How Low Can You Go? Take the Great Square Challenge

Cast your gaze up, up, up on the next dark, moonless night and stare into the Great Square of Pegasus. How many stars do you see? Zero? Two? Twenty? If you’d like to find out how dark your sky is, read on. 

The Great Square, one of the fall sky’s best known star patterns, rides high in the south at nightfall in mid-December. It forms part of the larger figure of Pegasus the Winged Horse. For our purposes today, we’re going to concentrate on what’s inside the square.

Bounded by Alpheratz (officially belonging to adjacent Andromeda), Scheat, Markab and Algenib, the Great Square is about 15° on a side or one-and-a-half balled fists held at arm’s length.

At first glance, the space appears empty, but a closer look from all but the most light polluted skies will reveal a pair 4th magnitude stars in the upper right quadrant of the square. Fourth magnitude is about the viewing limit from a bright suburban location.

Astronomers use the magnitude scale to measure star and planet brightness. Each magnitude is 2.5 times brighter than the one below it. Aldebaran, which shines at 1st magnitude, is 2.5 times brighter than a 2nd magnitude star, which in turn is 2.5 times brighter than a 3rd magnitude star and so on.

Moonlight and especially light pollution reduce the number of stars we can see in the night sky. This specially prepared map shows slices of sky based on amateur astronomer and author John Bortle's Dark Sky Scale. Classes range from 1 (excellent with stars fainter than 7th magnitude visible) to 9 (inner city with a limiting magnitude of 4). Click for more detailed descriptions of each class and rate your own sky. Credit: International Dark Sky Association
Moonlight and especially light pollution reduce the number of stars we can see in the night sky. This specially prepared map shows slices of sky based on amateur astronomer and author John Bortle’s Dark Sky Scale. Classes range from 1 (excellent with stars fainter than 7th magnitude visible) to 9 (inner city with a limiting magnitude of 4). Click for more detailed descriptions of each class and rate your own sky. Credit: International Dark Sky Association

A first magnitude star is 2.5 x 2.5 x 2.5 x 2.5 x 2.5 (about 100) times brighter than a 6th magnitude star. The bigger the magnitude number, the fainter the star. From cities, you might see 3rd magnitude stars if you can block out stray lighting, but a dark country sky will deliver the Holy Grail naked eye limit of magnitude 6. Skywatchers with utterly dark conditions might glimpse stars as faint 7.5. My own personal best is 6.5.

With each drop in magnitude the number of stars you can see increases exponentially. There are only 22 first magnitude or brighter stars compared to 5,946 stars down to magnitude 6.

What appears blank at first is filled with stars -- 26 of them down to magnitude 6.3 are visible inside the Great Square from a dark sky site. How many can you see? Click for a large version. Source: Stellarium
What appears blank at first is filled with stars — 26 of them down to magnitude 6.3 are visible inside the Great Square from a dark sky site. How many can you see? Click for a larger version. Source: Stellarium

Ready to stretch your sight  and rate your night sky? Step outside at nightfall and allow your eyes to dark-adapt for 20 minutes. With a copy of the map (above) in hand, start with the brightest stars and work your way to the faintest. Each every small step down the magnitude ladder prepares your eyes the next.

With a little effort you should be able to spot the four 4th magnitude range stars. At magnitude 5, you’ll work harder. Moving beyond 5.5 can be very challenging. I revert to averted vision to corral these fainties. Instead of staring directly at the star, play your eye around it. Look a bit to this side and that. This allows a rod-rich part of the retina that’s excellent at seeing faint stuff play through the scene and snatch up the faintest possible stars.

Magnitude scale showing the limits of the eye, binoculars and telescopes. Credit: Dr. Michael Bolte, UCO/Lick Observatory
Magnitude scale showing the limits of the eye, binoculars and telescopes. Credit: Dr. Michael Bolte, UCO/Lick Observatory

From my house I can pick out about dozen points of light inside the Square on a moonless night. How many will you see? Once you know your magnitude limit, compare your result to John Bortle’s Dark Sky Scale … and weep. No, just kidding. But his Class 1 excellent sky includes a description of seeing stars down to magnitude 8 and the summer Milky Way casting shadows.

Hard to believe that before about 1790, when gas lighting was introduced in England, Class 1 skies were the norm across virtually the entire planet. Nowadays, most of us have to drive a hundred miles or more to experience true, untrammeled darkness.

Have fun with the challenge and let us know in the comments area how you do. Here’s hoping you find the Great Square far from vacant.

Catch Comet Lemmon While You Can

If you honed your observing chops on Comet PANSTARRS this spring, consider dropping in on Comet Lemmon, now returning to the dawn sky. Southern hemisphere observers saw this comet at its brightest in March when it briefly became dimly visible with the naked eye. It’s now faded to around magnitude 6, the same as the faintest stars you can see under a rural sky.

Because it’s been “vacationing” in the southern constellations, northerners have had to wait until now to see it.

Comet Lemmon with gas (left) and dust tails on April 24. Click to see a short movie showing rapid changes in the comet's tail in 25 minutes. Credit: Gerald Rhemann
Comet Lemmon with gas (left) and dust tails on April 24. Click to see a short movie showing rapid changes in the comet’s tail in 25 minutes. Credit: Gerald Rhemann

Like PANSTARRS, C/2012 F6 Lemmon is visible in modest-sized binoculars (7x35s, 10x50s) as a small, fuzzy ball of light with perhaps a faint tail. Watch for it to slowly track along the eastern side of the Great Square of Pegasus for the remainder of April and May. It competes with twilight low in the eastern sky this week but gradually becomes better placed for viewing as May unfolds. The best time to look is about an hour and a half before sunrise now and 2 hours before sunrise by mid-May.

The waning moon interferes some until around May 5. On the 6th, watch for the thin lunar crescent moon to pass 8 degrees below the comet. Around that time, we’ll finally get a good view of Lemmon in a dark, moonles sky just before the start of dawn.

On May 6 a beautiful thin moon will be near Comet Lemmon at dawn. This map shows the sky about 1 1/4 hours before sunrise. Stellarium
On May 6 a beautiful thin moon will be near Comet Lemmon at dawn. This map shows the sky about 1 1/4 hours before sunrise. Stellarium

Comet Lemmon will fade from naked eye limit to a dim binocular smudge of 7.5 magnitude  by mid-May. If you have a telescope, look for a pair of tails – a short, diffuse one of dust particles and the straight, streak-like gas tail fluorescing in the sun’s ultraviolet light. The tails point approximately to the south-southwest. Catch this comet while you can!