How and When was the Speed of Light Measured?

Throughout the history of science, one of the most fundamental constants in physics, the speed of light, has fascinated scientists and scholars. This article discusses the methods and techniques used to measure the speed of light, starting from the pioneering work of Danish astronomer Ole Roemer in the 17th century and moving through to contemporary methods.

The Earliest Successful Measurement: Ole Roemer

The first reliable measurement of the speed of light is attributed to Ole Roemer, a Danish astronomer in the 1600s. By observing the moons of Jupiter, specifically the moon Io, he noticed that the timing of its eclipses varied depending on the Earth's position relative to the Sun. Roemer deduced that this variation was due to the varying distance between the Earth and Jupiter, providing the time needed for light to travel this distance.

In 1676, Roemer made a groundbreaking observation. He noted that when the Earth was on the opposite side of the Sun from Jupiter, the timing of Io’s eclipse was delayed by approximately 22 minutes. Conversely, when the Earth was moving closer to Jupiter, the eclipse appeared earlier. This was the first direct empirical evidence for the finite speed of light, which was a departure from the then-prevailing belief that light travels instantaneously.

Mathematical and Experimental Approaches

The measurement of the speed of light follows a mathematical and experimental approach. An experimental method involves the use of electromagnetic waves. By bouncing these waves from Earth and recording the signal’s return, the time taken for the round trip is measured. This round trip time, combined with the known distance, allows scientists to calculate the speed of light.

Theoretical and Current Methods

The speed of light in various media can be measured using various methods such as interference patterns. However, the speed through a vacuum is not measured but defined. This value is (299,792,458) meters per second, equivalent to (670,616,629) miles per second, based on the international definition.

Ole Roemer's Contribution

Danish astronomer Ole Roemer's work in the 1670s was groundbreaking. He used the timing of the eclipses of Jupiter by Io to measure the speed of light. Roemer's observations showed that when the Earth was on the far side of the Sun relative to Jupiter, Io's eclipse was delayed by about 22 minutes. This delay was due to the additional time it took for the light from the eclipse to travel the extra distance.

Roemer also had access to the latest astronomical tools and techniques. He worked with his colleague Christiaan Huygens to refine the calculations and determine that the speed of light was approximately 131,000 miles per second. More refined measurements later adjusted this value to around 186,000 miles per second.

Accuracy and Challenges

Roemer’s measurements were quite accurate for the time, but they required a high degree of precision. The timekeeping instruments available in the 1670s, such as weight-driven pendulum clocks, had limited accuracy. Even with these limitations, Roemer was able to measure the time difference with remarkable precision, achieving accuracy within a tenth of a part per million (ppm).

Conclusion

The measurement of the speed of light has evolved significantly over time, from the pioneering work of Ole Roemer to modern experimental methods and definitions. Each method has contributed to our understanding of one of the most fundamental constants in the universe.