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I assume at the Solstice - a definite latitude would have exactly 24 hours of daylight. What latitudes (North and South) are these - they might vary a little with longitude?

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The latitudes today are 66°33′46.1″ N and S, the Arctic and Antarctic Circles.

By definition, north of the Arctic circle, and south of the Antarctic circle, everywhere gets at least one day with no sunset, which I guess is what you mean by 'exactly 24 hours of daylight'?

From the linked article:

North of the Arctic Circle, the sun is above the horizon for 24 continuous hours at least once per year (and therefore visible at midnight) and below the horizon for 24 continuous hours at least once per year (and therefore not visible at noon).

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  • $\begingroup$ Doesn't this assume that daylight starts when the center of the Sun (and not the upper limb) has risen, and that there's no refraction at the horizon? $\endgroup$ – Barry Carter Feb 5 '16 at 17:58
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    $\begingroup$ You can interpret "exactly 24 hours of daylight" however you like. I thought the simplest interpretation was the one that defines the famous Circles. $\endgroup$ – kwinkunks Feb 5 '16 at 18:05
  • $\begingroup$ I interpreted it as sunrise to sunset time of exactly 24 hours, but I agree your answer is a good approximation (sorry, I'm an amateur astronomer, and a bit OCD about the definition of sunrise and sunset) $\endgroup$ – Barry Carter Feb 5 '16 at 18:06
  • $\begingroup$ @kwinkunks, the Arctic circle and the Tropic lines are generated due to the Sun transit. It is not related to the duration of the sunlight. Atmospheric refraction and reflexion allow us to see the Sun around 3º under their position on the sunset and sunrise. So when you think that the Sun is in the horizon, it is under it. $\endgroup$ – David García Bodego Oct 23 at 8:29
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It depends...

What do you understand as 24 hours daylight? Sun should be visible? There should be light, even if the Sun is not visible?

There are different interpretations:

Astronomical Twilight:Astronomical twilight is the darkest of the 3 twilight phases. It is the earliest stage of dawn in the morning and the last stage of dusk in the evening.

Nautical Twilight: Nautical twilight is the second twilight phase. Both the horizon and the brighter stars are usually visible at this time, making it possible to navigate at sea.

Civil Twilight: Civil twilight is the brightest of the 3 twilight phases. The Sun is just below the horizon, so there is generally enough natural light to carry out most outdoor activities

So if you consider the Civil twilight, at 61º lattitude, you will get one 24 hours daylight. (Close to Helsinki - Finland)

If you go to the Nautical one, then you will come south.

Check this link. You will enjoy.

Hope it helps!

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