in our model and watch the gap between day and night temperatures shrink, then see how the sea already keeps coastal towns mild.
Explore next
- Watch Try it What If? Lab What if Earth rotated faster? One spin of Earth gives us one day and one night. Spin Earth twice as fast in our model and the days and nights each shrink to about six hours, with less time for the ground to warm up or cool down.
- Watch Try it What If? Lab What if Earth stopped rotating? You might think a world that does not spin would have no day and night. But Earth also travels around the Sun. Stop the spin in our model and the Sun still crosses the sky, just once a year: six months of daylight, then six months of night.
- Watch What If? Lab What if a day lasted 48 hours? Earth spins once about every 24 hours, and almost every living thing has a body clock tuned to that day. Imagine a day twice as long: long, bright days and long, dark nights. What would happen to your sleep, to plants and to animals?
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch Try it What If? Lab What if the whole Earth were covered by ocean? Water soaks up a lot of heat before it gets hot, and gives it back slowly. Turn a patch of damp ground into ocean in our model and watch the gap between day and night temperatures shrink, then see how the sea already keeps coastal towns mild.
- Watch Try it What If? Lab What if Earth had no Moon? The Moon's pull stretches Earth's oceans and gives us most of our tides. Take the Moon away in our tide model and see what is left: the Sun still makes tides, but they are less than half as big as the Moon's.
- Watch Try it What If? Lab What if Earth had two moons? Our Moon goes round Earth in a neat, repeating loop. Add a second moon in our orbit model and watch what changes: the two moons tug on each other as well as on Earth, so their paths start to wobble.
- Watch Try it What If? Lab What if the Sun disappeared? The Sun cannot vanish, and it will keep shining for billions of years. But imagining it gone answers a real question: what keeps Earth going round? Switch off the Sun's pull in our model and Earth stops curving and travels on in a straight line.
For parents & teachers
What children will learn
- Day length controls how long the ground is heated and how long it cools.
- Longer days and nights make bigger temperature swings in the model.
- Water stores more heat than sand, so it changes temperature more slowly.
- The Moon and Venus show how air and spin together shape a world's temperatures.
Recommended ages: 5 to 17 · About 15 minutes
Talk about it
- How would plants, animals and people cope with three days of sunshine and three days of darkness?
- Why might a desert get hotter by day and colder by night than a seaside town?
- Why does the model say its long-day temperatures with air are too extreme?
Try at home or in class
- Put a thermometer in a cup of dry sand and another in a cup of water in the sun. Check every 15 minutes, then move both into shade and keep checking.
- Mark a sunny spot on a sidewalk and feel it in the morning, at noon and after sunset (carefully, with the back of your hand).
Key vocabulary: rotation, solar day, temperature swing, heat capacity, atmosphere
Where these facts come from
- Earth Fact Sheet , NASA Goddard Space Flight Center
- Moon Fact Sheet , NASA Goddard Space Flight Center
- Planetary Fact Sheet (Metric) , NASA Goddard Space Flight Center
- Venus Facts , NASA Science
- Climate and Earth's Energy Budget , NASA Earth Observatory
- Astronomy 2e, §4.6 Ocean Tides and the Moon , OpenStax
- Atmospheric dynamics of Earth-like tidally locked aquaplanets (Merlis and Schneider, Journal of Advances in Modeling Earth Systems) , arXiv (California Institute of Technology authors)
Checked, not yet human-reviewed. Last updated October 23, 2026. Spotted a mistake? Tell us.