Why is it light at night: several main reasons for this phenomenon. Why is the sky blue and the sunset red
Sometimes at night we have the opportunity to observe such a phenomenon in which the sky seems not dark enough. And today we will consider questions about why the sky is bright at night.
Why is it light at night in winter
In the winter period of the year, we are used not only to the fact that it starts to get dark much earlier than in summer, but also to the fact that the weather is usually such that even in the daytime the daylight hours seem less bright. Despite this, sometimes we have the opportunity to observe fairly bright nights, so we need to consider why the sky is bright at night in winter.
There are two reasons for lighter skies at night:
- If you notice that the night is not as dark as usual, and there are precipitations in the form of snow outside, you can be sure that snow is the reason for such a bright sky. Snowflakes reflect the light of lanterns, as well as moonlight, due to which there is an illusion of a more illuminated night sky;
- If the sky is bright enough, and there is no precipitation, then strong and low cloudiness can be considered the cause of such a phenomenon. Pay attention to the clouds and clouds - they are lower than usual. For this reason, it is the clouds that act as light reflectors from the earth, which leads to the appearance of the illusion of a bright sky.
Why is it as bright as day at night
If, when asking a question about the night illumination of the Earth's surface, you were directly interested in information about the so-called "White Nights", which are observed, for example, on the territory of St. Petersburg, then in this situation the answer will be completely different.
To begin with, it is worth noting that such white nights are observed not only in St. Petersburg, but also in many other parts of our planet. For example, it is quite possible that someone will be interested in the question of why it is light in Greenland at night, since a similar phenomenon is also present there.
The reason for the emergence of such a phenomenon is considered to be events of a planetary scale. The fact is that at a certain point in time, due to the fact that the Earth revolves around the Sun along a certain trajectory, and also makes revolutions around its own axis, our planet is on such a trajectory that even at night the Sun in the territory, for example, St. Petersburg or Greenland sits not much below the horizon. Accordingly, even at night, sunlight is scattered over the surface of the Earth and in the above-mentioned territories, a kind of twilight is observed instead of the usual night.
The world around us is full of amazing wonders, but we often do not pay attention to them. Admiring the clear blue of the spring sky or the bright colors of the sunset, we do not even think about why the sky changes color with the change of time of day.
We are accustomed to bright blue on a fine sunny day and to the fact that in autumn the sky becomes hazy gray, losing its bright colors. But if you ask a modern person about why this happens, then the vast majority of us, once armed with school knowledge of physics, are unlikely to be able to answer this simple question. Meanwhile, there is nothing complicated in the explanation.
What is color?
From the school physics course, we should know that differences in the color perception of objects depend on the wavelength of light. Our eye can only distinguish a fairly narrow range of wave radiation, with blue being the shortest and red being the longest. Between these two primary colors lies our entire palette of color perception, expressed by wave radiation in different ranges.
A white sunbeam actually consists of waves of all color ranges, which is easy to verify by passing it through a glass prism - you probably remember this school experience. In order to remember the sequence of changing wavelengths, i.e. the sequence of colors in the spectrum of daylight, invented a funny phrase about a hunter that each of us learned in school: Every Hunter Wants to Know, etc. 
Since red light waves are the longest, they are the least susceptible to scattering during transmission. Therefore, when you need to visually highlight an object, they use mainly red color, which is clearly visible from afar in any weather.
Therefore, a stop signal or any other warning light is red, not green or blue.
Why does the sky turn red at sunset?
In the evening hours before sunset, the sun's rays fall on the surface of the earth at an angle, and not directly. They have to overcome a much thicker layer of the atmosphere than in the daytime, when the surface of the earth is illuminated by the direct rays of the sun.
At this time, the atmosphere acts as a color filter, which scatters the rays of almost the entire visible range, except for the red ones, which are the longest and therefore most resistant to interference. All other light waves are either scattered or absorbed by water vapor and dust particles present in the atmosphere.
The lower the sun drops in relation to the horizon, the thicker the layer of the atmosphere the light rays have to overcome. Therefore, their color is increasingly shifted towards the red part of the spectrum. A folk sign is associated with this phenomenon, saying that a red sunset portends a strong wind the next day. 
The wind originates in the high layers of the atmosphere and at a great distance from the observer. Oblique solar rays highlight the outlined zone of atmospheric radiation, in which there is much more dust and vapor than in a calm atmosphere. Therefore, before a windy day, we see a particularly red, bright sunset.
Why is the sky blue during the day?
Differences in the length of light waves also explain the pure blue of the daytime sky. When the sun's rays fall directly on the surface of the earth, the layer of the atmosphere they overcome has the smallest thickness.
