The goal of this project is to research and replicate Ole Rømer’s method of measuring the speed of light to get a more accurate measurement. When observed, the time between the eclipses of Io, one of Jupiter’s moons, was inconsistent and seemed to vary based on the distance between Earth and Jupiter, the closer Jupiter was to Earth, the closer together the times of the eclipses were, and the further Jupiter was from Earth, the longer the time period between the eclipses. Using the difference between the time of the orbit as seen from Earth when Jupiter was closer, and when Jupiter was further he predicted that Io’s eclipse would be later than expected (in his case, he predicted 10 minutes) to prove his theory. His prediction was correct. Another scientist then calculated the speed of light using Romer’s discoveries. He ended up with a final calculation of 220,000 km/s, while the now accepted actual speed is 299,792.458 km/s.


Io’s Orbit around Jupiter: 1.769137786 days = 42.4593069 hours



Now that I have researched Rømer’s method, I am using TheSkyX Pro program to replicate the night sky back in 1676 when Rømer did his experiment and to show exactly when we can see Jupiter in the future at a certain time. The program shows what you will see in the sky at any time on any date from your location, and specifically what you can see with a small telescope, a big telescope, and the naked eye. 


I did my first trial of determining the speed of light through TheSkyX Pro program. I came up with 369,944 km/s, about 23.4% off of the true speed of light. This first trial was slightly more accurate than Romer’s final measurement, except Romer’s value was about 26.6% under the true speed of light while my first measurement was over. This first trial was to just get in the ballpark of the right number and make sure there were no huge slip-ups or things I wasn’t accounting for. Now that I know I am generally properly replicating his method, I am going to do some tests on the SkyX program simulating the time he did it in 1668-1678


To get a more accurate measurement that also allows the observations to be realistically done when the time comes, I have decided to change my method a bit to where they will be more similar to how Romer did his observations. I am going to take multiple observations from a few months before and after Jupiter’s opposition, this should give more accuracy in both the end result and the replication of how it was originally done. This is because you can only see Io’s immersions for a couple months before opposition and Io’s emersions for a couple months after opposition.


I have finished simulating the observations from 1672, many of them are very close in time to Romer’s observations but there is still a fair difference between the observations I got from SkyX and Romer’s documented observations. (Romer’s observations are below).


It turns out that I am not able to replicate Romer’s experiment in the time of the 1670’s because the SkyX program shows what really happens and not what you see from Earth, and the records I’ve found of Romer’s observations don’t have enough of the details I need to get an accurate measurement. I am going to start doing a series of observations hopefully in a few weeks because Jupiter is at opposition on May 9th, and as I stated in a previous post I am only able to get observations of immersions a couple months before and emersions a couple months after opposition, so from around March to July. I am going to take these observations I take and then take the difference of them compared to what I see on the SkyX because the SkyX shows what’s actually and not what seems to be happening from Earth.


May 8, 2018 – Immersion at 5:05:37 pm

Date      Apparent        Apparent      Distance  Distance  App.   Ang.    Phase   S.E.      S.E.   P.A.                Solar
(0 UT)       R.A.            Declination   to Earth   to Sun      Mag.  Diam.  Angle  Long     Lat    Axis   Ls        Elong

Mar 09   15 24 35.40   -17 24 39.5   4.92446   5.42115   -1.8     40.0     9.5       270.7   -3.4   15.0   266.3   115.1W

Observation Dates:

March 13, 2018 – 3:02:28 am | Distance: 4.86 au

Romer’s Observations:


“Bradley’s Discovery of Stellar Aberration.” CSeligman, Accessed 9 Nov. 2017.

The Editors of Encyclopædia Britannica, editor. “Armand-Hippolyte-Louis Fizeau.” Britannica, 20 July 1998.

“Galileo Galilei Biography.” The Famous People, 27 Dec. 2016, Accessed 7 Nov. 2017.

“Geocentric Ephemeris for Jupiter : 2018  Coordinated Universal Time (UTC).” Astro Pixels, Fred Espenak,

“How the speed of light was first measured.” Today I Found Out, Accessed 10 Nov. 2017.

“Ole Rømer.” Encyclopedia Britannica, 20 July 1998, Accessed 11 Sept. 2017.

“The Position of Jupiter in the Night Sky, 2014 to 2018.” Naked Eye Planets, edited by Martin  J. Powell, Martin J Powell, July 2014,

“Profile: Ole Roemer and the Speed of Light.” American Museum of Natural History, Accessed 12 Sept. 2017.

Smith, Margaret. “Ole Rømer and the Speed of Light.” JSTOR Daily, 7 Oct. 2014. JSTOR, Accessed 12 Sept. 2017.

“Speed of Light Historical Measurements.” Speed-Light, Accessed 12 Sept. 2017.

نور، این «ذره – موج» پُر از زیبایی و راز” [“Measurement of the Speed of Light”]. Hamneshinebahar, 2012, Accessed 4 Nov. 2017.