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Length and Brightness of Twilight

-- Brian | January 18, 2021

Photo of ten VLA antennas pointing at a cloudy sky at twilight.

Question:

Hello, I live in Schenectady New York, 42.8142° N, 73.9396° W. I notice that the summer night is brighter than the winter night, but I read that you must be farther north to notice twilight at night in the summer. Do you know why the sky is brighter at certain times of year than others? Thank you!

-- Brian

Answer:

There is a really nice way to calculate and visualize the durations of twilight from any location, and specifically for Schenectady NY, using the Time and Date Sun Graph. As you can see from the sun graph calculation, the difference in duration of astronomical twilight between December and July is about 45 minutes (longer in July). Dependent upon when you observe the sky brightness, and if it is during a period of twilight (astronomical, nautical, or civil), this could be an explanation for the differing brightness of the night sky you observe.

-- Jeff Mangum

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