World
What If Earth Rotated Twice as Fast?
What does a shorter day do to ground speed and apparent weight?
A 24-hour day is a habit, not a law. Earth's sidereal spin is a little under that, and the solar day is the 86 400 seconds we count. Multiply the spin and two things move at once: the Sun crosses the sky faster, and the ground at the equator has to travel farther per second to complete the turn. There is also a quieter change. Rotation already reduces apparent weight a little through a centrifugal term, ω² R cos²(latitude). Faster spin, larger term, slightly lighter footsteps at low latitudes. At four times today's ω that term is no longer a rounding error. Over long times the planet would also grow more oblate. This page does the first-order numbers and leaves the new figure of the Earth to geology.
Change this
Results update as you move the controls.
2 is a 12-hour-ish day.
Your result
11.97 hours
new length of a sidereal day
today 23.934 hours sidereal
- Ground speed at this latitude
- 579 m/s
- Centrifugal term
- 0.053 m/s²
- Apparent g
- 9.754 m/s²
- Apparent g today, same latitude
- 9.794 m/s²
- Change in apparent weight
- -0.4%
2,085 km/h
What does that mean?
At 2× today's spin, a sidereal day is 11.97 hours. At 51.5°N the ground moves east at 579 m/s (2,085 km/h), versus 290 m/s now. Apparent g is 9.754 m/s² here, down from 9.794 m/s² today, because the centrifugal term rose to 0.053 m/s². That is a lighter footstep, not weightlessness. A fatter equator would take geological time.
At a glance
- Day now (sidereal hours)23.93 h
- Day in this run11.97 h
Compare
| Equator speed now | 465 m/s |
|---|---|
| Equator speed in this run | 930 m/s |
| 51.5°N | 579 m/s |
Compare scenarios
The day is just 2π / ω
Double ω and the sidereal day halves. Clocks that track the Sun would show something close to a 12-hour solar day at 2×. Biology that entrains to light would be asked to fit two cycles where it now fits one. This experiment does not redesign sleep. It prints the new length of day and the new ground speed at your latitude, using the same cosine law as the stopped-spinning page, only in the other direction.
A little less apparent g
Effective gravity in the rotating frame is g minus the local centrifugal acceleration. At the equator today that subtraction is about 3.4 cm/s², a third of a percent. It grows as the square of the spin. At 2× it is four times today's centrifugal term; at 4× it is sixteen times. Poles do not get the discount: cos(90°) is zero. Nobody is flung into space at 2×. At much higher multiples the equatorial effective g could approach zero, which is a different, more dramatic slider than this one allows.
Oblateness is a long job
Earth is already an ellipsoid because it spins: WGS 84 is not a sphere. Spin it faster and, given time, rock and ocean would adjust toward a fatter equator. That is hydrostatic equilibrium on a geological clock, not an afternoon. Weather would also meet a stronger Coriolis parameter. This model will mention those and will not run a GCM or a viscoelastic Earth. If you want zero spin instead of more spin, the stopped-spinning experiment is the counterpart.
How we calculated this
Present ω is 2π / sidereal day. New ω is that times the spin multiple. Day length is 2π / ω_new in hours. Ground speed is ω_new × R_eq × cos(latitude). Apparent g is gn minus ω² R cos²(lat), never printed as negative without comment (the slider stops at 4×, where equatorial g_eff is still positive). Oblateness and Coriolis are qualitative. WGS 84 equatorial radius and NIST gn.
Go further
A curated rabbit hole from this question. Each link is a real experiment, not a random suggestion.
You might also like
What If Earth Stopped Spinning?
Remove Earth's spin over a time you choose and see the leftover eastward speed, the winds, and the new length of a day.
Explore
What If Earth's Gravity Doubled?
Scale surface gravity and compare weight and jump height. Same takeoff energy, higher g, lower flight.
Explore
What If Earth Had Two Moons?
Add a second moon with a mass ratio and a distance in lunar units. Tidal forces add in a simplified way — not a three-body simulation.
Explore
What If You Fell Through the Earth?
Uniform-density Earth, no friction: about 42 minutes to the other side. With friction, you do not oscillate.
Explore
What If Earth Was Twice as Large?
Enlarge Earth's radius two ways: keep density (g rises with R) or keep mass (g falls as 1/R²). The assumption is the whole experiment.
Explore
What If Gravity Stopped?
Weight vanishes; velocity does not. Earth goes straight, air expands, people float. A thought experiment with a timer.
Explore