The First Velocity of Light
Who first confirmed that light has a finite speed?
Ole Christensen Rømer (1644 - 1710) was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light...
The determination of longitude is a significant practical problem in cartography and navigation. Philip III of Spain offered a prize for a method to determine the longitude of a ship out of sight of land, and Galileo proposed a method of establishing the time of day, and thus longitude, based on the times of the eclipses of the moons of Jupiter, in essence using the Jovian system as a cosmic clock; this method was not significantly improved until accurate mechanical clocks were developed in the eighteenth century. Galileo proposed this method to the Spanish crown (1616–1617) but it proved to be impractical, because of the inaccuracies of Galileo's timetables and the difficulty of observing the eclipses on a ship. However, with refinements the method could be made to work on land.
After studies in Copenhagen, Rømer joined the observatory of Uraniborg on the island of Hven, near Copenhagen, in 1671. Over a period of several months, Jean Picard and Rømer observed about 140 eclipses of Jupiter's moon Io, while in Paris Giovanni Domenico Cassini observed the same eclipses. By comparing the times of the eclipses, the difference in longitude of Paris to Uranienborg was calculated.
Cassini had observed the moons of Jupiter between 1666 and 1668, and discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed. In 1672 Rømer went to Paris and continued observing the satellites of Jupiter as Cassini's assistant. Rømer added his own observations to Cassini's and observed that times between eclipses (particularly those of Io) got shorter as Earth approached Jupiter, and longer as Earth moved farther away. Cassini made an announcement to the Academy of Sciences on 22 August 1676:
This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.[3]
Oddly, Cassini seems to have abandoned this reasoning, which Rømer adopted and set about buttressing in an irrefutable manner, using a selected number of observations performed by Picard and himself between 1671 and 1677. Rømer presented his results to the French Academy of Sciences, and it was summarised soon after by an anonymous reporter in a short paper, Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des sciences, published 7 December 1676 in the Journal des sçavans. Unfortunately the paper bears the stamp of the reporter failing to understand Rømer's presentation, and as the reporter resorted to cryptic phrasings to hide his lack of understanding, he obfuscated Rømer's reasoning in the process. Unfortunately Rømer himself never published his results.[4]
By trial and error, during eight years of observations Rømer worked out how to account for the retardation of light when reckoning the ephemeris of Io. He calculated the delay as a proportion of the angle corresponding to a given Earth's position with respect to Jupiter, Δt=22*(α⁄180°)[minutes]. When the angle α is 180° the delay becomes 22 minutes, which may be interpreted as the time necessary for the light to cross a distance equal to the diameter of the Earth's orbit, H to E.[6] (Actually, Jupiter is not visible from the conjunction point E.) That interpretation makes it possible to calculate the strict result of Rømer's observations:
The ratio of the speed of light to the speed with which Earth orbits the sun, which is the ratio of the duration of a year divided by pi as compared to the 22 minutes
365·24·60⁄pi·22 ≈ 7,600.
In comparison the modern value is circa 299,792 km s-1⁄29.8 km s-1 ≈ 10,100.[7]
Rømer neither calculated this ratio, nor did he give a value for the speed of light. However, many others calculated a speed from his data, the first being Christiaan Huygens; after corresponding with Rømer and eliciting more data, Huygens deduced that light travelled 16 2⁄3 Earth diameters per second.[8]http://en.wikipedia.org/wiki/Ole_R%C3%B8mer
In this way the first velocity of light was calculated as about 277,000 km/sec while it is believed to be 299,792.458 km/sec today.
The knowledge that light has a finite speed had no significance in people's lives in 1676 or the 17th century and even in the 18th century or till radio communication was invented in the late 19th century. But, the invention of wireless radio needed something more mathematical than the simple knowledge that light has an finite speed.
So, the big events in the late 17th century:
1652: Cape Town founded by the Dutch East India Company in South Africa.
1664: British troops capture New Amsterdam and rename it New York.
1665: Isaac Newton discovers universal gravitation.
1665–1667: The Second Anglo-Dutch War fought between England and the United Provinces.
1667–1699: The Great Turkish War halts the Ottoman Empire's expansion into Europe.
1676: Rømer published the discovery of a finite speed of light.
1687: Isaac Newton publishes Philosophiae Naturalis Principia Mathematica.
1694: The Bank of England is established.
1703: Saint Petersburg founded by Peter the Great. Russian capital until 1918.
1704: End of Japan's Genroku period.
1707: After Aurangzeb's death, the Mughal Empire enters a long decline and the Maratha Empire slowly replaces it.
1707: Mount Fuji erupts in Japan.
1709: Great Frost of 1709, coldest winter in 500 years.
(to be continued...)
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Luk 12:13 And one of the company said unto him, Master, speak to my brother, that he divide the inheritance with me.
Luk 12:14 And he said unto him, Man, who made me a judge or a divider over you?
Luk 12:15 And he said unto them, Take heed, and beware of covetousness: for a man's life consisteth not in the abundance of the things which he possesseth.