The Story of Fahrenheit and Celsius: Two Men, Two Scales

Daniel Gabriel Fahrenheit (1686–1736)

Daniel Gabriel Fahrenheit was born in Danzig (now Gdansk, Poland) in 1686, the eldest of five children in a prosperous merchant family. Both his parents died of mushroom poisoning on the same day in 1701, when he was fifteen. He was sent to Amsterdam to learn the merchant trade but became obsessed with scientific instruments — particularly thermometers and barometers. He spent years traveling across Europe meeting instrument makers and scientists before settling in the Netherlands and establishing himself as the most skilled thermometer maker of his age.

Before Fahrenheit, thermometers existed but were nearly useless for comparison. Every instrument maker used a different scale. A reading of "50°" on one thermometer meant nothing when compared to "50°" on another. Fahrenheit solved this by inventing reproducible, standardized reference points and by replacing alcohol-based thermometers with mercury. Mercury's uniform thermal expansion, higher boiling point, and visibility in a glass tube made it vastly superior for precision measurement. Fahrenheit thermometers became the European standard.

The Three Reference Points

Fahrenheit established his scale using three reference points, which he described in a 1724 paper in the Philosophical Transactions of the Royal Society:

1. 0°F — the temperature of a brine solution (a mixture of ice, water, and ammonium chloride). Fahrenheit used this as his lowest reliably reproducible fixed point. He believed this was approximately the coldest temperature achievable in the laboratory.

2. 32°F — the freezing point of pure water.

3. 96°F — the temperature of the human body (specifically, measured in the mouth or under the armpit). Fahrenheit later described this as "the temperature of a healthy man."

The scale was designed so that these three points fell at round numbers. The gap between 0° and 96° was divided into 96 equal parts, which allowed convenient subdivision — 96 is divisible by 2, 3, 4, 6, 8, 12, 16, 24, 32, and 48.

Why 32° for Freezing and 212° for Boiling?

The 32° and 212° values are consequences of the original three-point calibration, not deliberate choices. Once Fahrenheit fixed 0° as the brine point and 96° as body temperature, the freezing point of pure water fell naturally at approximately 32° on that scale.

The boiling point of water at 212° was not part of Fahrenheit's original design either. After his death, scientists recalibrated the scale using two fixed points — the freezing and boiling points of pure water at standard atmospheric pressure — and adjusted the degree size slightly so those points fell exactly at 32° and 212°. This moved body temperature from 96° to approximately 98.6°F (37°C exactly), the value still cited today.

The gap from 32° to 212° is exactly 180°. This is not a coincidence of design; it is a result of retroactive calibration that preserved the original zero point while anchoring the upper end to a reproducible physical phenomenon.

Anders Celsius (1701–1744)

Anders Celsius was born in Uppsala, Sweden in 1701, into a family of scientists. His grandfather Magnus Celsius was a mathematician; his uncle Olof Celsius was a botanist who worked with Linnaeus. Anders became a professor of astronomy at Uppsala University at age 29 and was one of the most respected scientists in Scandinavia.

His scientific work ranged across astronomy, atmospheric optics, and the aurora borealis. He participated in an expedition to Lapland in 1736–37 that confirmed Newton's prediction that the Earth is flattened at the poles — a significant validation of Newtonian physics. He founded the Uppsala Astronomical Observatory in 1741.

The Original Inverted Scale (100° = Freezing)

Celsius published his temperature scale in 1742 in a paper titled "Observations on two persistent degrees on a thermometer." His choice of fixed points was elegant: the freezing and boiling points of water at standard atmospheric pressure, which are more reproducible than body temperature or brine solutions. The scale had 100 equal divisions between these points — making it decimal and therefore easy to calculate with.

Here is the counterintuitive part: Celsius's original scale was inverted relative to the one that bears his name today. He placed 100° at the freezing point of water and 0° at the boiling point. This may have been a practical choice — for an astronomer in Sweden, working outdoors in winter, a thermometer where temperature increases as the mercury falls (toward 100°) may have been easier to read. Or it may have reflected a preference for defining the scale from water's behavior rather than from an arbitrary zero.

The inverted scale was used during Celsius's lifetime and for a few years after his death.

