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To help compare different orders of magnitude, the following list describes various speed levels between approximately 2.2 × 10 −18 m/s and 3.0 × 10 8 m/s (the speed of light). Values in bold are exact.
The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour).
Rømer starts with an order of magnitude demonstration that the speed of light must be so great that it takes much less than one second to travel a distance equal to Earth's diameter. The point L on the diagram represents the second quadrature of Jupiter, when the angle between Jupiter and the Sun (as seen from Earth) is 90°.
The speed of light in vacuum is defined to be exactly 299 792 458 m/s (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.
[6] [7] Huygens reports on a letter by Ole Christensen Rømer, dated from 1677, where the speed of light is said to be at least 100,000 times faster than the speed of sound, and possibly six times higher. In the latter case, the speed found by Rømer (214,000 km /s) was of the same order of magnitude as the speed of light admitted today. [5]
Light-time correction occurs in principle during the observation of any moving object, because the speed of light is finite. The magnitude and direction of the displacement in position depends upon the distance of the object from the observer and the motion of the object, and is measured at the instant at which the object's light reaches the ...
The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place, only one clock is needed to measure the total time; thus, this speed can be experimentally determined independently of any clock synchronization scheme.
1690 – Christiaan Huygens gives the first estimate of the speed of light in air or vacuum, based on Rømer’s work. The result is equivalent to about 2×10 8 m/s in modern units, correct only to the order of magnitude. 1727 – James Bradley correctly identifies the peculiar behaviour of γ Draconis as stellar aberration.