The Speed of Light series consist of five parts. Quick access links are here.
Part 1 | Earliest Ideas
Part 2 | The Eclipses of Io
Part 3 | Chopping Light Beams
Part 4 | Done With Mirrors
Part 5 | Michelson and Morley
In this post, the first successful calculation of the speed of light is described. It is an interesting story of astronomy.
Galileo and the Moons of Jupiter
Galileo Galilei was the first to record observations of Jupiter and its four largest moons through a telescope. This set the stage for later efforts by the Danish Astronomer Ole Christensen Rømer to measure the motions of Io, the closest of the moons to Jupiter. This post is about those efforts by Rømer and how they ultimately led to the realization that light had a finite and incredibly fast speed.
The telescope first appeared in the Netherlands in October 1608. The national government in The Hague received patent applications by Hans Lipperhey of Middelburg. Later, one was received from Jacob Metius of Alkmaar. He called it a device for “seeing faraway things as though nearby.” The telescopes had a convex and concave lens in a slender tube two to three feet long. Objects were magnified three or four times. Lenses of this type were in common use in eyeglasses.
News spread rapidly about the telescope. Soon, they appeared in many cities. Low powered versions could be purchased in Paris and cities in Italy. Thomas Harriot used a 6x telescope to view the Moon in August 1609. Galileo made his first 3x telescope in June or July 1609. He made an 8x one in August. His 20x version was used the first times in October or November 1609. This 20x telescope was used to discover the four moons of Jupiter. This short video uses a desktop software program to explain how the view might have appeared to Galileo. It should help you get a better sense of perspective on the rest of this post.
Latitude and Longitude
One of the huge challenges of the time was to know the correct longitude and time when sailing the seas. Time was used, along with the sextant, to navigate the seas. The sextant could accurately establish your position north or south by the locations in the sky of familiar objects such as the Moon, Sun, and stars. If Polaris, the north star, is directly overhead, you are at the north pole latitude. If it is near the horizon, you are near the equatorial latitude.
Where you are east or west is a more difficult question to answer. That is only determined by knowing the precise time. The time determines your longitude. Navigators and astronomers in the 1600s were trying to build a clock mechanism that would work accurately for long periods of time at sea. They also wanted a way to view some object in the heavens in order to know the exact time. That is why the Galilean moon Io is important. Astronomers hoped the consistent and regular orbit of Io might be useful as a clock. This brief video illustrates that idea and specifically what value Rømer measured.
Accidental Discovery by Rømer
Rømer set out to measure as accurately as possible the time between consecutive eclipses of Io by Jupiter. Sometimes, people set out to study something and end up re-directed in an unplanned direction leading to new insights and discoveries that are extremely important. Such was the case for Rømer.
He was at the Paris Observatory beginning in 1672 specifically to time the Io eclipse dates and times. During the time of closest approach of Earth to Jupiter, Io would have been larger and more easily seen and timed through a telescope. This short video shows the relative distances of the orbits of the two planets. The orbit of Io is too small to see in this simulated view. The video covers slightly more than a year of Earth time.
These two snapshots two months apart in time show Earth nearest to Jupiter and in the process of passing it. Two months of time would allow many orbits of Io to be timed. Rømer found the orbit time between Io eclipses to be 42.456 hours, less than two days between them. He potentially could have witnessed nearly 30 eclipses if clear skies always prevailed. Not likely at Paris.
It was hoped his data would allow accurate predictions of specific eclipse times at future dates in the year. Ships at sea would carry the ephemeris of eclipse times as seen from the Paris longitude on their journey. When they needed to know their own longitude, an observation of eclipse of Io would be made telescopically. Reference to the ephemeris for Paris time would be compared to the local time of the ship. If the ship saw the Io eclipse at 8 pm, and the Paris ephemeris said 12 pm, the 4 hour difference translates into 60˚ west of Paris longitude. Earth rotates 360˚ in 24 hours. That is 15˚ per hour toward the east. Paris would be more advanced in time than points west.
The key was to use the regular period of time of Io eclipses in order to predict the times on future dates throughout the year. Rømer could calculate 42.456 hours ahead from each one of the eclipses and predict all future dates in the year.
Also key was the ability to carry an accurate clock on a ship in order to know the correct time of a local place. Shipboard spring wound clocks were subject to large errors. That problem was solved 100 years later.
As Rømer worked to make the accurate eclipse timings he noticed something strange about his values. As the Earth moved in orbit to the distances farther from Jupiter as in these next two snapshots, observed eclipse times didn’t match the predicted eclipse times. The observations were several minutes later than the predictions as the dates increased. They reached a maximum of 22 minutes late when Earth was farthest away as in the second image, compared to when Earth was closest to Jupiter shown above.
The following is based upon notes from the web site of the Round Tower at the Univ. of Copenhagen, Europe’s oldest functioning astronomy observatory. Details about Ole Rømer are found at this link.
Rømer announced his results to the French Academie of des Sciences in September 1676.
The extra distance of travel of the light from the Io eclipse by Jupiter was taking longer to arrive because of the additional distance it must travel to reach Earth. He predicted an eclipse on November 9 would be delayed 10 minutes, which it was.
Enter Christiaan Huygens of Holland
Huygens read Rømer’s paper in September 1677. He wrote a letter to Rømer asking for more information. Huygens apparently understood the concept that light traveled across the diameter of the Earth’s orbit in 22 minutes. It had a finite speed.
In 1678, Huygens spoke to the Academie des Sciences and presented a paper called “Traite de lumiere“. In that treatise, he used the diameter of Earth’s orbit known at the time and Rømer’s value for the 22 minutes of light delay as it crossed the orbit diameter in order to calculate the speed of light. Ole Rømer is credited with the measurements leading to the concept that light had finite speed. Huygens did the arithmetic. He was the first person known to publish a value for the speed of light calculated from those measurements. His calculation was expressed in unusual terrestrial units as 16 2/3 earth-diameters/second, as well as others not used today.
Using today’s value for the diameter of the Earth of 12,742 km, the speed converts to 212,400 km/sec. The accepted value for the speed of light is 299,800 km/sec. The value calculated by Huygens from Rømer’s measurements was remarkably close to the correct value. It was wrong only because the diameter of the orbit of Earth was not known with sufficient accuracy.
Rømer went back to Denmark in 1681. He continued his career in science and government. He served as mayor and prefect of police of Copenhagen. He also served as head of the State Council. He is not remembered for his political offices. He is remembered for being the first person to measure the speed of light.