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In Denmark his name is written "Ole Rømer". Seindal, Wednesday, April 3, 2002

My book explicitly says that he never calculated the speed of light and that the orbit of Earth and Jupiter was not known in his time. The article said differently. Lir 02:23 Nov 19, 2002 (UTC)

Hello. You would need to state the title of "your" book. 86.52.114.223 (talk) 16:16, 20 January 2014 (UTC)[reply]

Dubious

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"Notable is also his definition of the new Danish mile. It was 24,000 Danish feet, which corresponds to 4 minutes of arc latitude, thus making navigation easier."

This is a conflation of the definitions of two different units of measure, the Danish land mile of 24,000 feet, and the geographical mile of four minutes of arc.

I'm pretty sure it hasn't always been mixed up this way in the article, so the first thing I'll do is check the history. Gene Nygaard (talk) 01:19, 3 March 2008 (UTC)[reply]

My recollections were faulty, and this has been incorrect since User:Egil added it over 3 years 1 month ago in this edit. So there isn't any better version to revert to, it just needs fixing. Gene Nygaard (talk) 01:25, 3 March 2008 (UTC)[reply]
I agree that this looks wrong and I've removed the mention of the "four minutes of arc"-mile. Rømer defined the Danish mile as 24,000 Danish feet which corresponded to about 7,532m, while the geographical mile of four minutes of arc is about 7,422m, so there are clearly two different miles here. For all I know, Rømer may have chosen the value of 24,000 to correspond roughly with 4 seconds of arc, but I can't find a source supporting that idea, so unless someone else can, this certainly does look like someone conflated the two miles by mistake. Hemmingsen 19:22, 25 October 2008 (UTC)[reply]

Section on determination of speed of light

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I think those other materials re basing on Romer's strategy should be placed somewhere... This article is a biography on the person. --Bentong Isles (talk) 05:21, 11 November 2008 (UTC)[reply]

The paragraph below:

"Assume the Earth is in L, at the second quadrature with Jupiter (i.e. ALB is 90°), and Io emerges from D. After one orbit of Io, 42.5 hours, the Earth is in K. Rømer reasoned that if light is not propagated instantaneously, the additional time it takes to reach K, that he reckoned about 3½ minutes, would explain the observed delay."

contains a mistake. For the Earth to be 3.5 light-minutes more distant from Jupiter than it was at position L, it must move around the Sun for approximately 25 days; during this time, Io would make about 14.2 orbits of Jupiter. Not a single orbit, as the paragraph above states.

Sincerely, A Ph.D. Physicist —Preceding unsigned comment added by Tenorio40 (talk • contribs) 01:43, 20 June 2009 (UTC)[reply]

I was hoping that someone would rewrite the entire light speed calculations part, since it is not very good, but I suppose correcting small errors is better than nothing. I shall make an attempt.DanielDemaret (talk) 08:31, 20 June 2009 (UTC)[reply]

I made that small change. However, I think that the explanation right now still contains several irrelevant notions that should be left out, and that some more relevant details should be put in. An example of a better explanation is here: http://www.is.wayne.edu/mnissani/a&s/light.htm. If n-oone protests, no-one changes this and I feel like it sometime I shall then rewrite that small paragraph. If I feel like it. DanielDemaret (talk) 08:47, 20 June 2009 (UTC) Since there are many different kinds of "minutes" and the articele does not specify nor make it clear which kind, it is confusing.DanielDemaret (talk) 08:54, 20 June 2009 (UTC)[reply]


Daniel - Since your improvement of June 20, 2009 it is much better. But I agree with you, the entire light-speed calculation section could be improved further. If you don't mind doing it, please do, since the Romer measurement was very interesting and important in the history of science. This does not matter now, but in the original mistaken paragraph (with only one orbit of IO around Jupiter), the Earth would have only moved away from L by 15 light-seconds after 42.5 hours, which is 14 times less than 3.5 light-minutes. But, as I mentioned, this is now irrelevant. I encourage you to rewrite the paragraph. Thanks in advance. Richard. —Preceding unsigned comment added by Tenorio40 (talk • contribs) 18:30, 22 June 2009 (UTC)[reply]

