While we think the units which describe the speed of light are "weird", perhaps we're looking through the wrong end of the telescope.
We had certain units for distance long before we calculated the speed of light and we have applied these units to the almost inconceivable speed of light.
The French originated the meter in the 1790s as one/ten-millionth of the distance from the equator to the north pole along a meridian through Paris. It is realistically represented by the distance between two marks on an iron bar kept in Paris. The International Bureau of Weights and Measures, created in 1875, upgraded the bar to one made of 90 percent platinum/10 percent iridium alloy.
In 1960 the meter was redefined as 1,650,763.73 wavelengths of orange-red light, in a vacuum, produced by burning the element krypton (Kr-86). More recently (1984), the Geneva Conference on Weights and Measures has defined the meter as the distance light travels, in a vacuum, in 1/299,792,458 seconds with time measured by a cesium-133 atomic clock which emits pulses of radiation at very rapid, regular intervals.
None of the definitions changed the length of the meter, but merely allowed this length to be duplicated more precisely.
The U. S. Congress legalized the use of the metric system in 1866 on the basis that one meter is exactly equal to 39.37 inches. In 1959 a number of English-speaking countries agreed that an inch is exactly equal to 2.54 centimeters so that the International foot is exactly equal to 0.3048 meters. The United States retained the old 1866 equivalency and called it the U. S. Survey foot so that 1 U. S. Survey foot equals 1.000002 International feet.
In the English-speaking world, 16th – 21st century, and in the Sudan , 20th century, a unit of distance = 5280 feet = 1760 yards = 8 furlongs, approximately 1.609 344 kilometers. Often referred to as the statute mile, from its having been defined in a statute* proclaimed by Elizabeth I of England (1592-93) which forbade building within 3 miles of the gates of London.
Although as early as 1607 legal authorities like John Cowell** regarded the 5280-foot mile as the one intended in contracts and statutes, it did not in fact become the legal mile throughout the United Kingdom until the passage of the
Act** establishing the imperial system of weights and measures in 1824. Scotland, however, adopted the 1760-yard mile only in 1685.
In the 16th century and before miles other than the statute mile were certainly in use in Great Britain (see old English mile, below). Those miles were gradually superseded largely through government use of the 5280-foot mile. The Letter Office and later postal services relied on the statute mile. The postal act of 1660***, for example, paid contractors carrying letters 3 pence per 5280-foot mile for each horse. Roads improved by the Turnpike Acts had milestones placed at 5280-foot intervals. Such measures accustomed people to thinking of a mile as 5280 feet.
* And that a Myle shalbe reckoned and taken in this manner and note otherwise, That is to saye, a Myle to conteyne Eight Furlongs, and everie Furlonge to conteyne Fortie Luggs or Poles, and every Lugg or Pole to conteyne Sixtene Foote and Halfe.
“An Acte againste newe Buyldinges,” 35 Elizabeth chapter 6, sec 8. (1592 – 1593) First known legal definition of the mile.
**
Mile (milliare) is a quantitie of a thousand paces, otherwise described to containe eight furlongs, and euery furlong to conteine forty lugs or poles, and euery lugge or pole to containe 16. foote and a halfe.
anno 35. El. cap 6.
John Cowell.
The Interpreter: or Booke containing the Signification of Words: Wherein is set forth the true meaning of all, or the most part of such Words and Termes, as are mentioned in the Lawe Writers, or Statutes of this... etc.
Cambridge: John Legate, 1607.
*** V. And it shall and may be lawfull to and for such Post Master Generall and his Deputy and Deputyes to aske demand take and receive of every person that he or they shall furnish and provide with Horses Furniture and Guide to ride post in any of the Post roads as aforesaid Three pence of English money for each Horses hire or postage for every English mile and Foure pence for the Guide for every Stage.
An Act for Erecting and Establishing a Post Office. (12 Car. II., cap. 35, 1660)
Statutes of the Realm. Volume 5: 1628-80.
London: 1819.
Let's turn to measuring the speed of light.
Galileo Galilei was the first person to attempt to measure the speed of light, in the early 1600s. Galileo and an assistant each stood on a different hilltop with a known distance between them, the plan was for Galileo to open the shutter of a lamp and then for his assistant to open the shutter of a lamp as soon as he saw the light from Galileo's.
Using the distance between the hilltops and his pulse as a timer, Galileo planned to measure the speed of light. He and his assistant tried this with different distances between them, but no matter how far apart they were, he could measure no difference in the amount of time it took for the light to travel between them.
