World
What If Earth Rotated Twice as Fast?
Short answer: your day would be about 12 hours, the ground at the equator would move twice as fast, and you’d feel very slightly lighter at low latitudes. At 2× spin the centrifugal lightening is four times today’s, still a small fraction of g.
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.
Written and checked by Aivora Digital · Last updated:
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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
Today’s spin rate is one turn per sidereal day; the new rate is that times the spin multiple. Day length = one sidereal day (about 23.93 hours) ÷ the multiple. Ground speed = spin rate × Earth’s equatorial radius × cos(latitude). Apparent gravity = standard gravity minus the spin effect (spin rate² × radius × cos²(latitude)). The slider stops at 4×, where gravity at the equator is still positive. Oblateness and Coriolis are qualitative. WGS 84 equatorial radius and NIST gn.
Assumptions and limitations
This is a simplified estimate. Actual results depend on factors the model does not include.
This is a simplified thought experiment. It uses transparent assumptions so you can see what changes — it is not a full physical simulation of the real world.
- The planet's radius is held at WGS 84; it does not instantly become more oblate.
- Apparent g uses gn minus the centrifugal term; other effective-g corrections are ignored.
- Solar day is scaled with the spin multiple (sidereal/solar distinction is noted, not fully expanded).
- Atmosphere keeps up with the new ground speed in this model.
- 4× is still below orbital speed at the equator, so nothing is flung off.
Sources
- IERS — Earth rotation
Sidereal day used to set Earth's present angular velocity.
- NOAA NGS — WGS 84
Equatorial radius used for surface speed and centrifugal terms.
- NIST CODATA — standard acceleration of gravity
gn = 9.80665 m/s², the conventional standard gravity (also ISO 80000-3).
- NASA Earth fact sheet
Mass, radius, rotation period and orbital speed used as the baseline Earth.
Go further
Related experiments that pick up where this one leaves off.
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