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The scale on your thermometer carries Anders Celsius's name, but if you could use his original instrument you would find it running backwards: water boiled at 0 and froze at 100. Celsius was an astronomer who wanted thermometers in Paris and Uppsala to agree, and his real contribution was choosing two fixed points and testing them. This post covers his life, how he built the scale, the conversions you need, and what a degree Celsius means today.
Who was Anders Celsius?
His MacTutor biography gives his dates as 11 November 1701 to 25 April 1744, both in Uppsala, Sweden. He came from a family of astronomers and mathematicians, and became professor of astronomy at Uppsala in 1730.
From 1732 he travelled through Europe, visiting observatories and academies. In 1733 he published 316 observations of the northern lights. In Paris he joined the debate about the shape of the Earth and took part in Maupertuis's expedition to Lapland in 1736 and 1737, which measured a degree of latitude near the Arctic Circle. The results supported Newton's view that Earth is flattened at the poles. Back home he built a new observatory in Uppsala, finished in 1741. He died of tuberculosis in 1744, aged 42.
How did Celsius build his scale?
Celsius explained his choice in a 1742 report, Observationer om twänne beständiga Grader på en Thermometer, roughly "observations on two constant degrees on a thermometer". MacTutor summarises his tests:
the freezing point did not change with latitude or with air pressure;
the boiling point did not depend on how long the water boiled or where the water came from;
the boiling point did depend on pressure, by about one degree for each inch of change in the barometer, so he set a standard barometer reading for it.
He then divided the interval between his two fixed points into 100 degrees, putting 0 at the boiling point and 100 at the freezing point. MacTutor records that he first used the scale on 25 December 1741 and that its direction was reversed after his death, giving the 0 for freezing and 100 for boiling that we use now. His stated aim was that thermometers made in different cities would read the same in the same air.
How do you convert between temperature scales?
The NCERT Class 11 thermal properties chapter gives the ice and steam points as 0 °C and 100 °C, and 32 °F and 212 °F. There are 100 intervals on one scale and 180 on the other, so (t_F − 32)/180 = t_C/100, which gives t_F = (9/5)t_C + 32. NCERT also gives T = t_C + 273.15 for the Kelvin scale. Thermal equilibrium and temperature are in the JEE (Main) 2026 syllabus.
Worked example 1: Celsius's original scale. On the original scale, call the reading θ. Freezing was 100 and boiling 0, so for an ideal linear thermometer θ = 100 − t_C. A temperature of 37 °C would have read 100 − 37 = 63 on Celsius's own instrument. A hot day of 45 °C would read 55, a lower number for a hotter day.
Worked example 2: any two-point scale. A badly marked thermometer reads 20 in melting ice and 180 in steam. What is the real temperature when it reads 100? Use the same method as NCERT's Fahrenheit relation: (100 − 20)/(180 − 20) = t_C/100, so t_C = 80/160 × 100 = 50 °C.
Worked example 3: the one useful coincidence. Where do the Celsius and Fahrenheit readings match? Set t_F = t_C = x: x = (9/5)x + 32, so −(4/5)x = 32 and x = −40.
Common mistakes:
Adding 32 to a temperature change. A change of 10 C° is a change of 18 F°. The +32 is only for readings.
Writing 273 instead of 273.15 when precision matters. For most JEE numericals 273 is fine, but read what the question gives.
Assuming the old fixed points still define the scale. They no longer do, as the next section shows.
Practise more conversions with our post on the zeroth law and temperature scales, and compare Celsius with the scale-maker before him in Daniel Fahrenheit and the mercury thermometer.
What does a degree Celsius mean today?
In 1948 the General Conference on Weights and Measures listed the unit as the "degree Celsius", symbol °C, in its Resolution 7 on unit symbols. Today the kelvin is defined by fixing the Boltzmann constant at exactly 1.380 649 × 10⁻²³ J/K, and the degree Celsius follows from it through T = t_C + 273.15.
NCERT points out a consequence: with these exact choices, water's freezing and boiling points at 1 atm are very close to, but not exactly, 0 °C and 100 °C. Celsius's fixed points became a convenient check, not the definition.
What can a student learn from Celsius's way of working?
Test your reference points. He did not assume freezing and boiling were fixed; he varied the conditions and checked.
Find what your result depends on. Discovering that boiling depends on pressure made his scale usable instead of misleading.
Design for others. He wanted a thermometer in Paris to agree with one in Uppsala. Writing a solution someone else can follow is the same habit.
Accept improvements to your work. His direction was flipped after his death, and the scale is better known for it.
If you mix up which formula applies to a reading and which to a change, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can rebuild the idea of a temperature scale from its fixed points.
Next step: write today's maximum temperature in °C, °F, kelvin and on Celsius's original scale, and check each answer against its fixed points.
