Ole Rømer's 1676 demonstration that light propagates
at a finite speed must have been a revelation to the
members of the French Royal Academy of Science. A young
and brilliant Danish "postdoc" at the Paris Observatory,
Rømer had unexpectedly answered a long−standing
fundamental question. Before his discovery, the likes
of René Descartes and Johannes Kepler had claimed
that light was an instantaneous phenomenon, and all
attempts to prove otherwise had failed.
Isaac Newton and especially Christiaan Huygens welcomed
Rømer's result; Huygens found it encouraging
in the development of his wave theory of light. There
were also a few ardent opponents, such as Robert Hooke
and Rømer's observatory colleague Jean Cassini.
What value of the speed of light did Rømer
actually report? I found 16 references, spanning the
years 1694−2003, that give values from 200 000
to 350 000 km/s. Such a range can hardly be attributed
to mistakes in the conversion of measurement units.
None of the sources I found quoted an original paper
or proceedings. The present French Academy of Sciences
led me to proceedings of a 1976 conference marking
the tricentennial of Rømer's discovery.1
Those proceedings include a copy of his only publication
about the speed of light.2
The sole message of that concise and tantalizing paper
is that the speed of light is finite, though incredibly
large. Rømer did not mention any specific value.
The first paragraph of Rømer's paper states
the question: Is light propagation an instantaneous
phenomenon or does it take time? The next paragraph
gives observations of Jupiter's innermost moon (the
one we now call Io) to show that light covers a distance
like Earth's diameter, "about 3000 lieues" (one lieue
= 4.448 km), in less than one second. Rømer's
reasoning was as follows: If light has a finite speed,
then when Earth is approaching Jupiter, Io's period
should appear shortened. Half a year later, when Earth
and Jupiter move apart, the moon's period should appear
to be longer. Io's actual period is about 42.5 hours,
during which time Earth traverses "at least 210 Earth
diameters." The two periods therefore, according to
Rømer, should differ by "nearly half a quarter
of an hour." But he did not observe a difference.
However, Rømer wrote, that does not mean
that light travel does not take time. Comparing the
time lapse of 40 successive periods of Earth's nearing
Jupiter with 40 periods while Earth is receding, he
observed a perceptible difference. Therefore, he stated,
light should traverse the diameter of Earth's orbit
around the Sun in 22 minutes. This retardation of
light showed up in all of the observations Rømer
had done at the Paris observatory since 1668.
With a good sense of dramatic timing, Rømer
played his ace in the next paragraph, where he illustrated
the effect of the proposed retardation of light. In
early September, he had predicted that Io's emerging
from Jupiter's shadow on 9 November would be 10 minutes
late with respect to a timetable he had made up from
August observations. The prediction appeared to be
correct, which convinced academy members that Rømer's
idea about a finite speed of light was correct.
The final paragraph of his paper explains that none
of the reasons normally used to account for irregularities
in the period of a moon or planet can explain the
observed deviations in the period of Jupiter's innermost
moon.
At best, the paper provides data to establish a
lower limit on the speed of light. Rømer says
that Earth travels in its orbit at least 210 Earth
diameters in 42.5 hours.3
If R is the radius of Earth's circular orbit, the
planet covers a distance of 2πR per year, which
puts the lower limit for the radius at 9.2 ×
107 km; consequently, the lower limit of
the speed of light is 140 000 km/s.
Probably the first person to actually calculate
the speed of light was Huygens in 1678. Rømer
communicated his results to Huygens in a letter before
the paper was published. In his famous Traité
de la Lumière,4
Huygens wrote that Rømer's results had not
yet been published. Huygen's book appeared in 1690,
but it had been written before 1678. In that year,
he presented his theory on light at the academy.
Like Rømer, Huygens seemed barely interested
in the exact value of the speed of light. He estimated
Earth's diameter to be 12 750 km and the diameter
of its orbit to be 24 000 Earth diameters. According
to Rømer's observations, light traverses this
distance in 22 minutes. To keep things simple, Huygens
rounded the speed down to 1000 diameters per minute,
or 212 400 km/s. Without the rounding, his speed of
light would have been 232 000 km/s. It was Edmund
Halley, in 1694, who found that Rømer's 22
minutes should instead be 17 minutes; thus Halley
gave the speed of light as approximately 300 000 km/s.
The conclusion must be that Rømer is not
directly responsible for any of the values of the
speed of light attributed to him. He probably was
aware that the data were uncertain. But he was the
first one to prove that the speed of light is finite—a
scientific breakthrough that is essential to modern
physics. If the speed of light were not finite, we
probably would have to stick to a platinum bar in
Paris for the standard meter.
References
1. R. Taton, ed., Rømer et
la Vitesse de la Lumière, Librairie Philosophique,
Paris (1978).
2. O. C. Rømer, Philos. Tran.,
xii, 893 (1677).
3. For the possible origin of these
data, see A. van Helden, J. Hist. Astr. 14,
137 (1983).
4. C. Huygens, Treatise on Light,
S. P. Thompson, trans., Dover, New York (1962). Original,
Traité de la Lumière, published
in 1690.
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