Looking up and around during the day, it’s easy to understand why the ancients believed that we live under a great dome of sky. Extrapolating to the night sky with the stars apparently rotating overhead, the sixth century Greek philosopher Anaximenes may have been the first to imagine that we live under a vast, rotating crystal sphere. This model of the heavens, in which fiery stars are somehow nailed to the sphere, became a fundamental principle of Western cosmology down to the time of Copernicus and Kepler. One reason why this was the dominant view for so long is simply because, at night, everything overhead looks as far away as everything else: Moon, planets and stars.
The exercise I’m proposing here is to go outside before dawn on clear night sometime this November and try to force your brain to appreciate the three-dimensionality of what, without additional information, makes everything look equidistant, as if on a dome. The easily recognizable constellation of Orion is a useful starting point, with the addition of several other bright stars and two planets: Mars and Jupiter.
First, orient yourself by looking up toward the southwest, away from bright lights, and you’ll see Orion the Hunter striding manfully across the sky. The three stars of his belt point roughly to Aldeberan (up and right) and Sirius (down and left). Look above Orion’s head and there’s Jupiter, almost overhead and brighter than anything else. Now locate dimmer and slightly rust-colored Mars by finding Gemini, the Heavenly Twins Castor and Pollux, off to the left; Mars is a little below this pair.
In the diagram, I’ve shown about how long it takes light from these celestial objects to reach us. The closest is Mars, whose reflected sunlight left the planet just four minutes ago, while it took a bit over 30 minutes for Jupiter’s light (also reflected sunlight) to get here. Now, a big jump, from minutes to years: Sirius is the brightest star in the night sky, more than 100,000 times farther away than Jupiter. Sirius is bright compared with other stars because it’s so close, light having left there in April of 2016.
Everything else in this sampling gets to be orders of magnitude more distant. Procyon, Castor and Pollux are a dozen or more light-years away, while the two outmost stars in Orion’s belt are around 2,000 light-years distant — light left them around the time that Anaximenes was trying to figure out just what the heck was going on up there.
Our knowledge of these distances didn’t come easily. Even after Kepler had confirmed Copernicus’ guess that Earth orbited the sun and the stars were much farther away than the known planets, medieval cosmologists were stymied by their lack of precise instruments versus the sheer distances involved. It wasn’t until the 1800s that star distances were first measured accurately, using stellar parallax. (Imagine blinking first one eye, then the other, noting that a nearby object moves relative to the background … except your eyes are nearly 200 million miles apart, the diameter of Earth’s orbit around the sun.)
Wishing you clear skies!
Barry Evans (he/him, barryevans9@yahoo.com) counts the stars as his friends, as are buyers of his latest literary effort The New Humbook.
This article appears in ‘Powerful’.
Keep watching Mars over the next few weeks relative to Castor & Pollux, as it goes into retrograde motion starting early December 7 thru February 23