I agree that there's probably enough material to create a separate article on Rømer's determination of the speed of light, especially as I want to add some more! But, in the meantime, a couple of comments here:
  • I'm not sure the note about the date of the opposition of Jupiter is really relevant, at least not if it is referenced to Dieter Egger's applet. If we want to find a date for the opposition in 1676, we should look to a professional ephemeris: someone at WikiProject Astronomy must have access to one, or there might be the relevant data in doi:10.1119/1.19020
  • Where does "Rømer's data" come from? There's no reference for it, and one recurring modern comment about Rømer's determination is precisely the lack of available experimental data to support it.
Physchim62 (talk) 12:02, 20 October 2009 (UTC)[reply]
Rømer's data pp. 4,1
Rømer's data pp. 2-3
I don't know what reference has been used for the eclipse data currently in the article or why the 1676 observations are picked out but they clearly originate in Rømer's own manuscript. While Rømer never published his experimental data and they were thought to be lost, Kirstine Meyer discovered Rømer's manuscript with all original data (and parts of his calculations) in 1913 and published them in 1915. I uploaded it to commons a couple of months ago, so next time you hear someone complaining about the lack of experimental data to support Rømer's determination, just point them the way to commons :) Hemmingsen 18:21, 20 October 2009 (UTC)[reply]
Wow! That's great! I can easily explain the concentration on 1676 values.
On 22 August 1676, Cassini made an announcement to the Royal Academy of Sciences that he would be updating his ephemeris of Io to take account of a new effect (or "irregularity" as he called it). He seems to have given the figure of "ten or eleven minutes" as the time it takes light to travel the distance detween the Earth and the Sun, and he certainly predicted that the emersion of Io from Jupiter's shadow on 16 November 1676 would occur ten minutes later than would otherwise be calculated.
For some reason, the eclipse of 16 November was never used – maybe it was just cloudy in Paris that night – but there was another eclipse on 9 November 1676 which also fitted with the new model. Rømer then presented the new calculations to the Royal Academy of Sciences on 21 November.
Hence, it is fairly clear that these are not Rømer's original results, the one's he used to make his 1676 predictions, as they have observations from 1677 and 1678. Nor is it clear that all of the observations are Rømer's – Rømer arrived in Paris in 1672, while the table has observatons from 1669 and 1671: in fact, it is widely accepted that Rømer also took advantage of obervations by his friend Picard. Rømer might have used some of his own results from Uranienborg, but he must have known that doing so would introduce a new factor – the difference in longitude – and so make his calculations less precise. Anyway, he was working in what was then the world centre for observations of Jupiter, so why not take advantage of the long set of observations that were available to him.
The document seems to be more of a convenient table of eclipse observations that he could use to try to refine his calculations. Still, it allows anyone (who can be bothered) to reconstruct what he should have seen on the given dates, and so rework the calculation. Physchim62 (talk) 11:32, 21 October 2009 (UTC)[reply]
In 1677 Huygens wrote to Rømer with some questions on how Rømer had determined his value of 22 minutes. Quoting from Rømer's reply from September 1677 (I am using a Danish translation in Friedrichsen, Per & Tortzen, Chr. (eds.) (2001) Ole Rømer of the Latin original, so the translation may be slightly rough)
I have collected and examined all observations of the eclipses of the first moon made by either Mr. Picard or myself since 1668. It is actually more than 70.
Of these I have selected and compared the following intervals.
  • As the Earth was moving away from Jupiter between March 1671 and May 1671.
  • Immersions as the Earth was approaching Jupiter between October 1671 and February 72.
  • Emersions as the Earth was moving away from Jupiter March 72 to June 72.
  • Immersions as the Earth was approaching Jupiter between November 72 to March 73.
  • Emersions as the Earth was approaching Jupiter April 73 to August 73.
  • Emersions as the Earth was moving away between July 1675 and October 1675.
  • Immersions as the Earth was approaching May 76 to June 76.
  • Emersions as the Earth was moving away August 76 to November 76.
  • Immersions as the Earth was approaching June 77 to July 77.
[...]
With regards to the determination of the famous 22 minutes I primarily chose observations from the years 71, 72 and 73, partially because we had more from that period, [...]
It is partially on the basis of that letter that Meyer concluded that the observations in the document are Rømer's original data, and it is also why I find picking only the 1676 observations is a bit arbitrary, since Rømer's focus apparently was on 71-73.
You are of entirely right that it probably isn't be the original document (the later observations could have been added later, though, I suppose).
Rømer apparently used observations made in both Paris, Uranienborg and Copenhagen (Rundetårn) and attempted to correct for the difference in longitude using the values on the top of page 2.
If anyone's interested, Meyer's 1915 article does reconstruct Rømer's calculations to significant detail so it shouldn't be necessary to rework them. Hemmingsen 12:59, 25 October 2009 (UTC)[reply]

One might add, near the end of the section, that the speed of light is nowadays defined as precisely 299,792,458 metres per second, which (the second being already well-defined) defines the length of the metre. Wikipedia will have the details. Date around 1975, I think. 94.30.84.71 (talk) 22:28, 18 June 2022 (UTC)[reply]

Confusing

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In the section on the speed of light the articles says:

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...

Then in the next paragraph it says:

...Huygens deduced that light travelled 16+2⁄3 Earth diameters per second, misinterpreting Rømer's value of 22 minutes as the time in which light traverses the diameter of the Earth's orbit.

What did Huygens misinterpret, the value of 22 minutes or what it means?

Anachronism?

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The article mentions that "Rømer joined the observatory of Uraniborg on the island of Hven, near Copenhagen, in 1671"; however, the article on Uraniborg mentions that "The institution was destroyed in 1601 after Tycho's death" and later "both Uraniborg and Stjerneborg were destroyed shortly after Tycho's death," without any mention of the observatory being rebuilt at any point until recently ("Uraniborg's grounds are currently being restored," which doesn't specify the date, but obviously in the 20th or 21st century).

Does anyone have any explanation for this apparent anachronism?

Thanks in advance.

CielProfond (talk) 03:10, 19 February 2017 (UTC)[reply]

Den Store Danske Encyklopædi tells the story of Ole Rømer as follows: He worked on Tycho Brahe’s manuscript at Copenhagen University when Jean Picard came to Denmark to measure the precise location of “Uraniborg Plads” (Uraniborg square), so probably where Uraniborg used to stand. Also the Encyclopædie Britannica states similarly that Picard was interested in Uraniborg’s precise location to compare Tycho Brahe’s measurements with those made elsewhere. Donik (talk) 07:56, 26 September 2018 (UTC)[reply]

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