50 Fun Facts About Celsius

The Celsius scale is named after an astronomer of which nationality?

Anders Celsius (1701–1744) proposed the first version in 1742.

In Anders Celsius's original 1742 scale, what temperature was 0 degrees?

He put freezing at 100; the scale was reversed within two years of his proposal.

Which French physicist of Lyon inverted Celsius's scale in 1743 to put freezing at 0 and boiling at 100?

He published a mercury 'Thermometer of Lyon' built by the craftsman Pierre Casati that used the modern direction.

Which botanist reversed the temperature scale in 1744 with a custom greenhouse thermometer?

His Hortus Upsaliensis of 1745 is the first known Swedish document to report temperatures the modern way up.

What was the scale commonly called in English before 1948?

From Latin centum, a hundred, and gradus, steps.

Why did the weights-and-measures authorities rename the scale in 1948?

In French, 'centigrade' also meant one hundredth of a gradian, an angle unit.

When did BBC Weather finally switch from the old name to 'Celsius'?

Australia had gone all-Celsius in its weather reports in September 1972.

What is absolute zero on the Celsius scale?

On the Fahrenheit scale it is −459.67 °F; on the Kelvin scale it is exactly 0 K.

Which other temperature scale is used within the International System of Units alongside Celsius?

The kelvin is the SI base unit; a degree Celsius has exactly the same size.

Since 1967, the degree Celsius has been the only SI unit whose full name has what feature?

That year 'degrees Kelvin' became simply 'kelvin', with a lower-case k and no degree sign.

According to the BIPM's rules, how should 30.2 degrees Celsius be written?

The no-space exception applies only to angles: degrees, minutes and seconds of arc.

Which countries still mainly use Fahrenheit rather than Celsius?

Palau, Micronesia, the Marshall Islands and the Cayman Islands are among the holdouts.

At what temperature do the Celsius and Fahrenheit scales give the same number?

A one-degree Fahrenheit interval equals five-ninths of a Celsius degree.

What did Daniel Fahrenheit use to fix the zero of his scale?

His upper point was his estimate of human body temperature, first 90 and then 96 degrees.

How many degrees separate freezing and boiling water on the Fahrenheit scale?

Water freezes at 32 °F and boils at 212 °F at standard pressure.

In which year did Daniel Fahrenheit propose his scale?

He visited England that year and was elected a Fellow of the Royal Society on 5 May.

Fahrenheit is credited with making the first successful thermometers filled with what?

They were more accurate than the spirit-filled instruments then in use, and consistent enough that different observers could compare readings.

Which Copenhagen astronomer's temperature scale inspired Fahrenheit after they met in 1708?

Rømer told him demand for accurate thermometers was high; Fahrenheit was born in Danzig to a German family and worked mostly in the Dutch Republic.

Who proposed the absolute temperature scale that bears his name in 1848?

Born William Thomson, he derived −273 °C for absolute zero from the expansion coefficient of gases.

How do you convert a Celsius temperature to the SI absolute scale?

A change of 1 °C is exactly a change of 1 K, so intervals convert one to one.

What temperature was defined as exactly 273.16 K when the absolute unit joined the SI in 1954?

That is 0.01 °C, the single condition at which ice, liquid water and vapour coexist.

The 2019 revision of the SI defined the base unit of temperature by fixing which physicist's constant?

Every 1 K change now corresponds to exactly 1.380649 × 10⁻²³ joules of thermal energy, decoupling the unit from water entirely.

Since the scale was decoupled from water, what is water's measured boiling point at one atmosphere?

On the ITS-90 calibration standard it comes out at roughly 99.974 °C; the melting point of ice is a hair below 0 °C.

Why is Celsius described as an interval scale rather than a ratio scale?

Kelvin, which starts at absolute zero, is a true ratio scale.

Absolute zero on the Fahrenheit scale is what?

The Rankine scale sets its zero there while keeping Fahrenheit-sized degrees.

Which law of thermodynamics implies that absolute zero cannot be reached in a finite number of steps?

It can be approached but never attained; each further reduction gets harder.

Which French physicist first raised the question of a lowest possible temperature in 1703?

He argued that the zero of his instrument would be where the 'spring' of the air vanished.

The Réaumur scale sets water's boiling temperature at how many degrees?

René Antoine Ferchault de Réaumur proposed it in 1730 using a diluted-alcohol thermometer.

The Rankine scale, proposed in 1859, combines absolute zero with degrees of which size?

W. J. M. Rankine was a University of Glasgow engineer; 1 °R equals 5/9 of a kelvin.

At which university was Anders Celsius professor of astronomy from 1730 until his death?

He founded the Uppsala Astronomical Observatory in 1741 with instruments bought on his European travels.

Anders Celsius was the first to suggest a connection between the aurora borealis and what?

He watched a compass needle and found bigger deflections during stronger auroras; he published 316 aurora observations in 1733.

Celsius joined a 1736 French Academy expedition to Lapland led by Maupertuis to measure what?

The result helped confirm Newton's view that the Earth is flattened at the poles.

What was the title of the 1742 paper in which Celsius described his temperature scale?

He showed the melting point of ice is essentially unaffected by pressure and mapped how boiling point varies with barometric pressure.

Who built the 'Linnaeus thermometer' with the modern Celsius direction in Stockholm?

He was Sweden's leading maker of scientific instruments; several Swedes are credited with independently flipping the scale.

Which Unicode character is discouraged in favour of the degree sign followed by a capital C?

It exists for compatibility with legacy encodings and vertical East Asian text.

To convert a temperature difference from Celsius to Fahrenheit, you multiply by what?

Intervals do not get the +32 offset; a Celsius interval converts to kelvin one to one.

Which craftsman built the 1743 'Thermometer of Lyon' that used the modern direction of the scale?

Christin published its design on 19 May 1743.

Which non-standard symbol is sometimes used to distinguish a temperature interval from a temperature?

'40 °C ± 3 K' is another way writers avoid the ambiguity.

What did the 1954 General Conference on Weights and Measures define one standard atmosphere to be?

Celsius had proposed calibrating his zero, the boiling point, at mean sea-level pressure.

Fahrenheit's parents died in 1701 after eating what?

He was placed under guardianship and sent to a merchant apprenticeship in Amsterdam, from which he later ran off.

In which decades did most English-speaking countries switch from Fahrenheit to Celsius?

The United States was the notable exception.

An altitude change of only about 28 cm shifts water's boiling temperature by how much?

That sensitivity is why the 16-millikelvin gap between old and new definitions of 100 °C matters so little in daily life.

The surname Celsius, a Latinised estate name, comes from a Latin word meaning what?

The estate at Doma was also known as Hojen or Hogen.

In which year did Anders Celsius found the Uppsala Astronomical Observatory?

It was equipped with instruments he bought on his long tour of European observatories.

How many observations of the aurora borealis did Celsius publish in his 1733 Nuremberg collection?

He compiled sightings by himself and others from 1716 to 1732.

Whose belief that Earth is flattened at the poles did Celsius's Lapland expedition confirm?

A companion expedition to Peru, near the equator, provided the comparison.

Celsius wrongly explained the slow rise of Scandinavia above sea level as being caused by what?

The land is actually rebounding since the ice of the last ice age melted.

Of what disease did Anders Celsius die in 1744, aged 42?

He had been secretary of the Royal Society of Sciences in Uppsala since 1725.

Which learned body's name did Celsius propose when it was founded in Stockholm in 1739?

Linnaeus and five others founded it, and Celsius was elected a member at the first meeting.

Celsius published magnitudes for roughly how many stars using his own photometric system?

His measurements had a mean error of about 0.4 magnitude.