US astronomer who was a pioneer in three practical areas
of astronomical research: visual photometry, stellar spectroscopy, and
stellar photography. He established an international astronomical colour
index: a measure of the apparent colour of a star and thus of its temperature.
Pickering was born in Boston, Massachusetts. He was director of the
Harvard College Observatory from 1876. Unusually for his generation,
he encouraged women to take up astronomy as a career.
As a basis for the more than 1.5 million photometric readings he carried
out, Pickering made two critical decisions. First, he adopted a scale
on which a change of one magnitude represented a change of a factor
of 2.512 in brightness. Second, choosing the Pole Star (Polaris), then
thought to be of constant brightness, as the standard magnitude and
arbitrarily assigning a value of 2.1 to it, he redesigned the photometer
to reflect a number of stars round the meridian at the same time so
that comparisons were immediately visible. In 1908 Harvard published
a catalogue with the magnitudes of more than 45,000 stars.
The Henry Draper Catalogue 1918 contained the spectra of no fewer than
225,000 stars, work begun by Pickering and classified according to the
new system devised by Annie Jump Cannon.
The first Photographic Map of the Entire Sky, published 1903, contained
photographs taken at Harvard and at its sister station in the southern
hemisphere, at Arequipa in Peru, where Pickering's brother William Pickering
(1858-1938) was director. In addition, Pickering built up a 300,000-plate
Harvard photographic library.