THE NUMBER
Exactly 299,792,458 m/s
NPL explains that the SI fixes the speed of light in vacuum at 299,792,458 metres per second, and since 1983 the metre has been defined by how far light travels.
See the numbersTHE SPEED OF LIGHT
Light is fast,
but not instant.
Sunlight left the Sun about eight minutes before it reaches you.
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299,792,458metres per second
For centuries many thought light arrived instantly. A moon of Jupiter running late proved otherwise in 1676. Since 1983 the speed of light has been a fixed number, and the metre is defined by it.
At a glance
- Light takes time
- Measurements closed in
- Now it defines length
- A speed that became a ruler
299,792,458 m/s
Speed of light in vacuum, exact
Since 1983 the metre has been the distance light travels in a vacuum in 1/299,792,458 of a second.
THE CONSTANT · NPL
≈ 8 min 20 s
Time for sunlight to reach Earth
Worked out from NASA's figure of about 150 million km to the Sun. A light-year, NASA says, is about 9.5 trillion km.
SUNLIGHT · NASA
TWO NUMBERS WORTH KNOWING
How far light
goes, how long.
One figure from the definition,
one worked out from NASA's distance to the Sun.
The 8 minutes 20 seconds is our calculation from NASA's rounded distance and the defined speed; the true time varies a little through the year as Earth's distance from the Sun changes.
A QUICK HISTORY CHECK
How close did
they get?
Pick a measurement to see the value it gave. The bar shows how close it came to today's exact figure of about 299,792 km per second.
Historical values from a University of Waterloo history of measuring the speed of light; the modern value from NPL.
THE SHORT ANSWER
Fixed, exact
and fast.
The speed of light in a vacuum is now a defined constant.
Getting there took three centuries of measuring.
THE FIRST CLUE
A moon running late
Timing eclipses of Io, Jupiter's largest moon, Rømer predicted in 1676 that one would come 10 minutes later than tables said. It did.
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MEASURING DISTANCE
Time the light
For long distances such as Earth to the Moon, NPL says you measure how long light takes to travel there with an accurate timer.
How it is used
THREE WAYS LIGHT MEASURES LENGTH
A speed that
became a ruler.
NPL describes how the fixed speed of light is turned into real measurements.
Time of flight
Send a pulse of light over the distance and time it: seconds multiplied by 299,792,458 gives metres. It works best over long distances, such as to the Moon.
Interferometry
For shorter lengths, count wavelengths of stable laser light. Lengths up to 100 metres can be measured this way to accuracies of up to 1 part in a few million.
From tiny to huge
NPL says lengths can now be traceably measured from a few picometres up to the distance of the Voyager 1 spacecraft, already over 2.1 × 10¹³ m away.
FROM ASTRONOMY TO THE LAB
A cogwheel
and a blender.
Early values came from astronomy and depended on the theories used: later analysis of Io's eclipses between 1848 and 1870 produced Sun-to-Earth light times anywhere from 496 to 501 seconds.
In 1848–49 a rotating cogwheel was used to measure the speed of light on Earth instead. NPL notes that in 2009 it recreated that experiment with a kitchen blender. Before light took over, the metre was meant to be one ten-millionth of the distance from the North Pole to the equator.
Questions
Questions people ask
How fast is light?
In a vacuum, exactly 299,792,458 metres per second. NPL explains that this value is fixed by the SI, and the metre is defined from it.
How long does sunlight take to reach Earth?
About 8 minutes and 20 seconds, working from NASA's figure of about 150 million km to the Sun. In 1809, a study of Jupiter's moons put it at 8 minutes 13.2 seconds.
Who first showed light has a speed?
The University of Waterloo history credits Rømer, who in 1676 predicted that an eclipse of Jupiter's moon Io would be 10 minutes late because light takes longer to reach us from farther away.
Why is the speed of light exact?
Since 1983 the metre has been defined as the distance light travels in 1/299,792,458 of a second, so the speed is fixed by definition rather than measured.
How is it used to measure distance?
By timing a light pulse over long distances, such as to the Moon, or by counting laser wavelengths over shorter ones, according to NPL.
Sources
Where the facts come from
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