Expert: Tom Whiting - 4/2/2009

Question
Hi. I'm interested to know the exact end of astronomical twilight. I read that there are three types of twilight - civil, nautical, and astronomical. They end when the sun at 6 12 and 18 degrees below the horizon respectively. Numbers like these make me think that they are rounded off. I would like to know with more precision the end of astronomical twilight, when all stars which will become visible have already done so.
Thank you very much.

Answer
Hi Yisroel,
From Canada, our great neighbor to the north....beautiful country
up there.
Well, No, those are not 'rounded off'....those are the accurate hard numbers for the sun's angle below the horizon for civil, nautical, and astronomical end of twilight, respectively.

You want them more accurate?  Then how about 6.000 degrees,
12.000 degrees, and 18.000 degrees respectively.  I'm joking of course -- not really...they are hard, whole number.... of degrees
of sun below the western horizon.

In fact your information is quite accurate....the only thing I can
add is civil twilight is when you can no longer read a newspaper,
nautical is when the brighter stars come out so sailors at sea can
begin to take accurate sextant fixes on the brighter 1st magnitude stars, and astronomical twilight end means that it's as dark as it's going to get, so an astro-photographer can start photographing the night sky without fear of fogging his film with twilight....
EXCEPT for the Zodiacal light phenomenon which in the springtime months tends to extend what you think is still western sky evening twilight in a dark sky, but it isn't 'twilight'.  It's the 30 degree wedge of Zodiacal light (google it)....and in the fall, it precedes first light in the east by about 90 minutes also.  In the early morning eastern sky in the autumn months it's nicknamed the "false dawn". But it's a completely different mechanism compared to the 'twilight' light of the sun. It's actually sunlight reflecting off all the interplanetary dust along the ecliptic and Zodiac constellations, thus it's name. But it can only be seen very well
in very dark, clear mooonless skies...not an easy task.

Yes, the end of astronomical twilight is when all the stars that
you are going to see, are visible. The actual time of astronomical
twilight will vary greatly with your latitude. The farther north
you are (in the summer months) the shallower the angle of sunset,
therefore the longer twilight extends into the night.
Here at 42 N. Latitude in Erie, on June 21 sunset is close to 9 pm
EDT and astronomical twilight lasts 2 hours until 11 pm EDT.
Farther north, it takes even longer until you get up to the
Arctic Circle, where there is no total darkness at all in our summer months.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

FOLLOW UP:
Oh, you may be interested in reading the last paragraph on this
website as it relates the Zodiacal light and words of Mohammed
relative to the prayer sessions in Islam....at
http://en.wikipedia.org/wiki/Zodiacal_light
Clear skies,
Tom