Where is the darkest sky on Earth?
For the majority of people, the night sky no longer truly exists. Walking outside on a clear moonless night and looking up into the heavens will likely yield views of a few stars here or, and perhaps a planet or two if you're lucky. But that's about it. No Milky Way, no zodiacal light or the carpet of stars once visible to anyone who looked up.
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As light pollution continues to eat away at the remaining dark skies on Earth, growing at around 10% annually (doubling in brightness every eight years), the views our ancestors once took for granted are nearly all but extinguished from most of the planet.
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For those who still wish to see the night sky in its purest form, that raises a question; where exactly should you go to see such a sky? And equally as important, when?
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As a first guess people might assume the middle of the Pacific Ocean, or perhaps Death Valley would have the best sky on earth, and while the view no doubt would be amazing, there's actually a lot more to it than that.
Artificial light pollution
When people think of light pollution, this is what comes to mind; town lights, street lights, cars, houses and everything else that comes from civilisation. If you live near a city you'll be very familiar with this, and your "night sky" will be a bright haze and not much else.
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This kind of light pollution can be roughly broken down into nine levels, the bortle scale:
Image 1: the bortle scale (credit Stellarium)
The bortle scale measures from one, a pristine night sky, to nine, an inner city sky. While the bortle scale is a somewhat crude method for estimating the light pollution, it works pretty well as a rule of thumb.
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Around bortle four or lower, the night sky begins to really shine, and by bortle two the Milky Way core is clear and detailed. Anything brighter than about bortle six, and the sky just about vanishes all together.
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A more technical way of measuring light pollution is with a sky quality meter (SQM) device, which gives you the exact brightness of the sky. The only issue with an SQM is that it measure the entire sky, which might include bright features like the Milky Way, zodiacal light or even distant light pollution domes, all of which can inflate the reading and make the sky appear brighter than it really is. If your sky is obstructed by trees or other objects, then this can artificially lower the reading.​ One way of solving this issue is to add a lens to your SQM, making it an SQM-L. This only captures light from a small region of the sky, so you can point it with more precision. Generally when talking about how bright the sky is somewhere, the number refers to the zenith (the very top of the sky directly above you).
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When using an SQM of either type, the value you get back is called mag/arcsec^2, quite a mouthful, but it sounds more complicated than it is.
To understand what it means, first hold a finger out at arms length. The width of your finger is now approximately one degree across (there are 360 degrees in a circle, so you would need 360 fingers to complete a full ring around you). Now imagine your finger width is divided up into 60 equal parts, each one of these is about the size of an arcminute, a tiny angle 1/60th of a degree across. The moon is about 30 arcminutes wide, or about half the width of you finger (0.5 degrees). Now imagine dividing one of those arcminutes into another 60 slices, each of these are an arcsecond.
A tiny square patch of sky that is one acrsecond across, is an arcsec^2. The "mag" refers to absolute magnitude, a measure of how bright something is. So mag/arcsec^2 just refers to the amount of light coming from each tiny little arcsec^2 in the sky (of which there are billions)
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A perfectly dark bortle one sky will read 22.0 mag/arcsec^2 from an SQM-L when pointed at the zenith, and an inner city sky would read <18.4 (smaller is brighter).​
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So, where in the world has the least amount of light pollution? A light pollution map is a great tool for this.
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Image 2: global light pollution (credit lightpollutionmap.info)
Unfortunately, the darkest skies are where people aren't. In terms of just artificial light pollution, the oceans win this, and the middle of the Pacific or any ocean would have perfectly dark skies. On land, some of the least light polluted locations would be:
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- Inner Tibet
- Outback of Australia
- Sahel region of the Sarah Desert
​- Siberia
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Any any other sparely populated area in between.
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For any casual stargazing, this is a great way to find dark skies and about all of the information you'll need. But for astronomer,