Understanding Astronomical Nights: Why There are No Longer Nights Even Though Summer Hasn’t Started Yet

Although there is still a month left until the traditional holiday season, nature is already giving us long days and shorter nights. From an astronomical perspective, from mid-May there is not even night anymore. Sounds intriguing? What does that really mean?

We grow up convinced of the uniqueness of two moments in a day. Midnight, when the date changes, and noon, which theoretically marks the middle of the day. However, this is only an approximation, because when we look at the position of the Sun, when our watches indicate noon, the Sun will not necessarily be where we expect it to be.

The earth rotates from west to east, but it does not know the concept of time zones, which we invented to better organize the functioning of societies. Zones with the same clock time cover large areas. Central European time is valid from the eastern borders of Poland to the Atlantic Ocean, but in practice only on the 15th meridian it is close to solar time, and at noon the sun can be seen more or less in the south.

In other parts of Europe at noon the sun is still on the eastern side of the sky, and at other times a little to the west. In Warsaw, at the end of May, the sun will rise, i.e. the actual middle of the day, when the clocks will indicate 12:30. In Poznań, it will be 12:48. The further to the west of Europe, the later we have the actual south. In Spain, it’s only around 14:00.

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And although it is said about the inhabitants of southern Europe that they like the nightlife, in the summer the further north you go, the shorter the nights are, and from an astronomical perspective there are no nights for some time. And it makes sense.

In the colloquial understanding, night is the time when the sun will hide behind the horizon and will no longer illuminate the sky. However, Earth has an atmosphere where the light rays coming from our daytime star are bent, and that changes things.

For this reason, we enjoy beautiful sunsets, and the setting Sun seems to us to be above the horizon when in fact it is already below it. This phenomenon, called atmospheric refraction, causes the apparent elevation of objects in the sky near the horizon, and additionally deforms their shape. The sun just above the horizon can be in the shape of a triangle, a trapezoid. At the last moment, when the apparent image disappears below the horizon, a so-called green flash can appear, a rare phenomenon that lasts for seconds. They are best seen where nothing obscures the horizon.

Even when the apparent image of the Sun is hidden below the horizon, the sky is still illuminated by its rays. This is the time when we have the so-called twilight. There are three types of it:

• civil, as long as the sun is not lower than 6 degrees below the horizon,
• nautical until the sun is below 12 degrees,
• astronomical, until the Sun is below 18 degrees.

When twilight ends, a night of that type begins. In the morning, when we have dawn, everything repeats in reverse order.

The sun during the year, every day at noon (sunny, not watch) is at a different height. In Poland in the summer it reaches about 62.5 degrees, in December it drops to about 15.5 degrees.

At night, it hides several dozen degrees below the horizon in the winter, but in spring and summer for places located at latitudes greater than about 49 degrees, i.e. in practice for the whole of Poland, in the summer the Sun does not set even in the middle of the night more than 18 degrees.

From an astronomical perspective, astronomical night does not begin then, and astronomical dusk turns into dawn.

In Warsaw and cities located at the same latitude, the time without astronomical night lasts from May 20 to July 24.

In the north of Poland this period is a bit longer and has already begun, in the south, only at the end of May, at the beginning of June, astronomical nights will end.

Astronomers chose this definition of the beginning of the night for a reason. Only when the sun is lower than 18 degrees below the horizon does the sky become maximally dark. It will not be dimmer for the rest of the astronomical night. We omit here the influence of city lighting, moonlight, the Milky Way, whose glow in places far from city lights is clearly visible.

When there is no astronomical night, it is worth taking pictures at night, trying to observe phenomena such as northern lights (in Poland they have been frequent recently). The sun moves low below the horizon from west to east, all the while creating a bright glow in the north side of the sky.

The further north of Poland we go on holiday, the brighter this glow will be. The end of June is the best time from this perspective.

If you go to the south of Europe or Africa in the summer, it will turn out that the nights are longer and darker than in Poland. There, astronomical night occurs all year round. On the other hand, when traveling in June and July to the north of our continent, for example to Scandinavia, you can experience the so-called civil white nights.

White night is by definition a time when there is no night of a given type. From an astronomical perspective in Poland we have astronomical white nights, but civil white nights are truly spectacular, when even in the middle of the night it is as bright as just after sunset.

Even further north, beyond the Arctic Circles, when the latitude exceeds 67.5 degrees, the sun may not be below the horizon all day. And we have a polar day. The closer to the pole, the longer it is. At the pole itself, it lasts about 6 months and slowly turns into the polar night, which also lasts about 6 months. Interestingly, the polar night is slightly longer than the polar day, due to the previously described phenomenon of refraction.

In the southern hemisphere of the Earth, the situation repeats with a shift of 6 months, but the lack of astronomical nights can be practically experienced only by the inhabitants of the southern tip of South America, and white nights and polar day by people staying in Antarctica.

The time without astronomical night repeats regularly every year. This is a great period from the perspective of spending free time outside after sunset, although not well-matched with the holiday season. At the end of May, the nights can still be cool, and when school ends, we have the shortest day of the year and half the period of astronomical white nights.

Astronomers do not like white nights, of course, because they make it difficult to observe the deep sky. And although it is warm even at night in summer, and many interesting constellations can be seen in the sky, winter is potentially a better time for observations. The atmosphere is then clearer and the night is long. And theoretically better for sleep.