At what latitudes does daytime last exactly 24 hours once in the year?

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?

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'?

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).

• 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?
– user967
Commented Feb 5, 2016 at 17:58
• You can interpret "exactly 24 hours of daylight" however you like. I thought the simplest interpretation was the one that defines the famous Circles. Commented Feb 5, 2016 at 18:05
• 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)
– user967
Commented Feb 5, 2016 at 18:06
• @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. Commented Oct 23, 2019 at 8:29
• Subtract a few arc minutes south from the Arctic Circle / north from the Antarctic Circle -- the Sun's disk is about 30 minutes across. Commented Nov 9, 2021 at 22:45

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.

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Hope it helps!

I believe the phrase specific words have specific meaning is pertinent here. I believe the question has been misunderstood in the answers as pertaining to daylight instead of daytime. This question is not referring to daylight hours, it is in reference to the fact that the sidereal day length varies by plus or minus ~4 minutes due to a few factors; asynchronous rotation of earth, earth's tilted axis, variable speed at para/aphelion positions, and elliptical orbit being the primary factors. The 24hr day we all use is the sidereal mean time or standard time.

It is an interesting question. I cannot purport that the following is absolutely a fact but would say that it is a reasonable presumption with high confidence.

It would likely be between the 23.5 degree latitudes known as the tropic of Cancer and Capricorn. I might even make the assumption that you would have to be one that latitude in conjunction with the corresponding equinox or solstice. Those assumptions are hesitant at best though. The lack of data published makes me consider that the interactions of celestial bodies in asynchronous and non tidally locked orbits, while also wobbling on axis may continually affect what days and latitudes the amount of time from apex to apex of the sun to be exactly 24 hrs.

• Fair point, but the OP mentions both 'daylight' and 'daytime' so at best it's ambiguous. I agree your question is arguably more interesting, though I think it's the solar (not sidereal) day that varies in the way you're describing. A function with the awesome name 'the equation of time' crosses the x-axis — denoting a solar day of exactly 24 hours — four times a year for a given location. Commented Nov 9, 2021 at 23:51
• This question would probably be best answered at Astronomy SE. Commented Nov 10, 2021 at 8:42
• In addition to @kwinkunks comment, a 24.0000000 hour day as determined by the equation of time is independent of the latitude. This contradicts the OP which definitely asks for the latitude. It seems the OP is not asking for the date when the solar day is closest to being 24 hours long. Commented Nov 10, 2021 at 18:23