What does an actinometer measure?

An actinometer measures the heat power from radiation, in particular the sun. The actinometer was invented by John Hershel who is more famously known for discovering Uranus and several of its moons (and some of Saturns!) The name actinometer comes from the Greek word "aktin" meaning "ray" and the English word meter meaning to measure. The actinometer measures the amount of energy that a known substance emits as a result of being hit with the radiating light both visible and UV.

An actinometer is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the ultraviolet and visible wavelength ranges. For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are physical devices giving a reading that can be correlated to the number of photons detected.

https://en.wikipedia.org/wiki/Actinometer

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It was John's father, William Herschel, who detected Uranus in March 1781. John's invention occurred in 1825.

https://en.wikipedia.org/wiki/Actinometer