Every day we observe various atmospheric phenomena. Most often we do not think about their features and nature. But it’s worth taking a closer look around - and we will notice that many of these phenomena are much more interesting than they seem at first glance. One clear example is twilight.
What is twilight? This definition refers to two time periods:
- the period of time between apparent sunset and nightfall;
- the period between night and visible sunrise.
So, we have answered the question of what twilight is. But it turns out that they are divided into three types:
- civil;
- navigation;
- astronomical.
In this article we will talk in detail about the features of each type of twilight.
Civil twilight
From the image above you can learn about Blue Hour and Golden Hour.
Civil twilight begins when the Sun sets and ends when its center has dropped 6 degrees below the horizon. This type of twilight is the lightest. For this reason, small stars cannot be distinguished in the sky. But under favorable atmospheric conditions, the brightest celestial bodies can be observed. So, in cloudless weather during civil twilight, you can see Venus in the sky without using optical instruments.
During this period of time, you can see all ground objects almost as well as in the daytime. Therefore, in the morning civil twilight you can begin, and in the evening, complete work carried out in the open air. By the way, that’s why they are called civilian - at this time you can see well without additional lighting.
In some latitudes, civil twilight can last throughout the night. In other words, evening twilight gradually turns into morning twilight instead of night. This phenomenon is usually called white nights. On the territory of Russia they can be observed in St. Petersburg, Perm, Yakutsk, Vorkuta and a number of other cities.
Navigational twilight
After the end of evening civil twilight, the time of navigational twilight comes. They can also be observed before the onset of morning civil twilight.
During this period, the center of the Sun is located 6-12 degrees below the horizon. During navigational twilight, vague outlines of ground objects can be distinguished. However, without artificial lighting it is not possible to see their details. During this period, it is impossible to distinguish the colors of objects. For these reasons, field work without the use of lighting devices is not carried out in such conditions.
But during this period of time you can see all the navigation stars. At the same time, the horizon line is visible. This allows navigators to navigate in space using a sextant, measuring the angles between the horizon and the stars - therefore, twilight is “navigational.”
The military often launches attacks during navigational twilight. This is due to the fact that during this period it is most difficult for the enemy to detect them.
Astronomical twilight
Astronomical twilight is the period of time when the center of the Sun is 12-18 degrees below the horizon. With their onset, astronomers can begin observing most celestial objects. During this period, not only large planets and stars are clearly visible, but also small luminaries. However, objects with a particularly weak and diffuse glow can only be seen on astronomical nights.
However, the difference between night and astronomical twilight is clear only to those who specialize in the study of celestial bodies. For an ordinary observer, the transition from astronomical twilight to night is completely invisible. In other words, they are perceived as part of the night.
We hope that in this article you found a detailed answer to the question of what twilight is. It remains only to note that the duration of twilight in general and each period in particular varies greatly, depending on the latitude at which they are observed.
Passes along latitude 49°. There the white night can be observed only once a year - on June 22. Further north, the duration of this period increases, and the nights themselves become increasingly bright.
Experts also call this phenomenon civil twilight. Actually, evening is the time when the sun is already below the horizon, but signs of sunset are still visible. The earth is illuminated by diffused light, i.e. The rays of the already hidden star are received by the upper layers of the atmosphere and are partially scattered, and partially reflected and illuminate the earth. Objects are clearly visible without artificial lighting, clearly distinguishable, but this is no longer daylight - in clear weather the first ones are visible in the sky.
Depending on the illumination or, strictly speaking, on the position of the sun relative to the horizon, experts distinguish civil, navigational and astronomical twilight.
Civil twilight lasts from the moment of visible sunset until the time when the angle between the horizon line and the center of the solar disk is 6°, from 6° to 12° - navigational twilight, from 12° to 18° - astronomical twilight.
Thus, a white night is a phenomenon when evening twilight smoothly turns into morning twilight, bypassing the night, i.e. period of minimum illumination of the earth's surface.
A bit of astronomy
If we consider the phenomenon from an astronomical point of view, we should remember that the earth’s axis is located at an angle to the ecliptic plane, i.e. to the plane of the planet’s orbit around the Sun, and this inclination does not change.
Actually, the angle of inclination of the earth's axis changes. She describes a circle in space and at different times “looks” at different places in the starry sky. However, the period of this movement, in human understanding, is very long - almost 26 thousand years.
Thus, as the Earth moves along its orbit, the Sun illuminates either the northern or the southern hemisphere. Moreover, the tilt of the earth's axis is such that at some points in the orbit the sun's rays fall almost perpendicularly to one of the poles. In the illuminated hemisphere it is summer. In the polar regions, at this time there is a polar day, when the sun does not disappear below the horizon for many days in a row.
The other hemisphere experiences winter because it is poorly lit. The sun's rays seem to glide across the surface of the Earth and heat it poorly. The pole is in the shadow, there is polar night. In the circumpolar regions of the illuminated hemisphere, although the Sun sets, it does not set for long and is close to the horizon. So close that it can illuminate the surface of the planet with its rays scattered in the atmosphere. White nights are coming.
White nights are the time when the evening twilight merges with the morning, and the darkness of the night never comes. This natural phenomenon can be observed in the northern regions located at a latitude of at least 60°33’. The length of the period of white nights depends on the geographical location.
