Ole Christensen Rømer
September 25th, 1644.
Ok, let's try this. Obviously, I'm not a scientist but go with me. Why do things travelling near the Speed of Light appear to slow down?
Time and space are the same thing.
A car at rest travels through time but not space (relative to the observer). A moving car is travelling both through time and through space (relative to the observer).
The car only has a finite amount of potential to move through space/time, so if it is moving through time, but not space, all of that potential is exhausted in moving through time. However, when it travels through space at the Speed of Light (if such a thing were possible), then nearly all of that potential would be spent travelling through space, leaving no potential for it to travel through time. Hence, it moves incredibly slowly through time.
No? Ok, how about his one!
If you are travelling through space at the Speed of Light in your car (if such a thing was possible) and you turned on the headlights, how fast would the light from the headlights be travelling?
The answer is ... drumroll ... The Speed of Light! Speed is distance over time and is measured relative to the observer, so no matter where you are standing and what speed you are going, the light will travel away from you at the Speed of Light.
No? ok, ok... here's one for the parts of the world (I'm looking at you, America!) that still like to measure everything in medieval:
What temperature does water freeze at and, what temperature does it boil at and who invented the thermometer that measures between these two points?
The answer is, of course, zero degrees Celsius to both* and Ole Rømer.
Ole Rømer, the Danish astronomer born on September 25, 1644, revolutionized the world's understanding of light and time. His contributions, particularly his groundbreaking discovery of the finite speed of light, laid the foundation for significant developments in astronomy and physics. Though he lived in an era when telescopes were still relatively primitive, his ingenuity in solving complex celestial problems distinguished him as one of the foremost astronomers of the 17th century.
Rømer was born in Aarhus, Denmark, and studied at the University of Copenhagen, where he immersed himself in mathematics and astronomy. His interest in astronomy deepened under the tutelage of the prominent Danish astronomer Erasmus Bartholin. Rømer's early studies were heavily influenced by the astronomical legacy of Tycho Brahe, whose observations had set a standard for precision in the field. At a time when scientific communities across Europe were flourishing, Denmark's astronomical traditions provided the perfect environment for Rømer's eventual rise to prominence.
His career took a decisive turn when he moved to Paris in 1672, where he worked at the Royal Observatory under the renowned astronomer Giovanni Cassini. Paris at that time was one of the epicenters of scientific progress in Europe, and Rømer's move there placed him in the heart of groundbreaking research. One of his primary tasks in Paris was to improve the accuracy of celestial measurements, which led him to investigate a key issue in astronomy: the irregularities observed in the timing of Jupiter's moons.
Jupiter's moons, particularly Io, had been the subject of much scrutiny. The satellites of Jupiter were used to determine longitudinal differences across the globe, and their precise orbits were crucial to improving navigation. However, astronomers observed that the timing of Io's eclipses—when the moon passed behind Jupiter—appeared inconsistent. Sometimes, these eclipses occurred earlier than predicted, while at other times, they were delayed.
It was Rømer who eventually solved this mystery in 1676, providing an explanation that would change the course of scientific history. He proposed that the discrepancies in Io's eclipses were not due to errors in observation or calculation but rather to the fact that light had a finite speed. When Earth was moving toward Jupiter, the light from Io's eclipses would reach Earth sooner, and when Earth was moving away, the light would take longer to arrive. This delay, Rømer argued, was due to the time it took for light to travel the vast distances between Earth and Jupiter.
This idea was revolutionary. Up until that point, most scientists, including luminaries like René Descartes, had believed that light travelled instantaneously. Rømer's calculations showed that the speed of light was finite and that it took about 22 minutes for light to cross the diameter of Earth's orbit around the Sun, a distance of roughly 186 million miles (300 million kilometres). From this, later astronomers would calculate that light travelled at approximately 186,000 miles per second (300,000 kilometres per second), a figure remarkably close to modern measurements.
Rømer's discovery had a profound impact not only on astronomy but also on physics. His work demonstrated the limits of observation and showed that celestial phenomena were not always instantaneous. This was a key step in the eventual development of the wave theory of light, which would later be expanded upon by scientists like Thomas Young and James Clerk Maxwell. Rømer's discovery of the speed of light also indirectly paved the way for Albert Einstein's theory of relativity, which fundamentally altered our understanding of space, time, and light.
In addition to his work on the speed of light, Rømer made other notable contributions to science and technology. He was appointed royal mathematician and astronomer in Denmark in 1681, a position that allowed him to implement significant reforms in Danish scientific practices. He improved the national calendar, correcting inaccuracies that had accumulated over the centuries. Rømer also worked on developing more accurate instruments for astronomical observations, enhancing the precision of measurements used in navigation and timekeeping.
One of his most lasting legacies was his invention of the first practical temperature scale. In the late 17th century, thermometry was still a developing field, and accurate temperature measurement was difficult to achieve. Rømer devised a temperature scale that used water's freezing and boiling points as reference points, much like the Fahrenheit and Celsius scales that came later. Though his temperature scale eventually fell out of use, it marked an important step toward the development of more accurate thermometers and temperature scales.
Rømer's influence extended beyond the realm of science. As an administrator, he was responsible for numerous public works in Copenhagen, including improving the city's water supply and overseeing the construction of new public buildings. His attention to practical matters earned him respect not only among scientists but also among the broader Danish public.
Despite his many contributions, Rømer's work was not always recognized in his own time. The idea that light travelled at a finite speed was controversial, and it took many years for the scientific community to fully accept it. Even after his discovery, some prominent figures, including Cassini himself, remained sceptical. It wasn't until later astronomers, such as James Bradley in the 18th century, provided further evidence for the finite speed of light that Rømer's contribution was universally acknowledged.
Rømer's legacy is perhaps best summed up by his ability to see beyond the limitations of the instruments of his time. His genius lay not just in his mathematical and observational skills but in his capacity to theorize about unseen forces and phenomena. He helped shift the scientific paradigm, showing that even something as seemingly instantaneous as light was governed by physical laws and could be measured.
By the time of his death in 1710, Rømer had established himself as one of the most important astronomers of his era. His contributions, especially his discovery of the speed of light, remain central to our understanding of the universe. Today, Rømer's work is remembered as a pivotal moment in the history of science, a moment when humanity first began to grasp the true nature of light and its role in the cosmos.
*The temperature at which water boils is dependent on local pressure, so it is possible to 'boil' water at any temperature.
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