Sky and Telescope - April 2015 - 35

William Herschel's

Extraordinary
Night of Discovery
Mark Bratton

Recreating the legendary sweep of April 11, 1785
clearly with his instruments. As we will see, he did make
occasional errors in interpretation, despite the superior
optics; for instance, he thought that the planetary nebula
M57 was a ring of stars.
The other factor contributing to Herschel's interest
was the success of his sister, Caroline, in her study of the
sky. He had built her a small telescope, encouraging her
to search for double stars and comets. She located Messier
objects and more, occasionally ﬁnding star clusters and
nebulae that had escaped the French astronomer's eye.
Over the course of a year of observing, she discovered
about a dozen star clusters and galaxies, occasionally noting in her logbook: "Messier has it not!" William realized
that if Caroline could ﬁnd new objects with her small telescope, a larger instrument, deliberately designed to search
the skies, would reveal many more. In 1783, he embarked
upon the construction of his 18.7-inch aperture, 20-foot
telescope, the most eﬀective instrument of its era.

William Herschel, 1738-1822

Caroline Herschel, 1750-1848

Sk yandTelescope.com April 2015

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There's little doubt that William Herschel was the most
signiﬁcant astronomer of the 18th century. His accomplishments included the discovery of Uranus, infrared
radiation, and four planetary satellites, as well as the
compilation of two extensive catalogues of double and
multiple stars. His most lasting achievement, however,
was his exhaustive search for undiscovered star clusters
and nebulae, a key component in his quest to understand
what he called "the construction of the heavens." In Herschel's time, astronomers were concerned principally with
the study of solar system objects. The search for clusters
and nebulae was, up to that point, a haphazard aﬀair, with
a total of only 138 recorded by all the observers in history.
Even the celebrated Charles Messier, responsible for the
discovery of 40, regarded nebulae as nuisance objects to
be avoided during the search for comets.
By contrast, William Herschel systematically searched
for nebulae, using the most powerful telescope in
existence up to that time, an 18.7-inch speculum reﬂector. Between 1783 and 1802, he surveyed the sky on 401
nights, conducting 1,112 sweeps and cataloguing 2,500
separate objects, almost all of them original discoveries.
He surveyed almost the entire sky north of -33º declination, and published three catalogues of objects in 1785,
1789, and 1802. These catalogues were the forerunners
of J. L. E. Dreyer's New General Catalogue of Nebulae and
Clusters of Stars, printed in 1888.
Herschel's interest in nebulae was piqued by two
developments. One was his systematic examination of
the clusters and nebulae catalogued by the French comet
hunter, Charles Messier, in 1782 and 1783. Herschel's
6.2-inch ("7-foot") and 12-inch ("Small 20-Foot") reﬂectors were superior to any of Messier's telescopes, and he
was delighted to discover that many of the objects that
Messier considered "nebula without stars" were resolved

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Sky and Telescope - April 2015

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