The current Moon is just the thinnest of crescents, and it won’t rise until just before dawn. If you have the weather and the opportunity to go to a dark location to stargaze, now is your time. 

The thing is, no matter how remote a location you may be able to make it to, the sky is never truly dark. This is true at many different levels: human technology generates light that ricochets around in our atmosphere, drowning out the light of the faintest stars and galaxies. Moonlight brightens the sky in the same way as sunlight, just to a significantly lesser degree. Even bright planets like Jupiter can, at their brightest, cast enough light to read by. These are all familiar sources of light. What we forget is our atmosphere actually has its own glow. 

In a new paper in The Astronomical Journal, a team led by Miguel R. Alarcón uses low cost, highly sensitive light detectors, called photometers, to measure the changing sky brightness across the minutes and days of the year, across the eleven years that make up our Sun’s solar cycle. During that eleven-year cycle, our Sun goes from a nice quiet star to a magnetically mixed up orb of plasma that periodically launches solar flares and coronal mass ejections across the solar system. 

Using their global network of photometers, this team was able to see variations in sky brightness triggered by the influx of solar particles. They were also able to see variations over tens of minutes to hours that were independent of site, season, time, or solar activity. This glow is coming from the upper atmosphere and, like so many other atmospheric effects, is not well understood, yet. 

Want to see a truly dark sky? Well, you’re going to need to go to space. Can’t quite afford that ticket? We got your back. The much more affordable and accessible Canary Islands, home of the Roque de Los Muchachos Observatory, is measured to be one of the darkest places on the planet, with only 2% of the sky brightness contributed by human sources.

More Information

IAC press release

“Natural NSB during solar minimum,” M. R. Alarcon, M. Serra-Ricart, S. Lemes-Perera, and M. Mallorquin, to be published in The Astrophysical Journal (preprint on arxiv.org)