Celsius. Or, should that be celsius? Centigrade, even? Or centigrade? It seems proper we should consider such words at this time of year when our Celsius/centigrade, Celsius/ Centigrade degrees of heat are generally at their warmest.

Anders Celsius (1701 to 1744) was the Swedish astronomer known for inventing the Celsius temperature scale. He also built the Uppsala Astronomical Observatory in 1740, the oldest astronomical observatory in Sweden. The family name is said to be a latinised version of where they came from.

Anders was born at Uppsala where his father Nils Celsius had been an astronomy professor as were his grandfathers. Anders became professor of astronomy at Uppsala University in 1730.

There he built the “Celsius thermometer”, with 0 for the boiling point of water and 100 for its freezing point. After his death from tuberculosis in 1744 this was reversed.

Celsius and centigrade are two names for essentially the same temperature scale (with some slight differences) and tend to be used as though interchangeable. The centigrade scale is based on dividing the temperature between which water freezes and boils into 100 equal gradients or degrees.

The word centigrade comes from the Latin centum translated as "100" and gradus translated as "steps". The centigrade scale was introduced in 1744 and remained the primary measure for temperature until 1948.

In that year, 1948, the Conference General des Poids et Measures decided to standardize units of measurement and a new name was chosen for the temperature scale: Celsius.

In the US particularly they still measure temperature in degrees Fahrenheit. This system was developed in 1724 by German physicist Daniel Gabriel Fahrenheit(1686-1736), after whom it is named.

It gives the temperature at which water freezes into ice as 32 degrees, and the boiling point of water at 212 degrees, a 180 degree separation, as defined at sea level and under standard atmospheric pressure.

Up to the 1970s Fahrenheit was the way of measuring air temperature in the English speaking world. Since the 1970s Celsius however has become the norm in these islands, as well as throughout the EU.

There is a formula for converting Celsius temperatures to Fahrenheit, and vice versa, to assist American relatives. But it’s not simple. From Celsius to Fahrenheit – multiply by nine, divide by five, and add 32. From Fahrenheit to Celsius – deduct 32, multiply by five, divide by nine, and put out the cat (the latter is optional!). inaword@irishtimes.com