page_title: Rømer's determination of the speed of light
section_title: Observations
hierarchical_section_title: Rømer's determination of the speed of light / Observations
caption_attribution_description: English: Page 1 (right) and 4 of the list of eclipses of the Jupiter moon Io that formed the basis of Ole Rømer's discovery of the finite speed of light. The Latin title Obser. Primi Jovialium Parisiis translates to "Observations of the first Jupiter moon in Paris". Page 1 lists emersions and immersions of Io observed between 1668 and 1678. The first observation (circled) is marked "should according to everyone's opinion be excluded". The top of page 4 lists observations of the fourth moon of Jupiter (Callisto). At the lower half are a couple of attempts to match the number of revolutions ("Rev.") of Io with an average orbiting time for three periods, 76-77, 71-72 and 72-73. Next to them are what according to Meyer p. 19 appears to calculations of an average value of the time it takes light to travel a distance equal to the Earth orbit radius (ca. 11 minutes). At the bottom, written upside down, are calculations of eclipses in Paris based on observed eclipses at Uranienborg. Dansk: Side 1 (til højre) og 4 af listen over formørkelser af Jupitermånen Io, der dannede grundlaget for Ole Rømers opdagelse af lysets tøven. Den latinske titel Obser. Primi Jovialium Parisiis kan oversættes til "Observationer af den første Jupitermåne i Paris". Side 1 er en liste over emersioner og immersioner af Io observeret mellem 1668 og 1678. Den første obersvation (indcirklet) er mærket med "bør efter alles mening ekskluderes". Toppen af side 4 består af observationer af Jupiters fjerde måne. På den nederste halvdel er forsøg på at matche antallet af omløb ("Rev.") af Io med en gennemsnitsomløbstid for tre perioder, 76-77, 71-72 and 72-73. Ved siden af, er ifølge Meyer s. 19 tilsyneladende beregninger af en gennemsnitsværdi af den tid, det tager lyset at gennemløbe en afstand lig jordbanens radius (cirka 11 minutter). Nederst, skrevet på hovedet, er beregninger af formørkelser i Paris baseret på observerede formørkelser i Uranienborg.
context_page_description: Rømer's determination of the speed of light was the demonstration in 1676 that light has a finite speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer, who was working at the Royal Observatory in Paris at the time.
  By timing the eclipses of the Jupiter moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second, about 26% lower than the true value of 299,792 km/s.
  Rømer's theory was controversial at the time he announced it, and he never convinced the director of the Paris Observatory, Giovanni Domenico Cassini, to fully accept it. However, it quickly gained support among other natural philosophers of the period, such as Christiaan Huygens and Isaac Newton. It was finally confirmed nearly two decades after Rømer's death, with the explanation in 1729 of stellar aberration by the English astronomer James Bradley.
context_section_description: Most of Rømer's papers were destroyed in the Copenhagen Fire of 1728, but one manuscript that survived contains a listing of about sixty observations of eclipses of Io from 1668 to 1678. In particular, it details two series of observations on either side of the oppositions of 2 March 1672 and 2 April 1673. Rømer comments in a letter to Christiaan Huygens dated 30 September 1677 that these observations from 1671–73 form the basis for his calculations.
  The surviving manuscript was written some time after January 1678, the date of the last recorded astronomical observation (an emergence of Io on 6 January), and so was later than Rømer's letter to Huygens. Rømer appears to have been collecting data on eclipses of the Galilean moons in the form of an aide-mémoire, possibly as he was preparing to return to Denmark in 1681. The document also records the observations around the opposition of 8 July 1676 that formed the basis for the announcement of Rømer's results.
caption: Rømer's aide-mémoire, written at some point after January 1678 and rediscovered in 1913. The timings of eclipses of Io appear on the right-hand side of this image, which would have been "page one" of the folded sheet. Click on image for an enlarged view.
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