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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.
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 transverse velocity of the beam along the path in the sky between the objects has an apparent speed greater than light, but this represents separate photons of light. No photons are traveling the path from object 1 to object 2; the photons in the light beam are traveling a radial path outward from the lighthouse, at the speed of light.
[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]
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).
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.
For instance, the Fizeau wheel could measure the speed of light to perhaps 5% accuracy, which was quite inadequate for measuring directly a first-order 0.01% change in the speed of light. A number of physicists therefore attempted to make measurements of indirect first-order effects not of the speed of light itself, but of variations in the ...
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.