Twilight and white nights: “Companions” of the summer solstice

Probably, everyone knows that in the summer in the circumpolar regions of our planet, the so-called white nights come. Of course, astronomers there are not happy about this, because at this time only the brightest celestial bodies can be observed — the Moon and the planets of the Solar System visible to the naked eye, that is, Mercury, Venus, Mars, Jupiter and in some cases Saturn. Less well-known is the fact that, from an astronomical point of view, the “white nights zone” extends quite far to the south, up to the territory of Ukraine. Why do we hardly see them then?

In fact, there is nothing unusual about white nights. We all know such a phenomenon as twilight. They come after sunset, fade away completely after a while, and begin again before sunrise. If the night is short enough, the evening twilight simply does not have time to end before the morning ones begin, so even at midnight the “night” darkness will not come.

Morning astronomical twilight. The firmament is still full of stars, and the naked eye would not be able to distinguish it from the “ordinary” night sky, but the camera confidently shows a bluish tint — these are the sun’s rays scattered by the molecules of the gases of the earth’s atmosphere, reaching the observer, even though the luminary  is still deep enough below the horizon. The author of the picture is Alex Chubukov, Astropolis Amateur Astronomers’ Club in Kyiv

This is a very simplified explanation, and the main thing that needs clarification here is the phrase “some time”. And it differs significantly for different geographical latitudes, and also depends on the time of year. Therefore, to describe the complex of phenomena accompanying sunrise and sunset, astronomers primarily use angular units, namely, the depth of immersion of the solar disk under the horizon.

After long observations and measurements, scientists came to the conclusion that twilight should be divided into three stages. The first one was called civil twilight. It lasts from the moment of sunset to its immersion by 6° (that is, by 12 diameters of the solar disk). At this time, the level of illumination allows us to do most household chores. The sky is still quite bright and only the brightest objects are visible on it — the Moon, as well as planets and stars that have a brightness above zero magnitude. Further on, more and more stars begin to appear, but at the same time it is still easy to notice the horizon — the boundary between the “terrestrial” and “celestial” hemispheres. This makes it easier to determine the coordinates of the observer by measuring the height of celestial bodies using a sextant or theodolite. Therefore, this stage was called nautical twilight. It ends when the sun sets to a “depth” of 12°.

Finally, it becomes so dark that the horizon merges with the sky, and almost all the stars visible to the naked eye “appear”. But not far from the place of sunset, there is still a piece of the “illuminated” sky — these are the sun’s rays, scattering on atmospheric gas molecules, getting around the globe and reaching the observer. Scientists cannot conduct full-fledged observations at this time, so it is called astronomical twilight. The “illumination” finally disappears when our luminary goes deeper by 18°. Only after that the night begins in the full sense of the word.

Why is it so difficult to convert these degrees into units of time, even knowing that during the daily rotation of the sky, the Sun shifts by the amount of its apparent diameter in a little more than two minutes? At the equator, where the apparent movement of the sun is almost perpendicular to the horizon, it’s really not difficult: civil twilight there always lasts 26 minutes (two minutes “adds” atmospheric refraction), nautical — 50, astronomical — 74. But at higher latitudes, this time increases noticeably, and the stronger it is, the further south or north we are moving away. And in areas lying closer to the poles than to the equator, the duration of twilight also depends on the time of year.

When the Sun crosses the celestial equator on the days of the equinox, at noon it passes the upper culmination at an altitude equal to the depth to which it “dives” at midnight during the lower culmination. It is quite simple to calculate it for a specific place: it is necessary to subtract the geographical latitude from the right angle, then subtract from the obtained value 23.5° (the inclination of the earth’s equator to the plane of the ecliptic) and another half degree of correction for refraction, since the atmosphere of our planet, being an optically active medium, refracts the sun’s rays and forces them to “bend around” the bulge of the earth surfaces.

If we make the appropriate calculations for Kyiv, which latitude is approximately equal to 50.5°, we will get the minimum possible immersion of the sun under the horizon at 15.5°. This is already less than necessary for the onset of astronomical twilight, that is, we can confidently say that near the summer solstice in the north of Ukraine they do not end — evening twilight gradually turns into morning, and over the northern part of the horizon, even at midnight, the glare of the star is noticeable. The further north they are, the brighter they are, and the longer the duration of the period of such “white nights”.

So why don’t the residents of Ukraine see them, and in general, for the most part, they haven’t heard anything about them? So why don’t the residents of Ukraine see them, and in general, for the most part, they haven’t heard anything about them? Recall that we are talking about astronomical twilight, which for an ordinary citizen is not much different from an “ordinary” night. In addition, in large cities, artificial lighting strongly illuminates the sky. But at the end of June, when the weather is clear and there is no Moon, it is worth going away from the city lights (preferably to a place with an open northern horizon) — and the “all-night” star will become quite obvious. 

Further north, at the 54th latitude (Minsk is located there), the nautical twilight does not end a couple of nights before and after the summer solstice, and it is already hard not to notice — even at midnight the sky remains bright, although you can see enough stars on it. Finally, at latitude 60, the Sun does not sink more than 6° below the horizon for several nights. Civil twilight continues here all night, which allows performing normal economic activities around the clock. Actually, this is the classic “white nights”. 

On the Arctic Circle (north or south) in the vicinity of the summer solstice, the Sun does not hide behind the horizon for a couple of days at all. In fact, thanks to the refraction already mentioned, this can be seen already at about 66th latitude. The closer to the pole, the longer this period of the “non-setting Sun” will be. Conversely, there is not a single night between the 48th degree of north and south latitude when astronomical twilight does not end: at least for a few minutes at midnight, “real darkness” comes. In Ukraine, the cities of Kryvyi Rih, Zaporizhia and Donetsk are located at this latitude.

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