49 Fun Facts About Celsius

Which cold-weather emergency sets in when the body's core temperature falls below 35 °C?

Hypothermia. Below 35 °C the body is losing heat faster than it can make it, and because the cold also dulls thinking, victims often fail to realise they need help.

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

The boiling point of water. Celsius put freezing at 100, so his thermometer's numbers climbed as the weather got colder; within a few years others had flipped it to the form used today.

Which Italian scientist is credited with inventing the thermoscope, a forerunner of the thermometer, around 1600?

Galileo Galilei. His glass bulb and tube showed a liquid rising and falling with the temperature, but with no scale it could only show a change, never measure one.

Which 1740s naturalist had a modern-style Celsius thermometer built for his greenhouses?

Carl Linnaeus. He is often named as the man who gave the scale its modern form, but several scientists and instrument makers adopted it at about the same time, and no single person can take the credit.

What was the Celsius scale commonly called in English before 1948?

Centigrade, meaning a hundred steps. It was renamed after Anders Celsius in 1948, partly because in France the word centigrade was already in use for a unit of angle.

What heats its surroundings to about 30,000 °C, roughly five times hotter than the Sun's surface?

A lightning bolt. The flash heats the gases in its path so suddenly that they burst outward as a shock wave, and that is the sound we hear as thunder.

Which planet has the Solar System's hottest surface, about 465 °C, though another lies closer to the Sun?

Venus. Its thick carbon-dioxide atmosphere traps heat in a runaway greenhouse effect, leaving the surface hot enough to melt lead.

What is absolute zero on the Celsius scale?

−273.15 °C. Nothing can be colder, because at absolute zero particles have as little motion as nature allows, which is why the Kelvin scale starts its count there.

Which element has the lowest boiling point of all, about −269 °C?

Helium. Even at that temperature it only turns liquid, and at ordinary pressure it will not freeze solid however far it is chilled.

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

An uppercase letter. SI unit names are written in lower case even when they honour a person, like newton and watt, and 'degrees Kelvin' fell into line in 1967 by becoming plain 'kelvin'.

Under SI rules '30 °C' takes a space; the only unit symbols allowed to touch their number appear in which of these?

A ship's latitude. The SI treats °C like any other unit symbol and sets it off with a space, but the degree, minute and second signs for angles sit directly against the number, as in 45°.

A 1975 US law backed moving to metric units, Celsius included; why did Fahrenheit survive?

The switch was voluntary. The Metric Conversion Act created a board to coordinate the change but gave nobody the power to require it, and the board itself was abolished in 1982.

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

−40. A Fahrenheit degree is only five-ninths the size of a Celsius degree, so the two scales close in on each other as it gets colder and meet exactly once, at minus forty.

By his own 1724 account, what did Daniel Fahrenheit use to set the zero of his scale?

An ice-and-salt mixture. Such a brine settles by itself at one steady chill, about −18 °C, so he could recreate his zero in a workshop without waiting for a cold day.

Fahrenheit originally fixed 96 degrees on his scale as the temperature of what?

The human body. When the scale was later re-pinned to water's freezing and boiling points at 32 and 212, normal body temperature slid from 96 to the familiar 98.6.

Which London body published Daniel Fahrenheit's 1724 thermometer papers?

Royal Society. Its Philosophical Transactions printed five papers by him in a single year, giving the benchmarks of his scale, including the 32 and 212 still used for water.

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

Mercury. It proved an excellent liquid for showing changes in temperature, and his thermometers agreed so closely that observers in different places could at last compare readings.

Which Copenhagen astronomer's temperature scale did Daniel Fahrenheit adapt into his own after visiting him?

Ole Rømer. After the visit, Fahrenheit split each degree of Rømer's scale into four, which is how his own finer scale began.

Lord Kelvin, namesake of the absolute temperature scale, took his title from a river in which city?

Glasgow. William Thomson was a professor at its university from 1846, and when he was made a baron in 1892 he chose the name of the River Kelvin, which runs through the university grounds.

Which process, devised in the 1860s to keep wine from spoiling, now heats milk to about 72 °C?

Pasteurisation. Louis Pasteur found that gentle heating killed the microbes that soured wine, and the method was extended to milk only later.

From 1954 to 2019, the kelvin and the degree Celsius were defined by which fixed point of water?

Its triple point. Ice, liquid water and vapour coexist at only one temperature, and the scale's zero was set exactly a hundredth of a degree below it; a physical constant took over in 2019.

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

Boltzmann's. His constant ties temperature to the energy of moving particles, so since 2019 the kelvin has rested on a fixed number rather than on any property of water.

At about 4 °C, liquid water reaches its maximum what, which is why lakes freeze from the top down?

Density. Water colder than 4 °C is lighter and stays on top, so ice forms as a lid over the lake; if the coldest water sank instead, lakes would freeze from the bottom up.

Calling 20 °C 'twice as hot' as 10 °C is a mistake because the Celsius scale lacks what?

A true zero. Zero Celsius is only where water freezes, so ratios mean nothing on it; counted from absolute zero in kelvins, 20 °C is just 3.5% hotter than 10 °C.

Instead of melting, dry ice undergoes which change of state at about −78 °C?

Sublimation. Dry ice is frozen carbon dioxide, which has no liquid state at normal atmospheric pressure, so instead of leaving a puddle the block turns straight into a cold gas.

