The Christmas Day that just passed, marked the 283rd anniversary of the creation of the much used Celsius temperature scale.
I had the pleasure of reading Ian Hembrow's biography of Celsius recently for Physics World and found many aspects of the story fascinating: not least the fact that Celsius was not working on the physics of heat and/or temperature when he created his famous scale. He was working on Gravity.
Isaac Newton's theory of Gravitation had suggested the geoid shape of the earth - the idea that it should bulge slightly at the equator and be flattened at the poles, but in the 1700s the theory was not fully accepted. If it could be shown that the earth conformed to this prediction, it would offer a strong confirmation of the theory, and Celsius was one of a group put together to take the measurements needed to do so.
A quick glance at a photo of the earth (not an option available to Celsius) will show that the effect is very, very slight. To check if it is true at all required extremely precise measurements of the distance between two lines of latitude on the surface of the earth: if that distance was greater near the poles than it was to the south, then it suggest a flattening of the earth.
I was initially confused on reading about this: had I not learned in Geography class back in the Mullingar CBS in the 1980s that the lines of latitude are - by their very definition - evenly spaced? I probably had - but that is an approximation of the reality that the position of a line of latitude is based on lines drawn to the centre of the earth - as shown in the diagram here, and explained in more depth in the associated video.
But it wasn't at all an easy thing to measure the distance between two lines of latitude with precision in the 1700s. Indeed it wouldn't be simple now.
To do so requires extremely skilled surveying. As any leaving cert student will tell you, if you know the measure of two angles in a triangle and the length of one side, you can quickly calculate all angles and lengths. The art of surveying basically comes down to selecting key points on a landscape, and measuring all the angles created by the ensuing network of triangles, and at least one length.
That's simple enough on paper, but to make those measurements in northern Sweden forced Celsius and his group to spend many months cutting down hundreds of trees near to the peak of several mountains, leaving only one tall, denuded tree at each peak so that it could be seen from the others. That gave them the angles they needed. To measure one length precisely they had to spend weeks camped on the frozen surface of the Torne river - the only straight, flat, line available to them.
When all that was done, they found that one degree of latitude spanned a distance almost 1.5km greater in Sweden than an equivalent measurement in France. Newton's theory was supported.
It was a huge achievement and added much to the reputation of the still young scientist. But when he returned to his Uppsala base to take up his duties as a professor there, he was not satisfied. He knew that a further confirmation of the theory was possible, using a simple pendulum.
Again, as any leaving cert student could tell you, the period of a pendulum is determined by its length and the acceleration caused by gravity - which itself varies with the distance to the equator. If his Uppsala laboratory was marginally closer to the equator than a laboratory in France would be - due to the flattening of the earth - then the period of a pendulum should vary between the two points. Though this was true, of course, only if the two pendulums had exactly the same length.
He carried out a series of measurements for the period over 1741. He was concerned that the length of the pendulum might vary slightly due to the changing temperatures so he kept his lab well warmed by a fire that winter to try to match the summer temperatures - but he still wanted to monitor the temperature and that was hard to do in 1741. He had a simple thermometer that had been sent to him by a scientific colleague, De Lisle, from Paris. De Lisle had used the boiling point of water as one reference point on his scale, but Celsius was unhappy with the other reference point - the temperature in De Lisle's basement in Paris.
So while working on Christmas Day (one can only imagine it wasn't his job to make the dinner) he substituted that with the freezing point of water - and the centigrade scale was born!
Interestingly it was initially upside down: the b.p. was 0oC and the m.p. 100oC. That was flipped sometime in the years following his death.
Its a very enjoyable book, by the way. Utterly accessible while not selling the science short.