What is the difference in time between early morning twilight and sunrise?

Question

In trying to use a GPS receiver to determine "sunrise", the answer given was not what I want. I need to determine the time at which early morning twilight begins (eg. still totally dark) when all I have is the time of sunrise.
Given the variables of :elevation on the earth , level surrounding terrain, time of the season, etc...What is the number of minutes I subtract from "sunrise", as given by a GPS unit, to arrive at that time at which early morning twilight will begin (but is still completely dark)?

Answer 1

You need to know your latitude for that. It is much longer in the arctic than it is in the tropics. The U.S. Naval observatory has a calculator for this, but they only tell you the time of civil twilight. I think you can just triple the time between sunset and end of civil twilight to get the time between sunset and end of astronomical twilight though since civil twilight is when the Sun is 6 degrees below the horizon and astronomical twilight is when the Sun is 18 degrees below the horizon. Of course this is for flat terrain, you would need to do a calculation for the change due to local mountains or whatever. See the 3rd source for help with that.

Answer 2

The previous responder answers your question pretty well. The general rule is that it is considered dark when the sun is 18 degrees below the horizon. As for how long before sunrise or after sunset the sun passes this point, it depends on the time of year and your latitude.

Answer 3

If it's still dark, it's not twilight yet. Twilight is the time between darkness and when the sun breaks the horizon. Time it and various times of the year and divide for an average.

Answer 4

Don't know of a formula, but that info is available on a pilot's briefing which anyone can phone in and get, pilot or not, at 800 briefing or something. Google it. Pilot weather briefing.

They talk about civilian twilight and when it starts exactly.