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Does astronomical twilight end and night begin near the summer solstice in London or is that too far north?
Astronomical twilight lasts all night in London during the summer solstice, however due to the urban light pollution it is hardly noticeable.
Does astronomical twilight end on nights near the summer solstice in Southampton England UK?
Astronomical twilight does not end on nights near the summer solstice in Southampton, England.
Is 51 degrees north far enough south for astronomical twilight to end on dates near the summer solstice?
The Southern tip of Ireland is about 51 degrees North. That is not far enough South for astronomical twilight to end on dates near the summer solstice. (The Sun never sets at latitudes above about 66.5 degrees, at the summer solstice. Astronomical twilight ends with the centre of the Sun 18 degrees below the horizon. That means you would need to be around 48.5 degrees North to get full darkness at the summer solstice.)
What is the cutoff - southernmost - latitude for astronomical dusk - start of twilight - near the summer solstice?
At the summer solstice the sun sets on the northern horizon at latitude 66.6 degrees north. At the same time astronomical twilight ends only at latitudes below 48.6 degrees north. It's 66.6 minus 18 because for astronomical twilight to end the Sun's centre must be 18 degrees below the horizon.
Is London England far enough south to get a completely dark sky - without astronomical twilight lasting all night near the summer solstice?
No.
Does astronomical twilight end near the summer solstice when observed from Greenwich England?
No. Around the Summer Solstice, "astronomical twilight" lasts all night for everywhere in England. The southernmost point in England is Lizard Point, in Cornwall, at 49 degrees 57 minutes North.(Call it 50 degrees even, to make the math easier.) Around the Summer Solstice on June 21, the Sun is at a declination of 23 degrees 26 minutes. (Let's round that off to 23.5 degrees.) At midnight, the Sun is at 23.5 degrees North over the Pacific Ocean. That means that from Lizard looking north at midnight, the Sun is (90-50) + (90-23.5) degrees = 40+ 66.5 degrees =106.5 degrees away. That means that the Sun is 106.5-90 degrees below the horizon. "Astronomical Twilight" is defined as the Sun being less than 18 degrees below the horizon. On June 21, the Sun is only 16.5 degrees below the horizon at Lizard, and less than that anywhere else in England. So around the Summer Solstice (about a week either side of it, actually) the Sun is never more than 18 degrees below the horizon from anywhere in England, Scotland or Ireland.
Does it get darker than astronomical twilight in London England on nights near the June solstice?
Wikipedia sez: "Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon." London, England is at 51 degrees north latitude. At midnight in the June Solstice, the Sun is at a declination of 23.5 degrees (approximately), so the over-the-pole angular distance is (90-51) + (90-23.5) degrees, or 105 degrees. The horizon is 90 degrees, so the Sun is 15 degrees below the northern horizon at midnight on June 21. So, no, it won't meet the definition for "darker than astronomical twilight".
Does it get darker than astronomical twilight at night in any part of Denmark around the June solstice?
No. The southern border of Denmark is at 54 degrees 44 minutes north.Wikipedia sez: "Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon."At midnight in the June Solstice, the Sun is at a declination of 23.5 degrees (approximately), so the over-the-pole angular distance is (90-54.5) + (90-23.5) degrees, or 102 degrees. The horizon is 90 degrees, so the Sun is 15 degrees below the northern horizon at midnight on June 21.So, no, it won't meet the definition for "darker than astronomical twilight".
Are there any places in Great Britain where astronomical twilight ends near the summer solstice?
No. For any location in England, on June 21, the Sun is never more than 18 degrees below the horizon, which is the definition of "astronomical twilight". If by "Great Britain" you mean to included British colonies and possessions, then yes; places like Gibraltar and Bermuda are far enough south so that the do experience complete darkness at night.
Do all latitudes have equal sunlight hours in one year disregarding twilight?
Yes, all latitudes have equal sunlight hours in one year disregarding twilight, if you mean daylight hours rather than direct sun, which may be affected by cloud cover.
Does it get darker than astronomical twilight in Newcastle-upon-Tyne England in June?
No because the latitude is 55 degrees N so the Sun never goes more than 11½ degrees below the northern horizon, so nautical twilight and astronomical twilight do not end.
Why do you have to be within 48.5 degrees of the equator to get dark skies - without twilight - all year round?
The definition of "astronomical twilight" is when the Sun is more than 18 degrees below the horizon. At latitudes above 48.5 degrees, the Sun is less than 18 degrees below the horizon, even looking straight north (or south, for the southern hemisphere) at midnight on the solstice.
