Does it get darker after the end of Astronomical twilight?

Forums Atmospheric Phenomena Does it get darker after the end of Astronomical twilight?

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    Tim Haymes

    I have been using a SQM-L for a number of years. I have observed on occasions in good conditions and no moon, that the reading continued to climb well beyond the end of astronomical twilight. Is this just a fluke of local transparency, or is there a more profound reason for the increased darkness ?

    I think i would have to make more frequent measurements to find a trend, but has anyone else observed this ?

    Tim Haymes,  Knowl Hill, Berkshire.


    I have continuous one minute interval readings from my mast mounted networked SQL for six years, also all sky camera images at the same rate on the clearest of nights. Clear dark sky brightness continues to darken through midnight and beyond, by up to 0.3 magnitudes per square arc second. My sky brightness on good clear nights is 21.1. Gradual sky darkening during the night is always the case and is so across Europe. In general I do not do deep sky imaging until after midnight, for this reason. A combination of temperature drop towards dew point during the night with increasing mist on the horizon, blocking out distant city lighting and blocking horizon cloud reflection of city lighting beyond the horizon which causes most rural light pollution; also gradual turning off of lights after midnight, even if none local.

    Chris Hooker

    The definition of astronomical twilight is the geometrical condition that the centre of the Sun is between 12 and 18 degrees below the observer’s horizon. In more practical terms it also means that when the Sun is lower than 18 degrees, the atmosphere above you is no longer illuminated directly by sunlight; the Sun is setting for another observer about 2000 km away. However, even if the Sun is more than 18 degrees below your horizon, the sky in your location could still be slightly illuminated by the brighter twilight sky in the direction of the hidden Sun. If locations 2000 km away were also in astronomical twilight, then the indirect illumination would be negligible, and your sky would be as dark as it can get. This would presumably require the Sun to be around 36 degrees below your horizon. Do you have any timings or measurements of how the sky quality changes, and when the effect you’ve seen ends?

    Tim Haymes

    There were two readings I took recently that prompted my question.  Thank you all, and I am pleased that i can feel confident of an improvement as the night goes on for the reasons given.  My two readings were on March 29th
    2103 UT  (Sun -21)  SQM  19.8
    2315 UT (Sun -33)  SQM   20.2

    The best sky readings here is 20.3


    Grant Privett

    Hi Chris,

    Any chance of seeing one of your SQM from a clear moonless nights? Would be curious to see the shape it takes.


    Dr Paul Leyland

    This is a question to which I genuinely don’t know the answer.  Is some component of sky glow caused by long-lived excitation and relaxation (aka phosphorescence) of molecular species in the upper atmosphere?  Some so-called forbidden transitions have a lifetime of the excited state measured in minutes or hours.  A commonly encountered example is found in alarm clocks where the hands and figures can remain glowing for hours after the lights have been switched off.

    This mechanism may be a factor in the sky become darker long after it has been directly illuminated as the phosphorescence fades.

    Nick James

    I very much doubt if you could detect any natural scattered sunlight in the sky after the end of astro twilight from anywhere in southern England. I’m not sure how stable the SQM photometer is but I imagine you are detecting man-made scattered light. You would need to have a very good sky for natural sources such as airglow to have a detectable effect.

    I attach a plot from “Sunsets, twilights and evening skies” by Aden and Meinel. This is a lovely book and has several chapters on twilight. Also, a plot from an imaging session on La Palma where the skies are really dark and, in this case, very transparent . This shows the median sky background for several imaging sessions of objects at different altitudes. Astro twilight begins at 04:52 and it is only just detectable even from a place with very dark skies.

    It’s all a bit academic for me in Chelmsford. On very good nights I can only just detect the sun in my skies when it is between 15-16 deg below the horizon!

    John O’Neill


    I find the sky continues to darker after the end of astronomical twilight when observing in Topsfield, MA, USA. I would put this down to the turning off of lights over the region. 

    By contrast when in Rush, Ireland I do not see this sharp decline. There, lights are no longer turned off.

    An interesting article, Some Thoughts on Twilight, from Sky & Telescope (Oct 1960 p.207) by Joseph Ashbrook talks about this subject. I believe this can be found at


    David Arditti

    In Berkshire I would have thought the overwhelming effect would be the switching off of lights. I notice this in London: it is much darker after midnight, though none of the surrounding local authorities switch their street lights off.

    Yes in theory the airglow, caused by ionised molecules, would have a decay through the night, but, as others have said, it’s not going to contribute much for us.

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