Sky and Telescope - May 2012 - (Page 18)

News Notes

Claudius Ptolemy (circa 150 A.D.) as imagined by a 16th-century artist.

Ancient Astronomers: Smarter Than We Knew?
Ancient astronomers were clever in lots of ways, and with so many of their texts now lost, they probably had more going on than we know. Now Bradley Schaefer (Louisiana State University) thinks he has found a new piece of astronomical knowledge they used: a formula for the dimming of starlight by Earth’s atmosphere. He says they apparently corrected for this eﬀect ages before anyone had a physical model for why it happens. The dimming is called atmospheric extinction. It happens because a star at a low altitude is seen through more of Earth’s atmosphere than a star overhead. This is why the Sun and Moon look dimmer near the horizon than when they’re high. If the amount of air straight up is deﬁned as “one air mass,” you look through 2 air masses when you look 30° above the horizon, 5.6 air masses at 10°, and 40 at the horizon. Any naked-eye skywatcher notices the eﬀect. But putting good numbers to it is another matter. Astronomers compiled the ﬁrst large star catalog about two millennia ago. Claudius Ptolemy of Alexandria included it in his great compendium of 13 books on astronomy known as the Almagest around 150 A.D. It classiﬁes 1,022 stars by their “magnitude,” setting the basis of the brightness-measuring system we use today. But historians have debated whether 18 May 2012 sky & telescope

Ptolemy observed the stars himself or copied the star data from a now-lost catalog made by Hipparchus of Rhodes about 300 years earlier. Schaefer had a brainstorm for a new way to try to ﬁnd out. Rhodes is at 36° north latitude; Alexandria is at 31°. So in the southern part of the sky, stars would appear 5° lower — and therefore dimmer — from Rhodes than from Alexandria. Would the magnitudes in the Almagest give away the latitude of the author? To take an extreme example, Canopus at its highest would have looked 4th or 5th magnitude to Hipparchus but 2nd magnitude to Ptolemy. Schaefer, however, discovered a problem. No such eﬀects appear at all. No matter how near the horizon stars were viewed (from whatever latitude), any eﬀect of extinction averages out to about zero. “Somehow somebody corrected the Almagest magnitudes for extinction,” says Schaefer. “It’s the only way.” When Schaefer examined catalogs from two later astronomers, al-Suﬁ in the 10th century and Tycho Brahe in the 16th, he also found extinction corrections. This is a surprise, because no records mention extinction at all until the 1700s. Says Schaefer, “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.” Several astronomers remain skeptical and argue that scatter in the data may undermine Schaefer’s result, which he has not yet published. Still, the consensus seems to be that he’s onto something. How did the ancients do it? Probably

they watched stars that traversed a large range of altitudes, determined how bright they appeared at diﬀerent heights compared to stars at greater heights, and compiled a correction table accordingly. Schaefer did this himself while vacationing in the American Southwest, with pretty good results. He says it was “nakedeye backyard astronomy to the rescue of historical astronomy.”

WIKIMEDIA COMMONS

End of an S&T Era

Clearing out the former editor in chief’s oﬃce.

The buildings that housed Sky & Telescope’s editorial oﬃces for nearly half a century bit the dust on February 24th, literally, while camera-wielding former occupants looked on with mixed emotions. The former houses at 48 and 50 Bay State Road, Cambridge, Mass., served as our oﬃces from 1957 until 2006 when we moved to more modern quarters nearby. Condos are going up in their place. But a proud memorial stands across the street: S&T’s third and much larger former building now houses the American Association of Variable Star Observers. ✦

1 air mass

a ir m

sse

5.6 air m
10

asses
S&T ILLUSTRATION

The lower a star appears over your horizon, the more air its light has to go through to reach you. The amount of air straight over your head (whatever your height above sea level) is called one air mass.

ROGER W. SINNOTT

Table of Contents for the Digital Edition of Sky and Telescope - May 2012

Sky and Telescope - May 2012
Table of Contents
Spectrum
Letters
75, 50 & 25 Years Ago
News Notes
Saturn’s Raging Superstorm
Stars Above, Earth Below: Astronomy in National Parks
The Remarkable Case of Comet Lovejoy
In This Section
May’s Sky at a Glance
Binocular Highlight
Planetary Almanac
Northern Hemisphere’s Sky
Sun, Moon, and Planets
Celestial Calendar
Exploring the Solar System
Deep-Sky Wonders
Going Deep
S&T Test Report
New Product Showcase
Telescope Workshop
Planetary Imaging with Your DSLR Camera
Gallery
Focal Point

Sky and Telescope - May 2012