What if Earth rotated faster?

Earth spins all the time. When your side faces the Sun, it is day.

When your side turns away, it is night. What if Earth spun faster?

Earth spins like a top. When your side of Earth faces the Sun, you have daytime; when it turns away, you have night. One full spin, from one sunrise to the next, takes 24 hours.

So what would happen if Earth spun faster, say twice as fast? Would we just get more sunrises, or would anything else change? In this lab you can speed up Earth's spin in a model and find out.

Earth's rotation sets the length of the solar day, the interval between one local noon and the next. It also sets how long each patch of ground is heated by sunshine and how long it cools in the dark.

This lab asks what a faster rotation would do to both: the timing of day and night, and the temperature swing between them, using an energy-balance model of one patch of ground on the equator.

Today, Earth takes about one whole day to spin once.

The ground warms up in the sunny afternoon. It cools down at night.

Earth turns once in 23 hours and 56 minutes. Because Earth also moves a little way around the Sun each day, it has to turn a tiny bit more to face the Sun again, which makes the 24-hour day we use.

In the model, a patch of damp ground on the equator warms to about 29 °C in the afternoon and cools to about 21 °C before dawn. That difference, about 7 degrees, is the day-night swing.

Earth's sidereal rotation period is 23.9345 hours; the solar day is 24 hours because of Earth's orbital motion, 1/S = 1/P − 1/Y. For an equatorial patch at an equinox with Earth-like air and damp soil (heat capacity 2 MJ/m²·K), the model's repeating day runs from about 21 °C to about 29 °C: a swing of about 7 K.

The air's greenhouse effect and heat sharing by winds and oceans both damp that swing in the model, as they do on the real Earth.

In our pretend lab, Olly makes Earth spin twice as fast.

Now days and nights are short. The ground has no time to get very hot or very cold.

Set one spin to 12 hours. Now sunrise to sunrise takes about 12 hours: six hours of daylight, then six hours of night. You would see two sunrises every 24 hours!

Watch the temperatures too. The hottest afternoon drops to about 27 °C and the coldest night rises to about 23 °C, so the swing shrinks to under 4 degrees. Spin even faster, at 4 hours, and the swing nearly disappears.

At P = 12 h the model's solar day is 12.02 h, with 6.0 h of daylight on the equator. The repeating temperature cycle runs from about 23 °C to about 27 °C, a swing of 3.6 K, half of today's. At P = 4 h the swing falls to about 1 K.

This is a thought experiment carried out with a model. Earth's spin cannot be doubled, and the model changes only the timing of sunlight, nothing else about the planet.

The ground warms up slowly, and it cools down slowly.

Short days and short nights do not give it time to change much.

Ground, water and air take time to warm up and to cool down, a bit like a heavy pan on a stove. If you switch the burner on and off every few seconds, the pan never gets very hot or very cold; it just stays warm.

A fast-spinning Earth is like that. The sunshine and the darkness each last only a few hours, so the ground does not have time to heat up fully or cool down fully.

The surface layer has a thermal time constant of roughly C/(4εσT³ + k): how long it takes to respond to a change in heating. When the solar day is short compared with that time, the surface integrates the sunshine and barely changes; when it is long, the surface approaches equilibrium with full sun by day and cools by radiation at night.

The model's limits: one patch on the equator at an equinox, fixed heat sharing, and no clouds, rain or winds that would themselves change with the spin. Its temperatures are for comparing worlds, not a climate prediction.

Jupiter is a giant planet that spins very fast. Its day is less than ten hours!

Long, long ago, Earth's days were shorter too.

Some planets really do spin fast. Jupiter turns once in less than 10 hours, even though it is far bigger than Earth. It is about 7 percent wider through its equator than through its poles, and planetary scientists say its rapid spin is one of the reasons.

Earth's own spin is changing too. The friction of the tides is slowly braking it, so each day is about 0.002 seconds longer than a day a century earlier. Tiny changes like that add up over millions of years, so long ago Earth's days were shorter.

Jupiter's rotation period is 9.93 hours and its ellipticity 0.065, compared with Earth's 0.0034. University of Arizona researchers working with Juno data attribute Jupiter's equatorial bulge to a combination of its rapid rotation, its internal structure and its winds. Whether a faster Earth would bulge more is an idea; the model does not include shape at all.

Tidal friction is lengthening Earth's day by about 0.002 s per century, so in the distant past Earth's day was shorter. The model also leaves out how winds and weather would respond to a different spin.