TOPS
Why a top stays up
Gravity tries to tip a spinning top, but instead it precesses, tracing a slow circle. As it slows, the circle speeds up until it falls.
Try the top checkA spinning wheel likes to keep pointing the same way.
That one habit keeps a top upright, let a French physicist watch the Earth turn in 1852, steers ships to true north and helps hold the Hubble Space Telescope steady.
Updated
Spin a topTHE SHORT ANSWER
A spinning wheel
holds its line.
Early makers saw the inertia of a spinning axis.
Everything else builds on it.
1852
Foucault's new word
Léon Foucault coined gyroscope in 1852 for an instrument to observe the Earth's rotation, a year after his famous pendulum.
See the history
SHIPS
North without a magnet
A gyrocompass uses a continuously driven gyroscope to find true north, which matters on steel ships where a magnetic compass struggles.
How ships use itA QUICK TOP CHECK
How fast is
the top spinning?
Pick a stage in a spinning top's life to see what happens. The bar shows how strongly it wobbles round in a circle (precesses).
Based on the HyperPhysics explanation of the spinning top from Georgia State University.
TWO NUMBERS WORTH KNOWING
An old word,
a very fast wheel.
One date from the history of the gyroscope,
one figure from a working one in orbit.
1852
Year Foucault coined “gyroscope”
The Institute of Navigation's museum says Foucault introduced the term in 1852. A similar device made by Bohnenberger in Tübingen dates from around 1813 and was simply called a “Machine”.
THE NAME · ION MUSEUM
19,200rpm
Spin of each Hubble gyro wheel
NASA says Hubble carries six gyroscopes in pairs, each with a wheel spinning at a constant 19,200 rpm on gas bearings. They measure how the telescope moves as it turns between targets.
HUBBLE · NASA
NASA says Hubble originally needed three of its six gyros working, and has been able to run in a two-gyro mode since autumn 2005.
FOUR THINGS TO NOTICE
Watch a top,
see the physics.
A spinning top or a toy gyroscope shows the same behaviour that ships and spacecraft rely on. These are things to look for next time you spin one.
Spin it hard, then wait
A fast spin keeps the axis steady. Watch how little it moves at first.
Watch for the slow circle
As it leans, the axis sweeps round in a circle instead of falling. That sweep is precession.
See the circle speed up
As the spin dies away, the circling gets faster and wider, just as the physics predicts.
Catch the final bob
Near the end the top bobs up and down as it circles, then falls over.
GYROCOMPASSES
True north,
from a spinning wheel.
On a steel ship full of electrical machinery, a magnetic compass is unreliable. A gyrocompass instead seeks an equilibrium direction under gravity and the Earth's daily rotation, which lines its axis up with true north.
Hermann Anschütz-Kaempfe built the first practical gyrocompass in 1906, tested on the German warship Undine. Elmer Sperry patented his own design in 1907, and Sperry compasses were in use on larger American battleships and submarines by the eve of the First World War.
Questions
Questions people ask
Why doesn't a spinning top fall over?
Gravity pulls on the leaning top, but because it is spinning the top precesses, sweeping round in a circle instead of toppling. The precession speeds up as the spin slows, and eventually the top bobs and falls.
Who invented the gyroscope?
A spinning-axis device was made by Johann Bohnenberger in Tübingen around 1813 and described in 1817. Léon Foucault coined the word gyroscope in 1852 for an instrument to observe the Earth's rotation.
How did Foucault's gyroscope show the Earth turning?
In the Science Museum's 1883 version, a disc is spun at high speed, then hung by a fine silk thread with a pointer. The slow horizontal drift is read with a scale and microscope.
What is the difference between a gyrocompass and a magnetic compass?
A gyrocompass uses a continuously driven gyroscope to seek true, geographic north, rather than magnetic north. It is used on ships and submarines, where steel hulls and machinery upset magnetic compasses.
Does the Hubble Space Telescope use gyroscopes?
Yes. NASA says Hubble has six gyroscopes in three units. Each wheel spins at 19,200 rpm and sits in a sealed cylinder floating in thick fluid, with electronics that detect tiny movements of its axis.
Sources
Where the facts come from
- Science Museum Group: Gyroscopes for demonstrating the Earth's rotation, 1883
- Institute of Navigation museum: The Machine of Bohnenberger
- HyperPhysics, Georgia State University: Spinning top
- Encyclopaedia Britannica: Gyrocompass
- EBSCO Research Starters: Anschütz-Kaempfe invents the first practical gyrocompass
- NASA Goddard: Hubble Servicing Mission 4, gyroscopes
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