Slipher, Earl C.

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(b Mulberry, Indiana, 25 March 1883: d Flagstaff, Arizona, 7 August 1964)

planetary astronomy.

After receiving his B.S. degree at Indiana University, Slipher joined the staff of the Lowell Observatory at Flagstaff, Arizona, in 1905: he worked there until the day before his death. Slipher was a pioneer in planetary photography, and the quality of his photographs has seldom been surpassed. He regularly observed the brighter planets during their favorable oppositions over a period of more than fifty-five years, and his photographic sequences that show long-term changes on Mars and Jupiter and the various aspects of Saturn are unique.

Slipher’s special interest, like that of Percival Lowell, was the study of Mars. He obtained almost 200,000 images of the planet, nearly half of them through telescopes during expeditions to Chile in 1907 and to South Africa in 1939, 1954, and 1956. At these sites he obtained sharper photographs because Mars crossed the meridian close to the observer’s zenith, where the turbulent effects of the atmosphere were minimal.

Slipher was one of the first to recognize that multiple-image printing could improve the quality of information extracted from a series of photographs taken within a sufficiently brief interval to avoid blurring because of planetary rotation. He would make a single print by accurately superimposing a number of unusually sharp individual images. He also was one of the first to standardize his plates for photometric measures: this practice, which he initiated in 1918, is now generally followed.

Slipher found that features on Mars’s surface, which normally are invisible when photographed in the violet, sometimes stand out as clearly as they do in yellow light. He referred to this phenomenon as “blue clearing.” His other discoveries about Mars include the “W” clouds, the appearance of a very large dust cloud in 1956, and many secular and transient changes.

The culmination of Slipher’s work on Mars was Mars, the Photographic Story (1962), a compilation of 512 photographs that graphically illustrated many facts known about the planet. The Brighter Planets was published two years later. Only two weeks before his death he saw a preliminary copy of the deluxe edition, which contained photographic reproductions instead of halftones.

Photographs and scientific discoveries made from space vehicles have, since Slipher’s death, drastically changed the thinking of many on the idea that canals, oases, and valleys of vegetation have been seen and sometimes photographed on Mars. Slipher shared this older point of view, which had been widely publicized many years before by Percival Lowell. These classical interpretations do not importantly detract, however, from the value of the unique and extensive series of photographs that is his legacy to planetary science. His photographic sequence of Venus, one image of which shows a complete halo around the planet, has been frequently reprinted; and his photograph taken on 4 December 1911, showing Mars about to be occulted by the moon, is a striking illustration of the relative apparent size and surface brightness of these two objects.

Slipher’s many honors included an honorary D.Sc. from the University of Arizona, Tucson, and an honorary LL.D. from Arizona State College (now Northern Arizona University in Flagstaff).


Slipher published a number of short papers usually based on photographs. The best of these were reproduced in the two books mentioned above. An interim summary of his work is “The Planets From Observations at the Lowell Observatory,” in Proceedings of the American Philosophical Society, 79 (1938), 441–470

An obituary by A. P. Fitzgerald is in Irish Astronomical Journal, 6 (1964), 297.

John S. Hall