The Twilight Zone

A few days ago I wrote about how the sun is setting earlier and earlier this month, and how we are losing so much sunlight in the month of August (See here). In that article, I also alluded to the fact that on the plus side, there would be more hours of darkness in which to observe. But how to astronomers measure the darkness? When does it get truly dark after the sun goes down?

We measure The Twilight Zone.

We’ve all seen and experienced twilight, whether in the morning, or the evening. Sometimes it’s called dusk in the evening, and dawn in the morning. But the common denominator is that it’s the light that is in the sky when the sun is below the horizon. Even though we do not see the sun from our location, it’s light illuminates the sky above us, indirectly.

In the morning, the deep dark of night slowly succumbs to the inexorable sunrise. It’s tenuous at first, but you can see things slowly becoming more and more distinct. In the evening, it’s just the opposite. The sun sets below the horizon, and slowly but surely the darkness approaches, making it harder and harder to see things without artificial illumination. Even though this time of “light transition” is gradual and looks seamless, astronomers want to know when it is really dark.

But first let’s figure out sunrise and sunset. The definition of sunrise and sunset is the time when the upper edge of the sun’s disc is on the horizon, without being hidden by local obstructions (in essence, you need a flat horizon). Once you get there, you are in The Twilight Zone.

As I said before, there are levels of twilight, ranging from bright to just that last bit (usually not really apparent to the eye) of light before “full dark.” Scientists have divided the twilight into three stages: Civil, Nautical, and Astronomical.

(For examples here, we will concern ourselves with evening twilight).

Civil Twilight begins when the sun has set, and ends when the center of the sun is 6° below the horizon. You can see things with clarity, and don’t really need any artificial light sources.

Nautical Twilight begins when Civil Twilight leaves off, and extends until the center of the sun’s disc is 12° below the horizon. By this time it’s dark enough that you can’t make things out clearly, but can see shapes. You definitely need lights to do things outside.

Astronomical Twilight begins when Nautical Twilight ends. The sun is 18° below the horizon, and it’s as dark as it’s going to be. It stays that way until the beginning of Astronomical Twilight before sunrise. Then the whole process is reversed: Astronomical, the Nautical, then Civil. And then voila! It’s sunrise.

It’s the Astronomical Twilight that astronomers wait for. It’s dark, the sky is as deep as it can be (unless the moon is out, then it’s not really dark), and it’s time to get out the telescopes. We use this time to the utmost advantage, and always wish for more. And depending on the time of year, we get that.

Tomorrow, we’ll spend time in the dark. But not always the same amount…


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