How low can you go? Take the Great Square challenge

December 10, 2014 by Bob King, Universe Today
Look high in the southern sky at nightfall to find the familiar giant square that forms the barrel of Pegasus the Flying Horse. The map shows the sky around 6:30 p.m. local time. Credit: Stellarium

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.

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.

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

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.

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.

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. Credit: Stellarium

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 down to 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.

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

Explore further: The great world wide star count

Related Stories

The great world wide star count

October 23, 2014

How many stars can you see at night? Right now people all over the world are being asked to go out and count them!

Bright nova in the constellation of Delphinus

August 23, 2013

A bright nova has recently been discovered in the constellation of Delphinus (The Dolphin). The discoverer was Koichi Itagaki, in Japan who used an 18 cm reflecting telescope with a CCD camera. The nova was confirmed late ...

A cosmic snake for Chinese New Year

February 11, 2013

Gong Hey Fat Choy! Today marks the beginning of the Chinese New Year and what better way to celebrate the Year of the Black Snake than with a look at an enormous shadowy cosmic serpent, the Snake Nebula!

Sail past Orion to the outer limits of the Milky Way

December 1, 2014

Several nights ago the chill of interstellar space refrigerated the countryside as temperatures fell well below zero. That didn't discourage the likes of Orion and his seasonal friends Gemini, Perseus and Auriga. They only ...

PanSTARRS K1, the comet that keeps going

September 30, 2014

Thank you K1 PanSTARRS for hanging in there! Some comets crumble and fade away. Others linger a few months and move on. But after looping across the night sky for more than a year, this one is nowhere near quitting. Matter ...

Recommended for you

Nanoscale Lamb wave-driven motors in nonliquid environments

March 19, 2019

Light driven movement is challenging in nonliquid environments as micro-sized objects can experience strong dry adhesion to contact surfaces and resist movement. In a recent study, Jinsheng Lu and co-workers at the College ...

OSIRIS-REx reveals asteroid Bennu has big surprises

March 19, 2019

A NASA spacecraft that will return a sample of a near-Earth asteroid named Bennu to Earth in 2023 made the first-ever close-up observations of particle plumes erupting from an asteroid's surface. Bennu also revealed itself ...

The powerful meteor that no one saw (except satellites)

March 19, 2019

At precisely 11:48 am on December 18, 2018, a large space rock heading straight for Earth at a speed of 19 miles per second exploded into a vast ball of fire as it entered the atmosphere, 15.9 miles above the Bering Sea.

Levitating objects with light

March 19, 2019

Researchers at Caltech have designed a way to levitate and propel objects using only light, by creating specific nanoscale patterning on the objects' surfaces.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.