[Chaos-l] [Fwd: Re: Skywatching canceled]

[Robert Nielsen]

Margareta,

Actually, it is quite simple ... although you need to be a rocket 
scientist to do the math ...

There are three "twilights" that I found on the web ... and each is 
based on how many degrees below the horizon the Sun is:

civil twilight - Sun is between 0 and 6 degrees below the horizon
nautical twilight - Sun is between 6 and 12 degrees below the horizon
astronomical twilight - Sun is between 12 and 18 degrees below the horizon

After 18 degrees ... I guess people don't think the Sun contributes 
anything to the night sky, except zodiacal light.   So if you know the 
angle of the Sun relative to where you are making the calculation (I 
*think* this matters) and the speed the observer is moving as the Earth 
rotates, then you can figure out when the twilights end (or begin ... 
because the opposite thing happens in the morning).

Anyone want to chime in and help me figure out whether the angle of the 
sun matters?

Robert

mot36 at earthlink.net wrote:
> I don't know about these phenomena you are talking about but I have ( out
> of profound ignorance) wondered how you measure twilight in such detail. 
> >From when to when?   From the limb of the sun touching the horizon to when
> it drops below?  Probably not, it couldn't be that simple.  Margareta
>
>