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Why does the Sun shine? Today we know the answer to the question and we alsoknow that earlier answers were quite wrong. The problem of the source of solarenergy became an important part of physics and astronomy only with theemergence of the law of energy conservation in the 1840s. The first theory ofsolar heat based on the new law, due to J. R. Mayer, assumed the heat to be theresult of meteors or asteroids falling into the Sun. A different and moresuccessful version of gravitation-to-heat energy conversion was proposed by H.Helmholtz in 1854 and further developed by W. Thomson. For more than fortyyears the once so celebrated Helmholtz-Thomson contraction theory was acceptedas the standard theory of solar heat despite its prediction of an age of theSun of only 20 million years. In between the gradual demise of this theory andthe radically different one based on nuclear processes there was a period inwhich radioactivity was considered a possible alternative to gravitationalcontraction. The essay discusses various pre-nuclear ideas of solar energyproduction, including the broader relevance of the question as it was conceivedin the Victorian era.

The Source of Solar Energy, ca. 1840-1910: From Meteoric Hypothesis to Radioactive Speculations