When the Sun is 18 degrees below the horizon.
Here's the formula: From sunset until the Sun is six degrees below the horizon is civil twillight. It's still light enough to read. When the Sun is six to twelve degrees below the horizon is nautical twilight. One can still see the horizon at sea; that is, tell when the sea stops and the sky starts. From twelve to eighteen degrees is astronomical twilight, which is when the stars start to come out. After the Sun is more than eighteen degrees below the horizon, it's officially Night.
Astronomical twilight lasts all night in London during the summer solstice, however due to the urban light pollution it is hardly noticeable.
Yes, absolutely. At the Arctic Circle in December, it barely gets LIGHT enough to be astronomical twilight. Then it gets dark again.
Twilight is the time of day that falls between sunrise, or sunset, and full night.
Twilight is the half-light between night and day. Twilight occurs a dusk and also t dawn.
No.
It is twilight or dusk
No it only happens south of 48½ degrees north latitude.
The line dividing illuminated areas from night areas is called the terminator.
Polar night occurs when the sun doesn't rise above the horizon for 24 hours, creating continuous darkness. Twilight, on the other hand, is the period before sunrise and after sunset when the sky is partially illuminated.
day night midnight noon astronomical twilight nautical twilight civil twilight sunrise sunset moonrise moonset morning afternoon evening ante meridiem (AM) post meridiem (PM) o'clock dawn dusk
The transition time between day and night is called twilight. This is when the sky gradually changes color as the sun sets or rises, creating a period of low light levels.