How Astronomer Percival Lowell Mistook His Own Eye For Spokes on Venus

Feb 2, 2021 0 comments

Percival Lowell, the American astronomer whose name bears an observatory in Arizona, made several very significant observations of the planets. His biggest contribution being the hunt for Planet X beyond the orbit of Neptune. Although his search was unsuccessful, Pluto was eventually discovered near the place Lowell had predicted the missing planet would be, using the very observatory Lowell founded to study Mars.

The red planet fascinated Lowell, and it was his observations of Mars and the inference he drew from it for which Lowell is remembered the most. Lowell was convinced that there are intelligent beings on Mars, for he could see through the telescope a maze of canal like structures on the surface of the planet. The astronomer theorized that an advanced civilization indigenous to Mars built the canals to bring water from the polar ice caps to the equatorial region in a last ditch attempt to survive in an inexorably drying planet.

map of Mars drawn by Italian astronomer Giovani Schiaparelli

A map of Mars drawn by Italian astronomer Giovani Schiaparelli in the 1880s, showing waterways on the planet’s surface. It was Schiaparelli who first proposed the canal theory. From the book “A Popular Handbook and Atlas of Astronomy” by William Peck, 1891.

Lowell’s vision of a dying race fighting an unwinnable war towards an inevitable end excited the public and influenced the likes of H.G. Wells to speculate how different the life of Martians would be from that of earthlings, and how their evolutionary progress might compare to us. Wells theorized that in a planet as hostile as Mars, creatures would have no digestive system and no hands, but tentacles, which they would use to put the blood of other beings in their veins to survive. Ludicrous as it may sound, these ideas culminated into one of Well’s most famous works—The War of the Worlds.

Lowell’s writing continued to influence science fiction writers well into the middle of the 20th century. Martian canals figure prominently in Red Planet by Robert A. Heinlein (1949) and The Martian Chronicles by Ray Bradbury (1950). Even today, the vision of Mars derived from his theories remains enshrined in the works of many authors and are still read widely as classics of science fiction.

map of Mars drawn by Percival Lowell

Percival Lowell’s interpretation of the canals of Mars.

The notion of canals and intelligent life on Mars was finally put to rest only in 1972, when NASA’s Mariner 9 spacecraft sent close-up pictures of Mars’s surface, showing that there is no such structures on the red planet.

Some of his other ideas took even longer to resolve.

The Spokes of Venus

Percival Lowell

Percival Lowell

In 1896, soon after Lowell had acquired a new 24-inch refracting telescope and installed it at his Flagstaff observatory, the controversial astronomer began studying Venus. Lowell noticed that when he stopped down the aperture of the telescope, a mysterious dark spot materialized on the planet’s surface with spoke-like structures emanating from it. Lowell knew that Venus had a thick atmosphere that’s impossible to penetrate optically, yet he saw these markings that could only have been on the surface. Stranger still, these features always seem to face the earth, which meant that Venus must be in synchronous rotation with the Sun—an inference which didn’t tally with other observations. The most puzzling was the fact that no one else, other than Lowell, could see these markings.

The scientific society derided Lowell’s findings but they couldn’t help but wonder exactly what Lowell had seen, and why only he could see it. The matter was left unsettled for more than a century until a couple of amateur astronomers proposed a theory: Lowell was gazing into his own eye!

Lowell often observed the planet high in the daytime sky with the telescope's lens stopped down to under 3 inches in diameter to reduce the effect of the turbulent daytime atmosphere. With that setup, Lowell shrank the telescope's exit pupil in front of his eye to a pinhole of diameter less than 0.5 millimeter, effectively turning the telescope into a giant opthalmoscope that optometrists use to examine eyes of patients.

spokes of venus

Percival Lowell’s drawing of Venus and its spokes (left) somewhat resemble the back of the human eye (right)

Andrew T. Young of San Diego State University explained that a small exit pupil often casts shadows of blood vessels on the retina, making them visible. “As the eye pupil moves with respect to the small telescope pinhole, the angle at which rays arrive at the retina changes, so that features [that are] within the eye but a little removed from the retina may cast shifting shadows on the latter and be seen.”

To state it plainly: what Lowell saw as spokes were actually shadows of the blood vessels and other structures in his own retina. So instead of mapping the surface of Venus, Lowell had been mapping the structures in his own eye. While this may sound all very strange, this phenomenon is a well-known annoyance among astronomers observing planets at very high magnification.

When the late Carl Sagan spoke at the University of Glasgow in 1985 during Gifford Lectures, he used Lowell’s example of an over-productive mind as a cautionary tale about the power of belief and yearning to trump science and reason. Ironically, Lowell is sometimes compared to Sagan himself. In a paper published in Space Science Reviews in 2007, the authors described Percival Lowell as “the most influential popularizer of planetary science in America before Sagan.”

# William Sheehan, Venus Spokes: An Explanation At Last?, Sky & Telescope
# Leon Jaroff, What Lowell Really Saw When He Watched Venus, New York Times
# Kat Eschner, The Bizarre Beliefs of Astronomer Percival Lowell, Smithsonian Magazine
# Kevin Zahnle et al, Emergence of a habitable planet, Space Science Reviews


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