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Why dark skies are dark

Greetings stargazers.

We are fortunate to live in one of the darker corners of the continental United States. Most of you could correctly explain this darkness is because of the absence of any nearby large metropolitan areas. But even with the remoteness of Durango, we can still ask, ‘why isn’t the night sky bright?’

This was the question in what is now often known as Olber’s paradox. Although named after the 19th century German astronomer, the puzzle is much older. It says that if the universe was truly infinite, static and had a uniform distribution of stars and galaxies, then there would be an infinite number of stars, and no matter which way you looked, your line of sight would eventually run into the surface of a star. Although the paradox was put forward long before the existence of interstellar dust clouds was known, those, too, should be lit up daylight bright.

The straightforward explanation for the paradox is that the universe is neither static or infinite – at least the part we can observe. This is our current cosmological understanding with the Big Bang and the observed expansion of the universe. Even if there is an infinite extent to the universe, there is only a finite portion that we can possibly observe given its finite age. Traveling away from the Earth at the speed of light for the age of the universe gives an idea of how this sphere of what we can see is defined. Although the Big Bang is estimated to have been only about 13.7 billion years ago, special relativity plays its part in calculating the estimated diameter of the observable universe to be around 94 billion light years across.

Since our sky isn’t stellar bright, the next question is – just how dark is it? The Bortle Scale is used to measure the relative darkness of a location. The scale runs from 1, which is exceptionally dark, to 9, which is what you might find in the middle of a big city. Tables show what objects you might expect to see at each Bortle level, but since everyone has different eyes and night vision, the actual darkness is better quantified if you have a sensitive meter. Some smartphone apps can give an adequate approximation of this number.

The units of sky brightness are given in magnitudes per square arc second. A value of 21 would be as if there were a 21st magnitude star in each square arcsecond of sky, which is about what you could find in Durango, and would rank as a 4 on the Bortle scale. The darkest sky I have seen was near Molas Pass. During that evening, there was a noticeable increase in brightness when the summer Milky Way rose.

This month

Venus and Mars are still prominent in the western evening sky on either side of Castor and Pollux, the twins of Gemini. Venus will reach its greatest eastern elongation on June 4th. That is the day that it is farthest from the sun from our perspective, and so highest in the sky during this orbit. Sirius, the brightest star in the sky, is a bit to the southeast of Venus. This is a good time to compare just how much brighter Venus is than any star.

Jupiter is now visible in the morning just before dawn. Mercury is at its greatest western elongation on the morning of the 29th.

This is the time of the year that Omega Centauri is barely visible above the southern horizon. If you have a clear view in that direction, this is by far the brightest globular cluster in either hemisphere. Although you can see it with your naked eye, it is truly spectacular when viewed through binoculars.

Useful links:

International Dark Sky Association


Bortle Dark-sky scale


Astronomy picture of the day


An Astronomer’s forecast for Durango


Old Fort Lewis Observatory



Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.