Seares, Frederick Hanley

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(b: near Cassopolis, Michigan, 17 May 1873; d. Honolulu, Hawaii, 20 July 1964)


Seares was associated with the Mount Wilson Observatory for thirty-six years, fifteen of them as assistant director. His principal contribution lay in the field of photographic photometry: he standardized the stellar magnitude system and extended it to include stars fainter than the eighteenth magnitude.

The son of Isaac Newton Seares and his wife, the former Ella Ardelia Swartwout, Seares was born on a farm in the southwest corner of Michigan. By the time he was fourteen, the family was living in California, where he attended the Pasadena high school. He received a B.S. degree (with honors) from the University of California in Berkeley in 1895. This was his highest earned degree, although he remained at Berkeley four more years, two with the title of fellow and two more as instructor in astronomy.

In 1896 Seares married Mabel Urmy. Their only child, Richard, was born in Paris in 1900, while Seares was studying at the Sorbonne; the previous year Seares attended the University of Berlin.

Returning to the United States in 1901, Seares served for eight years as professor of astronomy and director of the Laws Observatory at the University of Missouri in Columbia. Then, at the invitation of G. E. Hale, he returned to Pasadena as superintendent of the computing division of the Mount Wilson Solar Observatory, but also to do research with the then brand-new sixty-inch telescope.

In this latter connection Seares was soon photographing stars in “selected areas” of the sky that had been chosen by Kapteyn. This collaboration culminated, more than twenty years later, in a catalog listing 67,941 stars, located in 139 areas (none larger than 23’ of arc in diameter) systematically scattered over the sky north of declination —15°. Seares’s problem was to intercompare, in a reliable way, light signals from widely separated stars that differed by as much as ten million times in brightness. His method was to photograph each area several times on the same plate, using absorbing wire gauze screens and reduced apertures to introduce known reductions in light, up to eleven maginitudes, between successive images of each star. Relative magnitudes could then be read off, and a common zero point applied to give absolute values.

The preliminary results of Seares’s photometric program were so impressive that in 1922 he was elected president of the committee on stellar photometry of the International Astronomical Union. n 1932 the IAU adopted international magnitude standards based on his work. but Seares himself was not yet satisfied: he then began work on a catalog that appeared in 1941, providing data on 2,271 stars within 10° of the North celestial pole, to be used for both visual and photographic magnitude standards.

Seares also wrote a number of papers on galactic structure. His conclusion as to the relative brightness of our Milky Way compared to other “spiral nebulae” was influential in the stand taken by Shapley (his student at Missouri and his co-worker at Mount Wilson) in the Curtis-Shapley debate of 1920. Seares also investigated the way interstellar material obscures and reddens the light from distant stars.

Seares was elected to the American Philosophical Society in 1917, an associate of the Royal Astronomical Society in 1918, and to the National Academy of Sciences in 1919. He received an LL.D. degree from the University of California in 1930 and another from the University of Missouri in 1934. In 1940 he was awarded the Bruce Gold Medal of the Astronomical Society of the Pacific. That same year his wife died; two years later he married his longtime assistant Mary Cross Joyner, and with her eventually retired to Honolulu, where he died at age ninety-one.


I. Original Works. For a description of the Laws Observatory, see “Report of the Director of the Laws Observatory of the University of Missouri,” in Publications of the Astronomical Society of the Pacific, 15 (1903), 167–168.

Seares’s photometric techniques are described in “Photographic Photometry with the 60-inch Reflector of the Mount Wilson Solar Observatory,” in Astrophysical Journal, 39 (1914), 307–340; his so-called exposure ratio method for direct determination of the color index (a number to be subtracted from the photographic magnitude of a star to get its visual magnitude) is outlined in “A Simple Method of Determining the Colors of the Stars,” in Proceedings of the National Academy of Sciences of the United States of America, 2 (1916), 521–525. His paper entitled “The Surface Brightness of the Galactic System as Seen From a Distant External Point and a Comparison With Spiral Nebulae,” appeared in Astrophysical Journal, 52 (1920), 162–182.

The Mount Wilson Catalogue of Photographic Magnitudes in Selected Areas 1–139 is vol. IV of the Papers of the Mount Wilson Observatory and appeared in 1930 as Carnegie Institution of Washington Publication no 402: the authors are given as F. H. Seares, J. C. Kapteyn, and P. J. van Rhijn (although Kapteyn had been dean alsmost eight years), assisted by Mary C. Joyner and Myrtle L. Richmond, Magnitudes and Colors of Stars North of +80°, written with Frank E. Ross and Mary C. Joyner, appeared as vol. VI of the same series and as Carnegie Publication no. 532 (1941): this work is Seares’s final attempt to provide a set of absolute standard magnitudes in the North Polar region, replacing those printed in Transactions of the International Astronomical Union, 4 (1932), 140–152, that had been adopted as international standards.

Seares’s ideas on interstellar obscuration are summarized in “The Dust of Space,” based on his Bruce Gold Medal lecture, in Publications of the Astronomical Society of the Pacific, 52 (1940), 80–115.

A list of 161 publications by Seares follows Joy’s 1967 biographical memoir (see below).

II. Secondary Literature. For a contemporary discussion of Seares’s work, see Alfred H. Joy, “The Award of the Bruce Gold Medal to F. H. Seares,” in Publications of the Astronomical Society of the Pacific, 52 (1940), 69–79, with photograph facing p. 69. Joy also wrote the entry on Seares in Biographical Memoirs. National Academy of Sciences, 39 (1967), 417–431 with bibliography 432–444. An obituary by R. O. Redman appeared in Quarterly Journal of the Royal Astronomical Society, 7 (1966), 75–79.

Sally H. Dieke