It’s always darkest just before dawn: True/False?

Neville Farquhar wonders whether it really is. We’ve all trotted this phrase out in the face of misfortune — usually someone else’s — but how many of us have actually camped out overnight with a photometer? If you did you would discover that the darkest hour is not always just before dawn.

I don’t get a lot of camping in these days and to be honest I’m more likely to pack the marshmallows and leave the old photometer behind when I do. For astronomers, though, this question is of great professional interest. Here are people who like camping out under the stars so much that they’ve done away with the sleeping bags in favour of sophisticated, permanent observatories. These days astronomers can have their marshmallows and eat them too, telling ghost stories around the telescope while machines do all the heavy lifting.

All is not well, though. Not only does modern civilisation spread toxins across the landscape, it also leaks a lot of light, light that finds its way into telescope tubes and spoils the evening of many an astronomer. Even once-isolated observatories are not free from this light pollution as modern cities with their dazzling candle power encroach more and more.

Astronomers are retaliating from their mountain eyries by tracking down the worst offenders so they can direct their wrath where it will do the most good. As part of this they have been looking at how the brightness of the sky changes during the night. And although they are most interested in artificial light sources they also studied the natural light of the night sky to provide a benchmark for comparison. Early results hinted that the axiom was true but more recent results show that sometimes it’s darkest just before dawn, sometimes its not. The chief culprit on a moonless night is airglow, light given off by the atmosphere itself, and its fluctuations seem to be random.

So the darkest hour is only sometimes just before dawn and, as is so often the case, another familiar phrase turns out to be mere moonshine.

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