Romer to Reaumur

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.

The Réaumur scale was proposed by René Antoine Ferchault de Réaumur in 1730. Réaumur was a French scientist primarily known for his contributions to natural history, particularly entomology. His thermometer used dilute alcohol and was calibrated between the freezing and boiling points of water, with the scale divided according to the actual expansion of the alcohol.

Réaumur calibrated his scale based on the physical expansion of his thermometric fluid — a specific dilute alcohol mixture. Between water's freezing and boiling points, the fluid expanded by exactly 80 parts in 1000 of its volume at freezing. He used this empirical measurement directly as the degree count, making 80°Ré the boiling point. The number 80 was not chosen arbitrarily but measured.

The French Revolution's embrace of the metric system in the 1790s demanded decimal-friendly units. Celsius's 0-to-100 scale fit the decimal philosophy perfectly; Réaumur's 0-to-80 did not. The French Academy of Sciences standardized Celsius for scientific work, and it spread through Napoleonic Europe. Réaumur lingered in German-speaking regions and Russia into the mid-19th century but eventually yielded everywhere. The irony is that Réaumur was French — his own country was the first to abandon his scale.

Multiply the Réaumur value by 5/4 (or 1.25). For example, 16°Ré × 1.25 = 20°C. To convert Celsius to Réaumur, multiply by 4/5 (or 0.8). The two scales share the same zero (0°C = 0°Ré); they differ only in degree size, since 100°C = 80°Ré.

Classic French confectionery texts list sugar syrup stages in Réaumur: "thread" stage at 80°Ré (100°C), "soft ball" at 94°Ré (117°C), "hard crack" at 124°Ré (155°C). Pastry chefs memorized these benchmarks and tested with a thermometer or by dripping syrup into cold water. Some vintage French and Swiss recipe books still reference Réaumur temperatures — modern reprints add Celsius equivalents in brackets. It is the last practical domain where Réaumur had a lingering foothold.