Galileo concluded that the speed of light was too fast to be measured by this method, and he was correct. We now know the speed of light very precisely, and if Galileo and his assistant were on hilltops one mile apart, light would take 0.0000054 seconds to travel from one person to the other. It is understandable that Galileo was unable to measure this with his pulse!
The Danish Astronomer, Ole Römer was the next to try to measure the speed of light. In an experiment that made Römer determined that an experiment should always involve outer space. Thus, he based his observations on the movement of planets themselves, announcing his groundbreaking results on August 22, 1676.
Unfortunately, the exact calculations he used were lost in the Great Copenhagen Fire of 1728, but we have a pretty good account from other scientists from that time who used Römer’s numbers in their own work. The gist of it was that using a bunch of clever calculations involving the diameter of the Earth’s and Jupiter’s orbits, Römer was able to conclude, however, that it took around 22 minutes for light to cross the diameter of Earth’s orbit around the Sun. Christiaan Huygens later converted this to more commonplace numbers, showing that by Römer’s estimation, light traveled at about 220,000 kilometres per second, off by about 27%.
In 1848–49, Hippolyte Fizeau determined the speed of light between an intense light source and a mirror about 8 km distant. The light source was interrupted by a rotating cogwheel with 720 notches that could be rotated at a variable speed of up to hundreds of times a second. Fizeau adjusted the rotation speed of the cogwheel until light passing through one notch of the cogwheel would be completely eclipsed by the adjacent tooth. Spinning the cogwheel at 3, 5 and 7 times this basic rotation rate also resulted in eclipsing of the reflected light by the cogwheel teeth next in line. Given the rotational speed of the wheel and the distance between the wheel and the mirror, Fizeau was able to calculate a value of 315000 km/s for the speed of light. It was difficult for Fizeau to visually estimate the intensity minimum of the light being blocked by the adjacent teeth, and his value for light's speed was about 5% too high. Fizeau's paper appeared in
Comptes Rendus: Hebdomadaires de scéances de l’Academie de Sciences (Paris, Vol. 29 [July–December 1849], pp. 90–92).
In 1850, Fizeau engaged L.F.C. Breguet to build a rotary-mirror apparatus in which he split a beam of light into two beams, passing one through water while the other traveled through air. Beaten by Foucault by a mere seven weeks he confirmed that the speed of light was greater as it traveled through air, validating the wave theory of light.
Between 1877 and 1931, Albert A. Michelson made multiple measurements of the speed of light. His 1877–79 measurements were performed under the auspices of
Simon Newcomb, who was also working on measuring the speed of light. Michelson's setup incorporated several refinements on Foucault's original arrangement. As seen in the figure below, Michelson placed the rotating mirror R near the principal focus of lens L (
i.e. the focal point given incident parallel rays of light). If the rotating mirror R were exactly at the principal focus, the moving image of the slit would remain upon the distant plane mirror M (equal in diameter to lens L) as long as the axis of the pencil of light remained on the lens, this being true regardless of the RM distance. Michelson was thus able to increase the RM distance to nearly 2000 feet. To achieve a reasonable value for the RS distance, Michelson used an extremely long focal length lens (150 feet) and compromised on the design by placing R about 15 feet closer to L than the principal focus. This allowed an RS distance of between 28.5 to 33.3 feet. He used carefully calibrated tuning forks to monitor the rotation rate of the air-turbine-powered mirror R, and he would typically measure displacements of the slit image on the order of 115 mm. His 1879 figure for the speed of light, 299944±51 km/s, was within about 0.05% of the modern value. His 1926 repeat of the experiment incorporated still further refinements such as the use of polygonal prism-shaped rotating mirrors (enabling a brighter image) having from eight through sixteen facets and a 22 mile baseline surveyed to fractional parts-per-million accuracy. (See fire below) His figure of 299,796±4 km/s was only about 4 km/s higher than the current accepted value. Michelson's final 1931 attempt to measure the speed of light in vacuum was interrupted by his death. Although his experiment was completed posthumously by
F. G. Pease and F. Pearson, various factors militated against a measurement of highest accuracy, including an earthquake which disturbed the baseline measurement.
See:
Today anyone can use Google to search for the speed of light in a vaccuum and get an accurate result in seconds: 299 792 458 m/s. But who discovered the speed of light, and how did they do it? Galileo Galilei was the first person to attempt to measure the speed of light, in the early 1600s. Galileo…
lco.global
Before Ole Roemer's 1676 discovery, scientists assumed that light could not be measured.
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