The “official” time when the white nights begin in St. Petersburg is June 11, and the end day is July 2. The peak of white nights occurs on the three-day summer solstice, June 21-23, when the day length from sunrise to sunset is almost 19 hours (more precisely, 18 hours 51 minutes). The so-called “civil twilight” (during which you can clearly distinguish surrounding objects without additional lighting) at this time begins around midnight and ends at approximately 2 am.
But in fact, you can watch the white nights in St. Petersburg for longer. Evening twilight here merges with morning twilight, starting from May 25-26 and ending on July 16-17. At this time, the sun does not fall below 9 degrees below the horizon, and darkness as such does not occur. At one time in St. Petersburg, it was during this period that the night lighting was turned off - it was already quite light on the streets.
What happens during the white nights in St. Petersburg
During the white nights, St. Petersburg is very crowded: in the city center life is in full swing both at night and during the day. At this time, the traditional Scarlet Sails holiday takes place, something like a citywide graduation party for schoolchildren and other celebrations, music festivals and sports competitions take place.
When there are white nights in St. Petersburg, guests of the city are very actively offered a night program: bus and walking excursions, walks along rivers and canals. The “highlight” of the program is traditionally the impressive spectacle of the bridges being raised: the Neva embankments are very crowded at night at this time, and cars and tourist buses in the city center sometimes sit in traffic jams for a long time.
Many cafes and shops in the city center are open 24 hours a day at this time. Unfortunately for those who like to walk around St. Petersburg during the white nights, public transport does not switch to permanent 24-hour service: night buses run only on the night from Friday to Saturday and from Saturday to Sunday, and from the Admiralteyskaya metro station to the Sportivnaya station during the bridges are raised and the night train runs. In addition, on the night of the Scarlet Sails holiday, the metro does not close at all.
Many people believe that during the white nights in St. Petersburg it is as light at night as during the day. This is not entirely true: for example, in the “twilight hours” in an open space without additional lighting it is difficult to read a book (unless the font is very large), but it is quite possible to play badminton.
"Pitfalls" of white nights
When coming to St. Petersburg during the white nights, you need to keep in mind that late and short twilight can cause some discomfort. Firstly, during this period, many people lose their sense of time in the evenings, unable to “orient themselves by the sun.” Therefore, if you go for a walk around the city and want, for example, to catch the metro, set a “reminder” on your phone, otherwise night may come unnoticed.
In addition, a white night can be fraught with sleep disorders - not everyone is able to sleep fully in the light. In such cases, thick curtains and “adjusting” your personal schedule to daylight hours can help. If you have difficulty falling asleep, it is best to go to bed immediately after midnight, when dusk is just beginning to fall and the light level, although not very strong, is still declining.
And, not letting the darkness of the night
To golden skies
One dawn gives way to another
He hurries, giving the night half an hour.
A.S. Pushkin "The Bronze Horseman"
Many people try to get to St. Petersburg at the end of July to see the famous white nights, about which they talk and write a lot. White nights are a symbol of St. Petersburg, but this amazing natural phenomenon can be observed in Kazan, Arkhangelsk, Kirov, Samara, Pskov, and Syktyvkar.
Why are the nights as bright as days in some parts of the Earth?
We know that day turns into night due to the movement of our Earth. Night comes when the Sun, in relation to a certain area of the earth's surface, is below the horizon and there is not enough light. That's why it's dark at night.
The duration of the night period depends on the latitude of the location of a point on the earth's surface, the inclination of the planet's rotation axis relative to the plane of its orbit, and the angle formed by the rotation axis and the direction to the central body is also important.
White nights are nights in which natural illumination is reduced only until twilight. The sun does not set below the horizon at all, and we see a bright sky all day long. Such white nights are called “polar days”.
The Earth's axis is tilted, so the Sun illuminates our planet in different ways: in winter, the sun's rays do not reach the North, and in summer, a huge amount of heat and light is shed on us. In science, white nights are “civil twilight” - the period from the setting of the upper edge of the solar disk beyond the horizon until the sun’s descent below the horizon does not exceed several degrees. It happens that the Sun sinks just a little below the horizon at night (does not have time to completely descend), night smoothly turns into day without the onset of darkness. On the days of the summer solstice, the sun does not fall below the horizon at all, and at latitudes above 65º the entire northern polar region is illuminated - the polar day. And when the sun does not go down beyond the Arctic Circle for a long time, rays of scattered sunlight reach the subpolar regions.
There are no white nights from the equator to latitude 49º, but above 49º there is a “white night zone”. In the Volgograd and Rostov regions (they are located at latitude 49º) there is one white night per year (June 22). And higher up, to the north, the white nights become lighter and longer.
Muscovites can also experience white nights, but here they are not as bright as in other cities. For almost three months from May 12 to August 1, the sun does not set in Yakutia. White nights can be seen in the following cities: Murmansk, Norilsk, Vorkuta, Cherepovets, Vologda, Berezniki, Magadan, Megion, Khanty-Mansiysk, Kotlas, Nizhnevartovsk, Nefteyugansk, Surgut, Syktyvkar, Petrozavodsk, Yakutsk, Ukhta, Noyabrsk, Nadym, Arkhangelsk, Severodvinsk.
Throughout Iceland, Finland, Greenland and Antarctica, you can enjoy white nights at certain times, as do parts of Sweden, Canada, Norway, Estonia, Great Britain and Alaska.
website, when copying material in full or in part, a link to the source is required.