Astronomy at dusk. When do “white nights” occur in Ukraine?

…After the Sun disappears behind the horizon, darkness does not come immediately — the sky remains bright for some time, as the sun’s rays still illuminating the upper layers of the atmosphere, are partially scattered by them and reach those areas where the luminary itself is no longer visible. This phenomenon is called twilight, and on Earth it is the most pronounced and longest among the other planets of the Solar System. It greatly interferes with astronomers’ ability to observe faint celestial objects, and this becomes especially noticeable in summer at high latitudes, which include northern Ukraine.

Full-fledged astronomical observations are possible only after the Sun has dropped below the horizon by 18°, i.e., the so-called astronomical twilight ends. In general, the sky is already quite dark during twilight, and to the human eye it appears the same as during a “real” night, but if you look closely, you can still see a lighter area in the northwest — the twilight segment. Then it gradually weakens and shifts to the right (in terms of our latitude), at midnight it is exactly in the north, and then slowly brightens and “rises” in the northeast.

In Kyiv, the first of such “white nights” when the twilight segment does not actually disappear occurs on May 31 or June 1. On the following nights, the brightness of the sky gradually increases at midnight and reaches a maximum on the day of the summer solstice, which this year happens on June 20. Then all the phenomena described above become less and less pronounced every night, and around July 13-14, near the local midnight (which does not actually coincide with twelve o’clock, if only because we live on daylight saving time in summer), a “real” night finally comes for a few minutes.

It is clear that the closer the observer is to the pole, the longer the interval between the first and last “white night” will be, and the greater the brightness of the sky at midnight. On the Arctic Circle, on the day of the summer solstice, the sun’s disk does not disappear behind the horizon at all. In scientific terms, this means that we can observe not only its upper culmination, but also its lower one.

On the contrary, the closer to the equator, the shorter the period of “white nights” and the darker they are. At 48° north and south latitude, they occur only for a few days near the solstice and pass almost unnoticed. Between these latitudes, “real” nights are observed year-round, but they are shorter in summer and obviously longer in winter.

Although in our latitudes “white nights” are not as spectacular as, for example, in the Scandinavian countries, we still recommend that you watch the twilight segment move slowly and change its brightness at midnight. Unfortunately, such observations in Ukraine are currently hampered by the curfew, but on the other hand, a nice bonus is the opportunity to see the aurora, which has become noticeably more frequent with the increase in solar activity.