Who determined how fast the speed of light was?

7 Dec 2016

Image: sarra22/Shutterstock

The latest Google Doodle to commemorate a major breakthrough in science harks back to 1676, when Danish astronomer Ole Rømer demonstrated that light had a finite speed.

The determination of the speed of light 340 years ago was a monumental moment – enabling humankind to accurately measure and predict events in our solar system – but it utterly changed our view of science.

At that time, Danish astronomer Ole Rømer was trying to solve the issue that other astronomers were facing, unable to accurately predict eclipses of Jupiter’s moon, Io.

Using a process of trial and error over the course of eight years, Rømer was able to announce to the Royal Academy of Sciences in Paris that Io would appear from behind Jupiter 10 minutes after it had been anticipated.

While his efforts were focused on correcting any previous miscalculations, his findings were actually instrumental in determining that the speed of light was not infinite, but very much finite.

In his scientific research paper, now featured on Journal des Savants, Rømer said that “light seems to take about 10 to 11 minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit”.

Speed of light Ole Rømer

More than just an astronomer

In doing so, Rømer became the first astronomer to suggest that the speed of light was 200,000,000m per second. This discovery would eventually inspire the work of other scientific visionaries, such as Isaac Newton and Edmond Halley.

The following centuries sparked much debate within the scientific community regarding the validity of Rømer’s light speed calculation with many subsequent, more accurate calculations emerging.

However, in 1975 it was finally determined that the speed of light was 299,792,458m per second.

In the 20th century, one astronomer greatly affected by Rømer was Albert Einstein, whose theory of relativity was based on the fact that the speed of light would remain constant in the vacuum of space.

While he might be best known for his efforts in helping us to understand the cosmos, Rømer was also instrumental in improving the standard of living in Copenhagen, where he spent his later years.

This included introducing oil lamp street lighting and proper sewers to the city, drastically improving the healthcare of its citizens.

He is also credited with inventing the mercury thermometer in 1709, which was said to have inspired Daniel Gabriel Fahrenheit to invent the eponymous temperature scale still in use today.

Updated, 1.30pm, 7 December 2016: This article was amended to reflect that accurate measurements of the speed of light existed prior to 1975, but this date was determined as being the most accurate.

Colm Gorey was a senior journalist with Silicon Republic