Ancient Astronomers
© Casey Reed

There's no doubt ancient astronomers were clever folk. Realizing Earth was round, estimating the Sun's distance, discovering heliocentricity - it's quite a list. But Brad Schaefer (Louisiana State University) suggested at the recent American Astronomical Society meeting in Austin that we should add another light bulb to the glow shining from history: ancient astronomers may have corrected for dimming caused by the atmosphere, centuries before anyone came up with a physical model for it.

This dimming is called atmospheric extinction. Extinction happens because starlight has to pass through Earth's atmosphere in order to reach us. But the effect isn't uniform: if you spend time stargazing you've probably noticed that a star high up in the sky's dome looks brighter than it does as it slides toward the horizon. That's because light coming to us from near the horizon passes through more atmosphere than if it shines straight down from overhead. (The Sun looks redder at sunset and sunrise for the same reason.)

Astronomers have catalogued stars' magnitudes for at least two millennia, all the way back to an ancient document called the Almagest. It was the Almagest that Schaefer began with - but his goal wasn't to determine if astronomers in olden days accounted for extinction. He wanted to use the brightnesses reported in it to decide a long-standing debate over who wrote the catalog in the first place, Hipparchus of Rhodes (circa 150 BC) or Ptolemy of Alexandria (circa AD 150).

Some researchers have looked at the positions reported for the Almagest's 1,000-plus stars to try to distinguish between the theories, but the differences aren't conclusive. So Schaefer decided to try using atmospheric extinction to crack the case. Rhodes is at 36° north latitude and Alexandria at 31.2° N, which means the same stars will appear lower in the sky (and therefore dimmer) in Rhodes than they will in Alexandria. The star Canopus, for instance, is by modern calculations the second brightest star in the sky, after Sirius. Canopus should have looked 4th or 5th magnitude to Hipparchus but 2nd magnitude to Ptolemy, Schaefer says. (Astronomers' magnitude system is also ancient, based on Hipparchus's work: a magnitude 1 star is 2.5 times brighter than a star of magnitude 2.) By comparing the Almagest brightnesses against modern magnitudes - which are extrapolated to how they would appear outside the atmosphere - Schaefer should have been able to tell by the growing difference between the two values where the observer was. But he soon discovered a problem.

© Zachariel / Wikimedia CommonsFrontispiece illustration from a Venetian 1496 edition of the Almagest, depicting Ptolemy instructing the 15th-century astronomer Regiomontanus (also known as Johannes Müller von Königsberg). Above the men is the zodiac, encircling the celestial sphere.
"You would expect that as you look further south the Almagest magnitudes would start ... going up and up and up," he says. "But when you look and see the real data, you see that they are not going up and up and up." No matter how near the horizon you look, the error in the stars' magnitudes compared to today's values averages out to about zero. "Somehow somebody corrected the Almagest magnitudes for extinction. It's the only way."

When Schaefer looked at catalogs from two other renowned astronomers, al-Sufi (10th century) and Tycho Brahe (16th century), he also found extinction corrections.

The result has surprised many astronomers because there are no historical records mentioning extinction. The first physical explanation for extinction came in the 1700s from the French scientist Pierre Bouguer, and while extinction is obvious to an experienced observer, "it's rather surprising that [the ancients] did a sophisticated and pretty accurate correction for something they don't talk about and no one ever knew they knew about," Schaefer says.

A few astronomers do remain skeptical and argue that scatter in the data may undermine Schaefer's result, but the overall consensus is that "he's onto something."

How did the ancients do it? Not clear. Probably they watched stars that traversed large parts of the sky and determined how bright those stars appeared at different heights above the horizon. With those estimates in hand, they could eyeball stars that never rose very high and figure out how bright the stars really were based on their apparent magnitude and how far they were above the horizon. Schaefer says he did rough extinction calculations by eye while vacationing in the American Southwest that were pretty good. "It's naked-eye backyard astronomy to the rescue of historical astronomy," he laughs. Our readers are invited to try calculating for themselves.