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