How did Ole Christensen Rømer, who in 1676, shortly after Galileo's death, was said to have measured the speed of light as 220,000 km/s by timing the orbits of Io around Jupiter?

My only guess is the moon Io orbiting Jupiter is simply a clock. It pulses away with an orbital period of 42 hours and provides to the earth, 21 hours of light each orbit; this does not change. Now, as viewed from "above" we go counterclockwise around the sun, and as we move towards Jupiter, the duration of light from Io would be less than 21 hours, whereas if we are moving away from Jupiter, the duration of light from Io would be more than 21 hours. Suppose we measure the time from Io's first appearance from behind Jupiter to its reappearance and call that its observed orbital frequency $f$ as opposed to its actual orbital frequency ${f_o}$ which is 1/42 hours.

He must have figured out the Doppler effect and this equation... $$f = \left( {1 + \frac{{\Delta V}}{C}} \right){f_o} % MathType!MTEF!2!1!+- % faaagCart1ev2aaaKnaaaaWenf2ys9wBH5garuavP1wzZbItLDhis9 % wBH5garmWu51MyVXgaruWqVvNCPvMCaebbnrfifHhDYfgasaacH8sr % ps0lbbf9q8WrFfeuY-ribbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0- % yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dc9Gqpi0d % meaabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWGMbGaeyypa0Zaae % WaaeaacaaIXaGaey4kaSYaaSaaaeaacqqHuoarcaWGwbaabaGaam4q % aaaaaiaawIcacaGLPaaacaWGMbWaaSbaaSqaaiaad+gaaeqaaaaa!3BBB! $$

I hashed through the numbers myself and I got an answer of about 10 minutes more or less from 42 hours depending on which way the earth is heading with reference to Jupiter.

  • $\begingroup$ No I think he knew the delta distance. Earth Jipiter and simply divided that by an observed delta t, at different times of the Earth and Jupiter orbits.At least this the principle, different path length / different time. $\endgroup$
    – Alchimista
    Commented Feb 20, 2018 at 12:14
  • 4
    $\begingroup$ You can consider asking in History of Science and Mathematics site hsm.stackexchange.com if you don't receive enough attention here, but wait a few days at least. $\endgroup$
    – uhoh
    Commented Feb 20, 2018 at 12:42
  • $\begingroup$ It's all well explained on wikipedia $\endgroup$
    – Walter
    Commented Feb 21, 2018 at 22:51
  • $\begingroup$ @Walter I read it on Wikipedia; I found it hard to follow and something was missing (i.e. the Doppler effect.). He must have had the equation above or he wouldn't have been successful at producing a good estimate. $\endgroup$ Commented Feb 21, 2018 at 23:36
  • $\begingroup$ There are also several new and also excellent answers in Physics SE: On the Rømer experiments and the speed of light $\endgroup$
    – uhoh
    Commented Aug 13, 2019 at 1:51

1 Answer 1


Ole Rømer did not measure a change in the frequency of light. He measured an apparent change in the orbital period of Io, one of Jupiter's moons.

The orbit of Io can be measured very accurately by observing when it enters or leaves the shadow of Jupiter. When the Earth is moving away from Jupiter, Rømer noted that the orbit of Io appeared to be very slightly longer than when the Earth was moving parallel to Jupiter.

Suppose the orbital period of Io is $p$ seconds. If you are not moving and you observe an eclipse at time $t$, the next eclipse will be at time $t+p$. However, if you are moving away from Io, the next eclipse will be seen at $t+p+x$, where $x$ is the time that light takes to travel from your position at time $t$ to your position $p$ seconds later.

The delay over a single orbit was too small for him to measure (about 30 seconds). But the delay was cumulative, and over about 30 orbits of Io, the orbit was delayed by about a quarter of an hour. He noted:

[This] 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.

Of course, Rømer didn't know the actual size of the half-diameter of the terrestrial orbit (the astronomical unit) However putting the modern value of about 150 million km this gives a value of the speed of light at about 220000km/s, which is not so far from the modern value of about 300000km/s

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    $\begingroup$ Although this isn't a measurement of the frequency of light, I wonder if it would still be fair to call it an instance of the Doppler effect. I ask because it still seems to be measuring a perceived decrease in (Io's orbital) frequency resulting from a difference in velocity. Is that correct? $\endgroup$
    – senderle
    Commented Feb 20, 2018 at 21:19
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    $\begingroup$ You can reason that way, although note that Rømer was 100+ years before Doppler. It's not a typical example of the Doppler effect. $\endgroup$
    – James K
    Commented Feb 20, 2018 at 21:24
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    $\begingroup$ @senderle ...the change in frequency or wavelength of a wave for an observer who is moving relative to the wave source. So maybe not, but it's an interesting way to think about it! $\endgroup$
    – uhoh
    Commented Feb 21, 2018 at 6:13
  • $\begingroup$ @uhoh, what's a wave? :) $\endgroup$
    – senderle
    Commented Feb 21, 2018 at 16:20
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    $\begingroup$ But didn't Christiaan Huygens measure an AU as being (1.023)(AU modern value) in 1659? Romer made his measurements in 1676 so he might have known the length of an AU and hence the speed of the earth around the sun or delta v. $\endgroup$ Commented Feb 22, 2018 at 3:35

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