No, it’s not about the books. But yesterday I mentioned that we no longer have astronomical night, and I felt that some definitions were in order.

End of civil twilight, available light photo looking NNW at 10:10 pm last night.

Twilight is defined as the period between the first traces of scattered light in the sky and sunrise, and between sunset and total darkness (other than starlight and moonlight.) It is actually divided into three periods each in the morning and evening, with boundaries determined by how far the sun is below the horizon.

Civil twilight is the period between the sun being on the horizon (sunrise and sunset) and the sun being 6 degrees below the horizon. If the sky is clear and you have good vision, you can generally see well enough to drive without lights, though your car will certainly be more visible to others if you turn your headlights on. The brightest stars and planets become visible after sunset and fade out before sunrise during civil twilight, but it’s not a very good time for even casual astronomy, except for watching the new moon, Venus and Mercury. Local laws generally define the limit of civil twilight using the time (30 minutes before sunrise and after sunset, for instance) but that can be very far off at high latitudes. Here in Fairbanks, for instance, it never gets darker than civil twilight from May 27 to July 28.

Nautical twilight is the period when the sun is 6 degrees to 12 degrees below the horizon. It was originally defined as the time when it was possible for sailors to make star sights, as most of the stars were visible but so was the horizon at sea. Here in Fairbanks it never gets darker than nautical twilight from April 26 (two weeks from now!) to August 18.

Astronomical twilight is defined as the period when the sun is 12 degrees to 18 degrees below the horizon. Ideally, when the sun is more than 18 degrees below the horizon 6th magnitude stars should be visible. Sadly, this is no longer true anywhere in the vicinity of city lights, and in most places the difference between astronomical twilight and civil twilight is imperceptible. Here in Fairbanks, it does not get darker than astronomical twilight from April 9 to September 4. City lights are not a great problem up here, but aside from a brief period in September good astronomical viewing coincides with temperatures far too low for comfortable star gazing.

Twilight is very short at the equator, where the sun sinks or rises vertically at a rate of 15 degrees an hour . This gives only 24 minutes for each stage of twilight. Sunsets and sunrises seem very abrupt. At higher latitudes the sun seems to slant down to (or up from) the horizon, and twilight (and sunrise and sunset colors) can last much longer. I’ve tried to incorporate this in Homecoming — the sunsets and sunrises are very short on Marna’s tropical island, for instance.

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