Wednesday, December 11, 2013
My backyard summer skyscape is closed in by looming pines and firs. Looking south from the back door around 10 p.m. this time of year, the trees frame three constellations and give three stars prominent places in my backyard cosmography. I taught myself their names decades ago because on those recircling summer nights my mind would saunter off beyond the treetops and I found I needed a map. The constellations are Hercules, Cygnus and Aquila.
Altair, in the constellation Aquila.
One of the bright stars is Altair, in a sort of sidelong diamond of stars called Aquila, which at this time of year is sitting right in the crook of the fir trees 50 feet from the back door. It’s the 13th-brightest star in the sky, at magnitude 0.7. The name Altair comes from a phrase in the seventh- or eighth-century Arabic name for the constellation, Al Nasr al Ta’ir, the Flying Eagle, and means roughly “the rising one.” The Babylonians and Sumerians 3,000 years or more ago probably called it the Eagle Star, and so did the Greeks later on. “Aquila” is Latin for “eagle.” The star just above Altair is Tarazed, and dimmer just below it is Alshain, both taken from an ancient Persian name, Shahin tara zed, meaning in one translation “the star-striking falcon.”
Altair is a striking star, when you get to know it. It’s not as bright as Vega, which is the fifth-brightest star at magnitude .03 and almost directly overhead on late August evenings. If you don’t have any trees for stable points of orientation, you can use Vega to find Altair. Once you spot it, sweep your eyes southward to Altair.
Vega and Altair are two points of the Summer Triangle. The third is Deneb in Cygnus, the Swan. To find Deneb (1.25 magnitude, slightly dimmer than Altair and Vega), go back to Vega, then sweep your eyes to the left. The brightest one you see in that direction is Deneb. Once you’ve got it, you should be able to spot that it’s the tail on a configuration of five stars that make a cross (sometimes called the Northern Cross) and that also look startlingly like a swan or goose flying toward the southwest.
Deneb is a supergiant star, which means it has so much material it is not aging the way most stars do and it’s grown unusually huge. Its diameter is about 116 times the sun’s, and it’s 80,000 times as luminous. It seems dimmer to us because it’s more than 1,400 light-years from us. By comparison, Vega is less than 2 1/2 times the diameter of the sun, but it appears much brighter because it’s so much closer to us, 26 light-years away. “Closer” — strange word to apply to the distance a particle of light going 186,000 miles per second travels in 26 years.
Hercules is a little harder to pick out. It has no first-magnitude stars. But for reasons unknown, it fascinates me. Something about the “subtle magnetism in nature,” no doubt, (as a better saunterer than me put it) that we unconsciously yield to. On a clear night, sweep your eyes through the triangle from Deneb to Altair and go west. The central part of Hercules is an Arkansas-shaped trapezoid of stars. His limbs shoot off the corners of the trapezoid at odd angles. In the old drawings, his arms and shoulders are actually upside down to us.
The star at the lower left of the Keystone, as the trapezoid is sometimes called, is a magnitude 3.2 star called, since the Renaissance, Delta Hercules, and in recent times Sarin. How it got this name, no one is quite sure. It first appeared in a pioneering sky map, the Atlas Coeli Skalnate Pleso, which was created in the mid-20th century by the Czech astronomer Antonin Becvar. But where Becvar got the word (along with 11 other strange, untraceable names), no one knows. The Skalnate Pleso is an astronomical observatory in Slovakia where Becvar worked, and the phrase, which sounds like something out of a science fiction novel, means “rocky lake.”
Your imagination can run wild with words like that, out into fantasies on Becvar’s possible channels of information. The black wastes of evening sky beyond the south-southwesterly firs can be kind of unnerving unless you get some points of orientation so you know where you’re walking.
Dana Wilde lives in Troy. His writings on the stars and planets are collected in “Nebulae: A Backyard Cosmography,” available from Booklocker.com. Backyard Naturalist appears the second and fourth Thursdays of the month. You can contact him at email@example.com.