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

The third law of thermodynamics. It means cooling anything all the way to absolute zero would take an infinite number of steps, though laboratories have chilled atoms to within billionths of a degree of it.

Around 1700, Guillaume Amontons glimpsed the idea of absolute zero while studying the 'spring' of what?

Air. He saw that cooled air pushes back less, and reasoned that at some low enough temperature it would lose its spring altogether, a limit to cold proposed almost 150 years before Kelvin's scale.

Which metal has the highest melting point of all, about 3,400 °C?

Tungsten. That is why light-bulb filaments were made of it: they can glow white-hot without melting, yet the pure metal is soft enough to be cut with a hacksaw.

Which metal melts at just under 30 °C, so a solid lump of it turns liquid in a warm hand?

Gallium. Mendeleev predicted the metal and its low melting point from a gap in his periodic table, and its discovery in 1875 was an early triumph for the table.

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

Uppsala. Fame from an Arctic expedition helped him persuade the Swedish authorities to pay for a modern observatory there, which opened in 1741.

Anders Celsius was among the first to link the aurora borealis to what?

Changes in the Earth's magnetic field. With his assistant Olof Hiorter he watched a compass needle and found it swung furthest on the nights when the northern lights were strongest.

Anders Celsius campaigned for Sweden to adopt which calendar, which it finally did nine years after his death?

The Gregorian calendar. Sweden's own earlier attempt, dropping leap days one at a time from 1700, ended in such a muddle that 1712 was given a 30 February to undo it.

In his 1742 paper, Celsius showed that water's boiling point rises and falls with the reading of which instrument?

The barometer. Because boiling shifts with atmospheric pressure, he said that end of his scale must be fixed at one standard pressure, while he found that ice melts at the same point whatever the weather.

Daniel Fahrenheit was born in Danzig, a Baltic port that is now in which country?

Poland. Danzig is today's Gdańsk; his family were German merchants there and he did his life's work in the Dutch Republic, which is why he gets called Polish, German and Dutch by turns.

The faint afterglow of which event keeps even the emptiest space about 2.7 degrees above absolute zero?

The Big Bang. Its heat has been stretched and cooled by the expanding universe into a faint microwave glow that fills the whole sky.

Which gas law says a gas's volume shrinks steadily as it cools, hinting at a coldest possible temperature?

Charles's law. Extend the line downward and the gas would shrink to nothing at one particular temperature, which gave early physicists their estimates of absolute zero.

Near absolute zero, atoms can merge into a state of matter named after Einstein and which Indian physicist?

Satyendra Nath Bose. He sent Einstein his calculations in 1924 and Einstein extended them to atoms, but seventy years passed before a laboratory chilled atoms cold enough to make the condensate.

Which gas turns liquid when chilled to about −196 °C?

Nitrogen. It makes up about 78% of the atmosphere we breathe, yet it has to be chilled nearly 200 degrees below freezing before it will turn liquid.

Which part of the brain acts as the body's thermostat, holding it close to 37 °C?

The hypothalamus. Sensing that the body is running warm or cool, it sets off sweating or shivering to pull the temperature back toward its set level.

Orphaned at 15, Daniel Fahrenheit was sent to learn a trade in which Dutch city?

Amsterdam. Both his parents died on the same day in 1701, and the merchant's trade he was sent to learn lost out to scientific instruments, which he went on to make for a living.

The coldest permanently inhabited places on Earth, where winters have reached −67.8 °C, lie in which region?

Siberia. Verkhoyansk recorded it in 1892 and Oymyakon matched it to within a tenth of a degree in 1933; parts of Antarctica get far colder, but nobody makes a permanent home there.

The Sun's surface is about 5,500 °C; which outer layer of its atmosphere is mysteriously hotter, at over a million degrees?

The corona. Nobody has fully explained why the Sun's atmosphere gets hotter the farther it stretches from the surface; astrophysicists call it the coronal heating problem.

The surname Celsius comes from the Latin celsus, a word related to 'excel'; what does it mean?

High. An ancestor turned the name of the family home, Högen, 'the mound', into Latin and took it as a surname.

Tardigrades, microscopic animals that can survive being chilled below −200 °C, go by which nickname?

Water bears. They ride out such cold by shutting their metabolism down to a standstill, and in that state they have also survived heat well above boiling and the vacuum of space.

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

Isaac Newton. He had argued that the spinning Earth must be squashed at the poles, and the claim was disputed, so France sent one expedition to the Arctic and another to the equator to measure it.

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

Water evaporating. He was right that the shoreline was shifting but blamed a shrinking sea; in fact the land is still rising, freed from the weight of the ice that melted after the last ice age.

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

Tuberculosis. He died only two years after presenting his thermometer, never knowing the scale would carry his name; that became official in 1948, more than two centuries later.

Which learned body, founded in Stockholm in 1739, counted Anders Celsius among its first members?

Royal Swedish Academy of Sciences. It set out to spread useful knowledge in plain Swedish, and today it is the body that awards the Nobel Prizes in Physics and Chemistry.

Lord Kelvin, of the absolute temperature scale, was knighted in 1866 for his work on which engineering feat?

The Atlantic telegraph cable. He designed the sensitive mirror galvanometer that could read its faint signals, and the project made the physicist a fortune as well as a knight.