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 cataloged 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).
Written by Camille Carlisle, SKY & Telescope. Continue HERE
Image above: Frontispiece 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. Zachariel / Wikimedia Commons
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