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.