Editing Speed of light (section)

=== First measurement attempts ===

In 1629, [[Isaac Beeckman]] proposed an experiment in which a person observes the flash of a cannon reflecting off a mirror about one mile (1.6&nbsp;km) away. In 1638, [[Galileo Galilei]] proposed an experiment, with an apparent claim to having performed it some years earlier, to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. He was unable to distinguish whether light travel was instantaneous or not, but concluded that if it were not, it must nevertheless be extraordinarily rapid.<ref name=2newsciences>
{{Cite book
 |last=Galilei
 |first=G.
 |year=1954
 |orig-year=1638
 |title=Dialogues Concerning Two New Sciences
 |url=https://www.questia.com/read/88951396/dialogues-concerning-two-new-sciences
 |page=43
 |others=Crew, H.; de Salvio A. (trans.)
 |publisher=[[Dover Publications]]
 |isbn=978-0-486-60099-4
 |ref=Reference-Galileo-1954
 |access-date=29 January 2019
 |archive-date=30 January 2019
 |archive-url=https://web.archive.org/web/20190130053744/https://www.questia.com/read/88951396/dialogues-concerning-two-new-sciences
 |url-status=dead
}}</ref><ref name=boyer>
{{Cite journal
 |last=Boyer |first=C. B.
 |year=1941
 |title=Early Estimates of the Velocity of Light
 |journal=[[Isis (journal)|Isis]]
 |volume=33 |issue=1 |page=24
 |doi=10.1086/358523
 |s2cid=145400212
 |ref=boyer-1941
}}</ref> According to Galileo, the lanterns he used were "at a short distance, less than a mile". Assuming the distance was not too much shorter than a mile, and that "about a thirtieth of a second is the minimum time interval distinguishable by the unaided eye", Boyer notes that Galileo's experiment could at best be said to have established a lower limit of about 60 miles per second for the velocity of light.<ref name="boyer"/> In 1667, the [[Accademia del Cimento]] of Florence reported that it had performed Galileo's experiment, with the lanterns separated by about one&nbsp;mile, but no delay was observed.<ref>{{Cite journal|last1=Foschi|first1=Renato|last2=Leone|first2=Matteo|date=August 2009|title=Galileo, Measurement of the Velocity of Light, and the Reaction Times|url=http://journals.sagepub.com/doi/10.1068/p6263|journal=Perception|language=en|volume=38|issue=8|pages=1251–1259|doi=10.1068/p6263|pmid=19817156|hdl=2318/132957 |s2cid=11747908|issn=0301-0066|hdl-access=free}}</ref> The actual delay in this experiment would have been about 11 [[microsecond]]s.

[[File:Illustration from 1676 article on Ole Rømer's measurement of the speed of light.png|thumb|upright=0.8|Rømer's observations of the occultations of Io from Earth|alt=A diagram of a planet's orbit around the Sun and of a moon's orbit around another planet. The shadow of the latter planet is shaded.]]
[[Rømer's determination of the speed of light|The first quantitative estimate of the speed of light]] was made in 1676 by Ole Rømer.<ref name="cohen"/><ref name="roemer"/> From the observation that the periods of Jupiter's innermost moon [[Io (moon)|Io]] appeared to be shorter when the Earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and estimated that it takes light 22 minutes to cross the diameter of Earth's orbit. [[Christiaan Huygens]] combined this estimate with an estimate for the diameter of the Earth's orbit to obtain an estimate of speed of light of {{val|220000|u=km/s}}, which is 27% lower than the actual value.<ref name="Huygens 1690 8–9">
{{Cite book
 |last=Huygens |first=C.
 |year=1690
 |title=Traitée de la Lumière |language=fr
 |url=https://archive.org/details/bub_gb_kVxsaYdZaaoC
 |pages=[https://archive.org/details/bub_gb_kVxsaYdZaaoC/page/n19 8]–9
 |publisher=Pierre van der Aa
}}</ref>

In his 1704 book ''[[Opticks]]'', [[Isaac Newton]] reported Rømer's calculations of the finite speed of light and gave a value of "seven or eight minutes" for the time taken for light to travel from the Sun to the Earth (the modern value is 8&nbsp;minutes 19&nbsp;seconds).<ref>
{{Cite book
 |last=Newton |first=I.
 |year=1704
 |contribution=Prop. XI
 |title=Optiks
 |url=http://gallica.bnf.fr/ark:/12148/bpt6k3362k.image.f235.vignettesnaviguer
}} The text of Prop.&nbsp;XI is identical between the first (1704) and second (1719) editions.</ref> Newton queried whether Rømer's eclipse shadows were coloured. Hearing that they were not, he concluded the different colours travelled at the same speed. In 1729, [[James Bradley]] discovered [[aberration of light|stellar aberration]].<ref name="Bradley1729"/> From this effect he determined that light must travel 10,210 times faster than the Earth in its orbit (the modern figure is 10,066 times faster) or, equivalently, that it would take light 8&nbsp;minutes 12&nbsp;seconds to travel from the Sun to the Earth.<ref name="Bradley1729"/>