Linnaeus Flips the Scale

Carl Linnaeus, the Swedish botanist famous for creating the binomial system of biological classification, was a colleague of Celsius at Uppsala. In 1745, one year after Celsius died of tuberculosis at age 42, Linnaeus inverted the scale to its modern orientation: 0° = freezing, 100° = boiling. He did so for the practical reason that positive numbers increase with warmth, which matched intuitive experience.

The instrument maker Daniel Ekström, who built thermometers for both Celsius and Linnaeus, may have made the change earlier; the historical record is ambiguous. What is clear is that the modern Celsius scale — 0° freezing, 100° boiling — was in use at Uppsala within a year of Celsius's death and spread from there.

The scale was called "centigrade" (Latin: centum gradus, hundred steps) for most of its history. The name "Celsius" was officially adopted by international agreement in 1948 to avoid confusion with a French unit of angle measurement also called "grade."

Lord Kelvin and the Absolute Scale (1848)

In 1848, the Scottish physicist William Thomson (later Lord Kelvin) published a paper proposing an absolute temperature scale based on thermodynamic principles rather than the behavior of any specific substance. His insight was that temperature measures the average kinetic energy of particles, and that there is a theoretical minimum temperature — absolute zero — where particle motion ceases entirely.

Kelvin's scale uses the same degree size as Celsius but places its zero point at absolute zero: −273.15°C (or −459.67°F). This makes the Kelvin scale the natural unit for thermodynamics: all temperatures are positive, and ratios of temperatures are physically meaningful (twice the Kelvin temperature = twice the average kinetic energy).

Key equivalences: - 0 K = −273.15°C = −459.67°F (absolute zero; theoretical minimum) - 273.15 K = 0°C = 32°F (freezing point of water) - 373.15 K = 100°C = 212°F (boiling point of water at 1 atm) - 310.15 K = 37°C = 98.6°F (human body temperature)

Kelvin is the SI base unit of temperature and is used exclusively in physics, chemistry, and engineering thermodynamics. In the 2019 SI revision, the kelvin was redefined by fixing the value of the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin.

Where Each Scale Is Used Today

The division is sharper than the "three metric holdouts" story suggests for temperature. Even within the US, Celsius is used in medicine (body temperature as 37°C), chemistry, meteorology for research, and all scientific publication. The temperature converter handles all three scales, including the Celsius to Fahrenheit conversion most travelers need.

A useful reference: - −40°C / −40°F (the coincidence point — see below) - 0°C = 32°F (water freezes) - 20°C = 68°F (comfortable room temperature) - 37°C = 98.6°F (body temperature) - 100°C = 212°F (water boils at sea level)

The −40° Coincidence

At exactly −40°, the Fahrenheit and Celsius scales agree. This is not a design feature of either scale — it is an algebraic consequence of their relationship.

The conversion formula is: °F = (°C × 9/5) + 32

Setting °F = °C: °C = (°C × 9/5) + 32 °C − (°C × 9/5) = 32 °C × (1 − 9/5) = 32 °C × (−4/5) = 32 °C = −40

The intersection at −40° is the unique point where the two linear scales cross. It has practical relevance: −40° is the rated operating temperature for many automotive lubricants and batteries, and it appears on standardized cold-weather testing specifications. Engineers writing specifications that need to hold in both metric and non-metric contexts often use −40° as a convenient common reference.

Will Fahrenheit Eventually Disappear?

The same logic that argues against full US metrication applies specifically to temperature. Fahrenheit's scale, for all its historical oddity, has one genuine argument in its favor as an everyday scale: its range from 0° to 100° roughly spans the range of temperatures humans actually experience outdoors in most inhabited regions. In Fahrenheit terms, 0°F is very cold and 100°F is very hot. In Celsius terms, the equivalent range is −18°C to 38°C — numbers that do not map as intuitively onto "very cold" and "very hot."

This is not a strong enough argument to override the advantages of decimal consistency and global interoperability, but it explains why Fahrenheit persists in US popular culture even as the US has adopted Celsius in science and medicine.

What is more likely than Fahrenheit's elimination is continued bifurcation: Celsius for science, medicine, and international travel; Fahrenheit for US weather forecasts and casual conversation. The two systems already coexist in the US without significant confusion for most people. A full switch would require changing every thermostat, oven, weather display, and popular weather app — a massive infrastructure cost with limited benefit to the people who would bear it.