Middle Ages: the first measurement of the speed of light

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

As is often the case in science, its calculation was a by-product of other actions that made much more practical sense. The end of the Middle Ages, European ships sail the oceans in search of new lands and trade routes. Newly discovered islands need to be mapped, and for this it is important to know more or less exactly where they are. There were noticeable problems with this.

Geographic coordinates are two numeric values - latitude and longitude. With latitude, everything is relatively simple: you need to measure the height above the horizon of some known star. In the Northern Hemisphere, it will most likely be the North Star, in the Southern - one of the stars of the Southern Cross. During the day, the latitude can be determined by the Sun, but the error is significantly greater - the luminary is quite large, it is difficult to follow it due to its brightness, and the boundaries of its visible disk are blurred under the influence of the earth's atmosphere. However, this is a relatively straightforward task.

What time is it now

Longitude is much more intricate. The Earth rotates on its axis, and you can find out where we are, knowing the exact time at this point and the time in some place, the longitude of which we know. In the literature, they usually write "prime meridian", this is, in general, correct, since we are talking about the same thing. If with the local time everything is quite simple, then with the zero meridian it is much more complicated.

There was no watch capable of showing the exact time of the place from where they were taken away in the era of the great geographical discoveries. At that time, a watch movement equipped with a minute hand was considered a high-precision technique. The first chronometers suitable for determining longitude appeared in the middle of the 18th century, and before that, mariners had to do without them.

The oldest theoretically worked out method was the lunar distance method, proposed by the German mathematician Johann Werner in 1514. It was based on the fact that the Moon is moving rather quickly across the night sky and by measuring with a special device - a transverse rod - its displacement relative to some known stars, you can set the time. The practical implementation of Werner's method turned out to be very difficult, and it did not play a noticeable role in navigation.

In 1610, Galileo Galilei discovered the four largest moons of Jupiter. This was an important scientific event - within the capabilities of the then observational astronomy, one more, besides the Earth, a celestial body was found, around which its own satellites revolved. But the most important thing for contemporaries was that the movement of these satellites could be simultaneously and equally observed from all points on the Earth, where Jupiter is visible at that moment.

Galileo Galilei

Already in 1612, Galileo proposed to determine the exact time, and hence the longitude, by the movement of Io, one of the four satellites of Jupiter. It has many remarkable features that Galileo, of course, did not know about, but, most importantly, it is relatively easy to observe. Finding out when he entered the shadow of the planet, it was possible to accurately establish the time. But the very first attempts to compile tables of eclipses of Io (and other Galilean satellites) revealed that this time was shifted in an incomprehensible way for the science of that era. The reasons remained unclear for three quarters of a century.

Merchant's son

Ole Christensen Rømer was born into a Danish merchant family in 1644. Information about his youth is fragmentary - he did not give birth, and personal fame will come to him much later. It is known that he graduated from the University of Copenhagen, and, apparently, was noticeable for his intellect. In 1671, Roemer moved to Paris, became an employee of Cassini and very soon was elected to the Academy of Sciences - then this meeting of learned people was less elite than later.

Ole Roemer

Towards the end of the century, he returned to Denmark, continued to be a practicing astronomer, and died there in 1710. But all this will come later.

It is finite

And in 1676, he proposed uncomplicated, for modern times, calculations that immortalized his name. The crux of the matter is simple. Jupiter is about five times farther from the Sun than Earth. It makes one revolution around the Sun in about 12 Earth years (we are rounding the numbers for simplicity). This means that in half a year, the distance from Jupiter to Earth will change by about a third. And this more or less corresponds to the observed difference in the eclipse times of the Galilean satellites.

Io today

It is now very easy for us to understand the logic of this reasoning, but in the 17th century it was customary to think that the speed of light is infinite. But Roemer suggested that this is not so. According to his calculations, the speed of light was equal to about 220 thousand kilometers per second, which is a quarter lower than the value established today. But for the 17th century it was not bad at least.

Then it turns out that everything is not so simple, and after two centuries Laplace will take into account the gravitational influence of satellites on each other, but this is a completely different story.

Roemer's idea did not play a significant role in geographical discoveries. Observing the moons of Jupiter through a telescope installed on board the ship was, due to the rolling, almost impossible. And in the middle of the 18th century, the first chronometers were developed, suitable for determining longitude.