Delisle to Romer

Delisle calibrated his thermometer by observing how mercury contracted as it cooled from boiling water. He measured degrees of cooling rather than degrees of warmth — the scale counted how far the temperature had dropped from the boiling point. This made 0°De the starting reference (boiling) and larger numbers represent lower temperatures, the opposite of every other common scale.

The freezing point of water is 150°De. The boiling point is 0°De. The scale spans 150 degrees between freezing and boiling — contrast with 100 degrees in Celsius or 180 degrees in Fahrenheit. To convert: °De = (100 − °C) × 1.5, so 0°C gives (100 − 0) × 1.5 = 150°De.

Joseph-Nicolas Delisle invented the scale in St. Petersburg in 1732, where he had been invited to work by Peter the Great. The scale was adopted by Russian scientists and used throughout the 18th century, particularly by Mikhail Lomonosov. It was later standardized by Joseph-Nicolas's colleague Joseph-Adam Braun, who fixed the two reference points.

Subtract the Delisle value from 100, then multiply by 2/3. Formula: °C = 100 − (°De × 2/3). For example, 150°De → 100 − (150 × 2/3) = 100 − 100 = 0°C (freezing). For 0°De → 100 − 0 = 100°C (boiling). Remember the scale is inverted: higher Delisle = colder temperature.

Delisle is the only well-known inverted temperature scale, but the concept appears elsewhere. Astronomical magnitude runs backwards (brighter stars have lower numbers). The Scoville scale for chilli heat is not inverted but is logarithmically compressed in a way that confuses people similarly. In thermometry, Delisle stands alone — every other scale (Celsius, Fahrenheit, Kelvin, Rankine, Newton, Rømer, Réaumur) increases with heat. The inversion made Delisle unintuitive, which is a key reason it lost out to Celsius.

The Rømer scale was created by Danish astronomer Ole Christensen Rømer around 1701. Rømer is also famous for being the first person to measure the speed of light quantitatively in 1676, determining it by observing time variations in the eclipses of Jupiter's moon Io from different positions of Earth in its orbit.

Rømer set 0°Rø below the coldest temperature he expected in Denmark so that all practical outdoor measurements would be positive. This was a common design principle for early thermometric scales — avoiding negative values in everyday use. The 7.5 value arose from his calibration methodology using two fixed reference points and dividing the interval into 52.5 equal parts.

Normal body temperature (37°C) is approximately 22.5°Rø on the Rømer scale. Rømer himself used body temperature as one of his calibration reference points, which Fahrenheit later borrowed when constructing the Fahrenheit scale — translating Rømer's body temperature reference into his own 96°F calibration point.

Daniel Gabriel Fahrenheit visited Ole Rømer in Copenhagen in 1708 and observed his thermometers and calibration method. Fahrenheit adopted Rømer's idea of using two fixed reference points and the principle of avoiding negative temperatures in common conditions. He then redesigned the scale — multiplying Rømer's degrees by approximately 4 and shifting the zero — to achieve finer graduation and a different zero point.

Unlikely. Rømer's scale had an awkward 7.5°Rø freezing point and a 52.5-degree span — not easy to memorize or subdivide cleanly. Celsius's 0-to-100 design was simpler, aligned with the decimal metric system sweeping Europe, and gained powerful institutional backing from Swedish and French academies. Fahrenheit's scale — partly derived from Rømer's — won in the English-speaking world largely due to British imperial reach. Rømer's real legacy is indirect: inspiring Fahrenheit, who then dominated for 250 years.