Practice makes perfect – the initial measurement of the light speed

“It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.” Albert Einstein

The light has fascinated people for centuries from both scientific and philosophical perspectives. The history of light’s nature and its speed is so old that this topic was touched by Greek scientists Empedocles and Aristotle. Empedocles suggested that light moves, and it needs time to travel, while Aristotle believed that light travels instantaneously.

Who was right? Which theory was more correct? Is it possible to count the lights traveling time? The answer lies in the future; today, we know that light is a wave with a known frequency. Let’s take a journey that humanity had to take to obtain this knowledge.

Image credit: E. Klejman

Dual nature of the light 

The light nature has excited research since ancient times. According to Maxwell’s Theory light is a propagating electromagnetic wave called electromagnetic radiation [1], while Quantum Theory suggests that light consists of tiny portions of energy called photons [2].

When we treat light as a type of radiation, it’s range oscillates from long radio waves (even a few km) to extremely short waves like gamma rays. When we think about light as a group of particles then they interact with other types of particles and change the properties of these particles (i.e. energy). For example, electrons (particles, which are negatively charged). The process when photons interact with electrons is called excitation.

To visualize this process imagine a ball, which is jumping down from stairs step by step. The steps symbolize energy levels, while one jump made by a ball can be compared to the excitement. The photons have no electric charge; therefore, it is not possible to send wirelessly electric current via light.

How photons are made? They are created in natural processes like nuclear reactions occurring in the Sun or bioluminescence. The first one is based on interactions between atomic nuclei and radioactive decay, while the second one is based on the chemical reactions taking place in the organism, for example, glowing jellyfish [3].

Thanks to the dual nature of the light, it is possible that the data can be transferred within long distances, where the other forms of communication like electrical cables or WiFi are not possible. [4]. And in nature, the Sun is a good example,  it heats our planet using nothing else but energy carried by light moving in space with the highest possible velocity being, in fact,  the speed of light.

Measurements- determining the speed of light

Now, we know what light and its nature are? But, if it is possible to measure light? Light travels through space with a constant velocity called speed of light (C), while centuries ago, scientists thought that light passing through space has its speed and its magnitude is infinite. Unfortunately, early attempts to check were not successful [5].

One of the first measurements of the speed of light was taken by a well-known scientist – Galileo. During the night time, he placed two of his students with lamps on two hills, which were 2 km apart. When he tightened his lamp, his student had to wait until he saw the light, and afterward, he had to lighten his lamp.[6] The rest of the experiment was just basic math, but due to high measurement error caused by human reaction time, the experiment was unsuccessful.

Galileo set the speed of light to be more or less 30 000 m/s, which is, unfortunately, the wrong value. Nevertheless, it was one of the first attempts to measure the speed of light. Unfortunately, Galileo did not take into account the human error that affected his experiment. But a similar measuring method will be used in the future, showing how brilliant man Galileo was.

Another scientist attempting to measure the speed of light was Ole Rømer.[7] In 1676, he found out that when Earth is moving away from Jupiter, the circulation period of its natural satellite Io is increasing. He was looking at Io’s eclipses to determine its circulation period.

The observation did not work with calculations of Io’s circulation period. Over the half-year, the delay accumulated up to 22 minutes, and then it started to decrease as Earth moved towards Jupiter.  The scientist claimed that the delay occurred because light from Jupiter had to travel an additional distance of Earth’s orbit diameter before it reached our planet. He knew the length of Earth’s diameter and using this knowledge he calculated the speed of light. It can be seen in Figure 1; Unfortunately, his work was not as precise as he wanted it to be.

According to his experiment, the speed of light was equal to 214 000 000 meters per second. Despite the wrong result, results we’re getting closer to the value we know today. Rømers work made a huge impact on science because he proved that the magnitude of the speed of light is not infinite [8].

Figure 1. Visualization. Image credit: E. Klejman

An astronomer James Bradley also tackled the problem of measuring the speed of light. He found out that some stars are moving in the night sky in small ellipses during the year [9]. Later in 1727, he discovered the circulation of the Earth as a result of this phenomenon.

Let’s imagine if we run in the rain, drops of rain from our perspective will be falling not exactly down, but at the angle. The same things happen with the light (shown in Figure 2), and so when the earth travels in one direction it creates the angle. This angle is known as an angle of aberration. If we measure this angle then calculating the speed of light is possible due to trigonometric functions.

Figure 2. Light aberration. Image credit: E. Klejman

Bradley set the speed of light to be 301 000 000 meters per second. This result is phenomenal because the measurement error was less than 1%, which overall is a small mistake. His measurement was the first measurement of the speed of light, which could be called accurate; therefore, James Bradley’s legacy should never be forgotten.


The earliest measurements of the light speed may not be highly accurate, but they are evidence of human creativity. Scientists from centuries ago do not have high technologies, but they tried to accomplish something that has never been accomplished before, things that were believed to be impossible. It makes their work remarkable.

Currently, we of course know that the speed of light is equal to 299 792 458 meters per second. So Bradley did not make a gross mistake in estimating the speed of light value. Still, scientists want to have a more precise value. The development of technology and human thought in the future enabled us to carry out even more accurate measurements.

This article is z joint work of Karol Kuryłek (Faculty of Chemistry, University of Warsaw), Jan Kaczmarek (Faculty of Chemistry, University of Warsaw), Ewa Klejman, (Faculty of Chemistry, University of Warsaw), Agnieszka Pregowska (Institute of Fundamental Technology Research, Polish Academy of Sciences, and Magdalena Osial (Faculty of Chemistry, University of Warsaw) as a part of the Science Embassy project. Image Credit – E. Klejman.


[1] Thomson, J. J. (1936). The Nature of Light. Nature, 137(3458), 232-233. DOI:10.1038/137232a0

[2] Gurevich, A., Garipov, G., Almenova, A., Antonova, V., Chubenko, A., Kalikulov, O., Zybin, K. (2018). Simultaneous observation of lightning emission in different wave ranges of electromagnetic spectrum in Tien Shan mountains. Atmospheric Research, 211, 73-84. DOI:10.1016/j.atmosres.2018.04.018

[3] Freudenrich, C., & Harris, W. (2020, May 22). How Light Works. Retrieved November 18, 2020, from

[4] Thouand, G., & Marks, R. (2014). Bioluminescence: Fundamentals and Applications in Biotechnology – Volume 1. Berlin, Heidelberg: Springer Berlin Heidelberg.

[5] Aoki, K., & Mitsui, T. (2008). A tabletop experiment for the direct measurement of the speed of light. American Journal of Physics, 76(9), 812-815. DOI:10.1119/1.2919743

[6] Foschi, R., & Leone, M. (2009). Galileo, Measurement of the Velocity of Light, and the Reaction Times. Perception,38(8), 1251-1259. DOI:10.1068/p6263

[7] Helden, A. V. (1983). Roemer’s Speed of Light. Journal for the History of Astronomy, 14(2), 137-141. DOI:10.1177/002182868301400206

[8] Garlinska, M., Pregowska, A., Masztalerz, K., & Osial, M. (2020, October 22). From Mirrors to Free-Space Optical Communication—Historical Aspects in Data Transmission. Future Internet, 12(11),179. DOI:10.3390/fi12110179

[9] Williams, M. (1982). James Bradley and the eighteenth Century ‘Gap’ in attempts to measure annual stellar parallax. Notes and Records of the Royal Society of London, 37(1), 83-100. DOI:10.1098/rsnr.1982.0006


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