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By Victoria Carroll on

Measuring Sunshine

The recent sunny spells have got me thinking about some of my favourite objects in the meteorology collection – sunshine recorders.

Sunshine recorder, Campbell type, c. 1880
Campbell sunshine recorder c. 1880 (Science Museum / Science and Society)

John Francis Campbell (1821-1885), of the Hebridean island of Islay, designed the apparatus pictured above. You may be able to figure out how it works just from looking at it…

The idea is that the glass ball acts as a lens, focusing the sun’s rays onto an area within the wooden cup and scorching it. As the sun travels from east to west across the sky it burns an arc shape into the wooden bowl, indicating the duration and approximate intensity of sunshine for the day. A single wooden bowl can be used for six months, since every day the sun is slightly higher (leading up to mid summer) or lower (leading up to mid winter) in the sky, so it leaves a separate trace. Simple – but effective.

Campbell’s design was refined by the Cambridge Professor George Gabriel Stokes. Stokes devised a stand, into which special cards are inserted every day to serve as the recording part of the apparatus. This gives more accuracy than the simple wooden bowl.

Campbell-Stokes sunshine recorder, 1899
Campbell-Stokes sunshine recorder, 1899 (Science Museum / Science & Society)

The Met Office still use Campbell-Stokes sunshine recorders today to measure hours of sunshine, although these are gradually being replaced by electronic instruments.

Anyway, enough talk of measuring sunshine. Time to sit back and enjoy it!

Woman in a deckchair, c. 1935
Woman in a deckchair, c. 1935 (© NMeM / Kodak Collection / Science & Society)