Anders Celsius

In Uppsala we have one of the oldest coherent weather measurements in the world. Anders Celsius, 1701-1744, the unknown man with the famous name, was a professor at Uppsala University and developed the unit of temperature that we still use today: the Celsius scale.

Anders Celsius was also the one who, together with his professor Erik Burman, helped start Uppsala University's weather measurements on 12 January 1722; every day they went out and measured temperature, air pressure and wind direction.

Weather measurements, or meteorological measurements as it is called, are common today, but a few hundred years ago there was no systematic approach and several different temperature scales and thermometers were used, including the de I'lsle and Fahrenheit scales. It was only when Celsius developed the international temperature scale Celsius, the scale we still use today, that we got uniform values.

Weather data and statistics from the 18th century onwards are very rare. Besides our weather statistics in Uppsala, there are only a few places that have carried out similar measurements; Berlin 1719, Lund 1740, St. Petersburg 1743, Stockholm 1756 (as an example, some of them certainly have interruptions during the measurement period).

Today, measurements are carried out automatically; every day, every second, precipitation, temperature, air pressure, relative humidity, soil temperature and solar radiation are measured. But there are also manual measurements; every morning at least one of our meteorology PhD students goes out to the observation farm in Uppsala and measures precipitation and possible snow depth.

Researchers both in Sweden and internationally use our weather data and SMHI also uses our data as input to their weather forecast models and climate work.

So why is a sustained series of weather measurements so important? Well, when we have such a long series of measurements, we can see how the weather and temperature vary over time, and this naturally becomes another important piece of the puzzle in our understanding of the climate.

Celsius - a pioneer

°C is known to stand for degrees Celsius, but who was Celsius? Celsius was a pioneer in investigating the Earth and its changes through systematic observations and by starting long series of numerical data, including temperature. He started this work in Uppsala in 1722.

Anders Celsius lived from 1701 to 1744 and grew up in an astronomical family in Uppsala but in rather poor conditions. At the age of 28, he became a professor of astronomy, but his main interest was our own planet.

Celsius first travelled to Germany, Italy, France and England for four years, then joined a French scientific expedition to the Arctic Circle for a year, and then founded an observatory in Uppsala (next to his mother's restaurant). However, his life was short; he died, unmarried but with many friends, of lung disease at the age of 42. But what did he actually do?

Arctic Circle expedition and Newton's theories

Newton's theories on gravity and centrifugal force were controversial at the time. In Paris, there were heated debates for and against, and they were going to try to settle the question by determining whether the Earth was slightly flattened at the poles as a result of its rotation. Just then Celsius arrived, and he suggested that the proposed expedition should go to the Arctic Circle in Sweden, to Tornedalen.

Celsius now became a member of the French expedition, which carried out measurements at the Arctic Circle in 1736-1737, both against stars in the winter cold and between mountain peaks in the summer. Gravity was also measured there. The results were then compared with those around Paris.

The expedition itself did not succeed in solving the controversial problem, but together with earlier observations of other phenomena, it eventually tipped the balance of the debate in Newton's favour.

Observatory, zero meridian and gravity

Inspired by what he had seen in Paris and Greenwich, after returning from the Arctic Circle, Celsius convinced the University to establish an observatory in Uppsala in 1739. The building is still standing as the house 'on a slant' in the centre of Uppsala. Celsius worked here for the rest of his short life with his diligent assistants Olof Hiorter and Pehr Wargentin.

Celsius became involved in new methods of using celestial bodies to determine the latitude and longitude of the Earth's surface, important for mapping, and he made the Uppsala Observatory the first zero meridian in Sweden. Together with Wargentin, he made Uppsala an international leader in longitude determination.

Celsius was also able to determine gravity in Uppsala, with a pendulum clock set up in the observatory and constructed by a specialist in London. It was one of only a few gravity points in the world at the time.

Sea level and land uplift

Celsius had taken an early interest in the phenomenon then known as the water reduction, now known as the post-glacial land rise, and in 1743 succeeded in determining its speed for the first time. He used an old seal stone, a stone where seals could be shot long ago when they were resting near the mean sea level at the time. The result was just over 1 cm/year, about the right value.

Based on this value, Celsius then concluded that large areas of Sweden must have been below sea level a few thousand years ago. He brought science into the story.

For the benefit of future generations, Celsius had a special mean sea level mark carved into another seal stone in 1731. This proved valuable a hundred years later, when it could be used to show that the phenomenon was a land rise and not a water fall.

Temperature scale and temperature series

Since 1722, Celsius had not only measured temperature but compared different thermometers and temperature scales. He found none of them reliable and therefore, after much testing, constructed his own temperature scale in 1742, defined by the freezing and boiling points of water at normal air pressure. He divided the interval in between into 100 degrees. Today this is the international unit of temperature.

Even when it came to temperature, Celsius was thinking of the benefit to future generations; he expected a weather record to be kept in Uppsala in perpetuity. Today, the temperature series from Uppsala starting in 1722 is one of the longest in the world.

Already in the latter part of the 18th century, Wargentin was able to use the temperature series to study climate change, a now highly topical subject.

Magnetic fields and northern lights

When Celsius was in London, he ordered a large compass with an extra-long needle from the instrument specialist there. In Uppsala, he used it to study small variations in the direction of the magnetic field and wondered if they could be related to phenomena in the atmosphere.

In the winter of 1741, Celsius and his assistant Hiorter noted that the magnetic needle contrasted with the appearance of the northern lights in the sky. In addition, through a collaboration with London, Celsius showed that a similar magnetic needle there contrasted with the one in Uppsala. Hiorter and Celsius had discovered that there must be a connection between the aurora and the magnetic field.

Today, this connection is fundamental to the understanding of the auroral phenomenon and the role the sun plays in it.

Text by Martin Ekman, Associate Professor of Geophysics and author of the book "The Man behind 'Degrees Celsius': A Pioneer in Investigating the Earth and its Changes".

More information about the book