Astronomers recognize three kinds of twilight, which helps them to better pinpoint when objects in the sky are visible. It's still a bit light out during civil twilight, and this is when the brightest planets, such as Venus, appear. Nautical twilight ends when the horizon is no longer visible at sea, and astronomical twilight ends when there are no traces of sky glow left. It's hard to distinguish astronomical twilight from true night, but if you're looking for nebulae or galaxies, it's better to wait until night has officially started.
The Three Types Of Twilight
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Key Facts In This Video
During civil twilight, the sky is still fairly light, as the sun has only just dipped below the horizon. 00:19
Day formally becomes night when the sun is more than 18 degrees below the horizon. 00:50
During the parts of midsummer when the sun is especially high in the sky, it never technically becomes night in the UK. 01:01