Why is the sky dark at night?

[HenryC]

Our discussion of night sailing above and the beauties of the night sky brings to mind a few thoughts about stars and the night sky you might be interested in. 

The brightnesss of stars is denoted by  the term "magnitude".  A bright star is of first magnitude.  The faintest star visible to the typical human eye is about magnitude 6.  The scale is set up so that there are six magnitudes, or five magnitude intervals between stars of the first and sixth magnitudes-the fainter the star, the higher the magnitude.

Of course, human beings have a wide range of visual acuity, and some, particularly the young, can see fainter than magnitude six; still, six is the usually accepted visual limit of the typical dark-adapted human eye.  Although the eye perceives the difference between mag 1 and 6 as five levels, in reality, a first magnitude star is a hundred times brighter than than a sixth magnitude star.  Each magnitude is about two and a half times brighter than the next fainter one.This is because the human eye does not have a linear response, but operates on a mathematical power law.  What we see as equal intervals are equal ratios. After all, we have to function in bright sunlight as well as under starlight, so the human visual system must be able to handle a range of brightness levels that covers a spread of millions of times the faintest to the brightest.  There is no camera made that can handle that range of brightness levels!

Of course, there are brighter things in the sky than first magnitude. Both Rigel and Betegeuse, the brightest stars in the constellation of Orion are magnitude 0.18 and 0.45, and Alpha Centauri A, a very nearby star (only 4 light years away) very similar to our own sun appears a shade brighter than magnitude zero.  Brighter celestial bodies will have negative magnitudes. The brightest nighttime star in the sky is Sirius,  at mag -1.44, although some of the planets may get brighter than that.  The full moon is about mag -12, and the sun is mag -26.75.  Keep in mind, magnitude tells us how bright a star appears.  How bright it really is depends on how far away it is.  Incidentally, the North Star, Polaris, is about second magnitude, easily visible but not particularly bright.

It stands to reason, and you can easily confirm it by just looking for yourself, that the number of stars increases explosively as the magnitude rises.  For example, between magnitudes -0.5 and +0.49 (mag class 0), there are only 7 stars.  Between magnitudes  0.5 and 1.49 (mag class 1), there are 13 stars.  Note the rapid  increase in stars for each higher magnitude class.

mag class          # stars

2                       71
3                       192
4                       625
5                       1,963
6                       5,606
7                       15,565

Only people with exceptional eyesight can see anything fainter than mag 6, so there are only about 8500 stars visible to the naked eye.  And since you can only see half the sky at any one time, (the earth is in the way!), even the most spectacular night has only a few thousand stars.

The human eye is an incredibly efficient and sophisticated light detector, but it does have its limitations. For example, at low light levels we have very poor color perception, all faint stars appear white.  (In reality, star colors range from blue-white, to white, to yellow, to orange, to red, depending on their surface temperature).  Our sun is pale yellow-white. It also takes some time for our eyes to adapt to complete darkness.  There is quick improvement after just a few minutes, allowing us to see fainter stars, but it takes about a half- hour before we are completely dark-adapted.  Even the slightest amount of extraneous light can ruin your night vision, so if you want to really appreciate the night sky, you need no light, even your boat's navigation lights reflecting off the water or the sails can really ruin your dark adaptation.  Even a crescent moon will ruin it, too.  Astronomers hate the moon, but they hate artificial light pollution even more.

City lights, reflected off the dust floating in the atmosphere, can cause the sky to glow, and robs our eyes of the white-on-black contrast needed to bring out the faintest stars, as well as lowering the overall sensitivity of our retina at maximum dark adaptation.   I live in Fort Lauderdale, and I have to drive due west on Alligator Alley for about 40 miles into the middle of the Everglades to get a truly dark sky.  Much further than that, and the lights of the Gulf coast towns start screwing up the western sky.  A truly dark sky is totally black, and even the faintest stars can stand out against it. I remember when Hurricane Wilma blew into town, the storm blew the dust out to sea and knocked all the electricity out.  A cold front came through that night and blew all the clouds away, and for the first time in decades Fort Lauderdale had a truly dark night.  The stars came out and people, particularly the young, were wandering the streets looking up, many had never seen a night like that before, and they may never see another one again.

And then there's the Milky Way, our home galaxy, seen from within.  A whitish, cloudy band going from horizon to horizon, like a pale river in the sky.  It looks like a ragged cloud, but with binoculars it resolves into a mist of untold millions of stars, like diamond dust on black velvet. A standard sailor's 7x 50 binocular will take you down to about magnitude 11, so that the faintest stars you can see with unaided vision are now at first magnitude.  And not just stars, you can see glowing clouds of gas stretched out in the yawning gulfs between the stars, star clusters, black smears where thick clouds of dust obscure the stars behind them.  You are staring into the Void.  It is breathtakingly beautiful, but it is vast, empty, cold and indifferent to us.  The universe simply does not care about us.  It's a sobering thought, and once you've experienced it, you'll never really forget it.

For those who have studied astronomy, everything else seems irrelevant.