Why is the Sky Dark at Night?

After writing the last post, “Afraid for the Dark”, I was reminded of a seemingly simple question that surprisingly few people know the answer to.   The question is the title of this article: “Why is the Sky Dark at Night?”    It’s a question that concerns something that we can observe on a daily, or rather nightly, basis (although perhaps not as well as we could at one time), just like the effects of gravity.  Gravity, like night-time darkness, has been around for a long time.  It was only in the time of Isaac Newton, and his Principia of 1687, that we started to understand something of how gravity worked.  In the 20th century, Einstein showed, in the theory of General Relativity, that Newton’s understanding was not complete.

In the case of night-time darkness, it was the 19th century before the question was asked — and the answer was not obvious:

In 1826, the astronomer Heinrich Olbers asked, “Why is the sky dark at night?”   By his time, physicists had learned enough to realize that, in a stable, infinite universe with an even distribution of stars, the entire universe should gradually heat up.

In a very informal ongoing experiment, I’ve asked the question of a number of people I know; and only one, out of a few dozen, has come up with the currently accepted answer.  I’m not going to give you the answer here, because Paul Lutus, a modern Renaissance man, who has designed spacecraft components for NASA, written the Apple Writer word processing program, and sailed solo around the world, has written a really first rate explanation.    Before reading it, you should try to spend a bit of time trying to work out the answer for yourself, but be forewarned that your intuition probably won’t help, and in fact will probably be a hindrance.

Scientists and physicists had to learn quite a lot about the behavior of energy before they were even prepared to ask Olbers’ question. In fact, for millennia the dark night sky provided an answer to a question no one thought to ask.

I hope you enjoy reading the article.  I think this simple question, and its answer, is another great example of how science can reveal wonderful complexity that is beyond our ordinary imaginations.

Afraid for the Dark

A few days ago, I ran across an online mention of an article in Cosmos, an Australian science magazine, that talks about the ongoing problem of light pollution: the effective obscuring of the night sky caused by the glow of artificial lighting. A large number of people today never have the opportunity to see a natural night sky:

“The arc of the Milky Way seen from a truly dark location is part of our planet’s natural heritage,” said Connie Walker, and astronomer from the U.S. National Optical Astronomy Observatory in Tucson, Arizona.

Yet “more than one fifth of the world population, two thirds of the U.S. population and one half of the European Union population have already lost naked eye visibility of the Milky Way.”

People’s unfamiliarity with what the night sky really looks like can have amusing results, as reported in an essay in the New York Times:

When the Northridge earthquake knocked out power in Los Angeles in 1994, numerous calls came into emergency centers and even the Griffith Observatory from people who had poured into the streets in the predawn hours. They had looked into the dark sky to see what some anxiously described as a “giant silvery cloud” over the shaken city.

Not to worry, they were assured. It was merely the Milky Way, the vast galaxy that humans once knew so well — until the glare from electric light effectively erased most traces of it from urban and near-urban skies.

I can remember, as a kid, going out in the car with my Dad to “the country”, west of where we lived in Northern Virginia, to look at the sky and see the stars.  After considerable time in other places, I’m now back in a place that, while it’s not all that far from where I grew up, was most definitely in the country then.  (Actually, it was more like the back of the beyond — even the turnip truck didn’t come out here.)  I can remember being able to see and identify the constellations, and seeing the Milky Way.  Today, you are lucky to be able to see a handful of the brightest stars.

It’s not just places that are in or close to a large city, like Los Angeles or Northern Virginia, that have the problem.  Even places normally thought of as quite remote have noticable effects from glare.  In another article in December 2008, the New York Times reported that, at Dante’s View in Death Valley, the sky glow from Las Vegas is one of the brightest objects in the night sky:

The neon glow from Las Vegas and its fast-growing bedroom communities is stealing stars from the park’s eastern fringe. New research reveals that light pollution from Las Vegas increased 61 percent from 2001 to 2007, making it appear brighter than the planet Venus on clear nights as seen from Dante’s View.

An index called the Bortle Scale is sometimes used to measure the darkness of the night sky.  The scale range from 1, which corresponds to complete darkness, to 9, which would represent, say, midtown Manhattan or the Las Vegas Strip.  A New Yorker article talks about the almost complete absence of visible stars in the city (Bortle Class 9):

Today, a person standing on the observation deck of the Empire State Building on a cloudless night would be unable to discern much more than the moon, the brighter planets, and a handful of very bright stars—less than one per cent of what Galileo would have been able to see without a telescope.

Once again, just getting away from the most densely populated areas is not enough:

The very darkest places in the continental United States today are almost never darker than Class 2, and are increasingly threatened. For someone standing on the North Rim of the Grand Canyon on a moonless night, the brightest feature of the sky is not the Milky Way but the glow of Las Vegas, a hundred and seventy-five miles away.

Astronomers have long been concerned about the effects of light pollution on their ability to make optical observations.  Now there is some evidence that having a dark night’s sleep is important to our health.  A study [PDF] published in the journal Chronobiology presented statistical evidence that suggests a possible link between overly-light night skies and cancer.   There are other studies that have also indicated that excessive light at night is disruptive to the body:

Late last year, an arm of the World Health Organization announced its decision to classify graveyard shift work as a “probable carcinogen,” in part because of controlled epidemiological studies of nurses, flight attendants and others who work at night have found breast cancer rates 60 percent above normal.

Researchers are still trying to unravel how such an effect might occur.  It is, however, undoubtedly true that our current situation, at least in the developed world, is out of the ordinary from an evolutionary perspective.  For example, it was remarked by travelers in the 17th century (when the only night-time illumination was from firelight, candles, and so on) that at night  one could smell the proximity of a large city, such as London, long before one could see it.

There is now something of a movement to reclaim the night sky from light pollution.  It has gathered evidence to show that lighting that is properly designed to minimize “sky glow” can also save energy, and can be better in other ways, too.  A prospective burglar, for example, is much more noticeable if he can be seen using a flashlight trying to break in, without the observer being blinded by the glare of indiscriminate lighting:

Crawford pointed out a cluster of mailboxes across the street from his garage. The lighting near the mailboxes was of a type that Crawford calls “criminal-friendly”: it was almost painful to look at, and it turned the walkway behind the boxes into an impenetrable void. “The eye adapts to the brightest thing in sight,” he said. “When you have glare, the eye adapts to the glare, but then you can’t see anything darker.”

For this reason, airport runways are not illuminated like mall parking lots, but with edge and center-line lights that put the illumination where it can do the most good.

The great science fiction writer Isaac Asimov once wrote a story called Nightfall, about a planet in a multiple-star system where darkness fell, and the stars became visible, only once every thousand years.  In fact, even the existence of stars outside the local system was  unknown.  The story deals with the traumatic effect on the inhabitants when darkness finally comes.  I hope we can avoid life imitating art.