Scattering of light waves occurs when they collide with gas molecules that make up air, and in this situation, the short-wavelength light range is the most stable, i.e. blue and purple light waves. On a fine windless day, the sky acquires amazing depth and blueness. But why do we then see blue and not purple sky?
The fact is that the cells of the human eye, which are responsible for color perception, perceive blue much better than purple. Yet purple is too close to the edge of the perceptual range.
That is why we see the sky as bright blue if there are no scattering components in the atmosphere, except for air molecules. When a sufficiently large amount of dust appears in the atmosphere - for example, in a hot summer in a city - the sky seems to fade, losing its bright blue.
Gray sky of bad weather
Now it is clear why the autumn bad weather and winter slush make the sky hopelessly gray. A large amount of water vapor in the atmosphere leads to the dispersion of all components of the white light beam without exception. Light rays are crushed in the smallest droplets and water molecules, losing their direction and mixing over the entire range of the spectrum. 
Therefore, light rays reach the surface, as if passed through a giant diffuser. We perceive this phenomenon as a grayish-white color of the sky. As soon as moisture is removed from the atmosphere, the sky turns bright blue again.
It is difficult to answer the question why the sky is blue and the sunset is red.
Why is this happening?
Scientists for several centuries could not explain the blue color of the sky.
From the school course of physics, everyone knows that white light can be decomposed with the help of a prism into its constituent colors.
There is even a simple phrase to remember them:
The initial letters of the words of this phrase allow you to remember the order of colors in the spectrum: red, orange, yellow, green, blue, indigo, violet.
Scientists have suggested that the blue color of the sky is due to the fact that the blue component of the solar spectrum best reaches the Earth's surface, while other colors are absorbed by ozone or dust scattered in the atmosphere. The explanations were quite interesting, but they were not confirmed by experiments and calculations.
Attempts to explain the blue color of the sky did not stop, and in 1899 Lord Rayleigh put forward a theory that finally gave an answer to this question.
It turned out that the blue color of the sky is caused by the properties of air molecules. A certain amount of rays coming from the Sun reach the Earth's surface without interference, but most of them are absorbed by air molecules. By absorbing photons, air molecules are charged (excited) and already emit photons themselves. But these photons have a different wavelength, and photons that give blue color predominate among them. That is why the sky looks blue: the more sunny the day is and the less cloudy, the more saturated this blue color of the sky becomes.
But if the sky is blue, why does it turn crimson at sunset? The reason for this is very simple. The red component of the solar spectrum is much worse absorbed by air molecules than other colors. During the day, the sun's rays enter the Earth's atmosphere at an angle that directly depends on the latitude at which the observer is located. At the equator this angle will be close to a straight line, closer to the poles it will decrease. As the Sun moves, the layer of air that light rays need to pass through before reaching the observer's eye increases - after all, the Sun is no longer overhead, but tends to the horizon. A thick layer of air absorbs most of the rays of the solar spectrum, but the red rays reach the observer almost without loss. That is why the sunset looks red.
Read this:
November 6, 2011 The sunset over Los Angeles was almost blood red and the Sun was huge. The sky surrounding the Sun was also bright orange-red. It was an amazing sight. People stopped along the road to watch it. I assume this is Planet X coming closer? And the reddening was due to the tail, and the increase in the Sun is also due to the red color of the dust? [and from another] Nov 5, 2011 This photo was taken just before sunrise near Kokomo, Indiana. Since late summer last year, I have often seen pink clouds like these, and increasingly blood red pre-dawn skies on clear days. November 3, 2011 This overcast day was taken about an hour after sunrise, note that the sun is peeking through the clouds and that the clouds near the horizon are pink. About two and a half hours after sunrise, slightly pink clouds could still be seen near the horizon, as in this photograph, although at that moment I had not yet taken a single picture. The pink color usually fades shortly after dawn. It was overcast this afternoon and I noticed that the clouds turn pink a couple of hours before sunset. If the tail of Planet X began to reach Earth, would the clouds become more pink during the daytime or the sky more red when it is a little hazy and overcast?
Mankind is accustomed to the fact that the rising and setting Sun is larger than the noon, and the Sun at sunrise and sunset, as well as the surrounding clouds, are orange. We have explained that this is due to light being more easily deflected in the red region of the spectrum, so that red light rays bend primarily over the horizon due to Earth's gravity, while light from other parts of the spectrum does not. Light from this part of the spectrum, which radiates from the Sun in all directions, is bent by Earth's gravity so that light that normally travels from either side of an observer on Earth is deflected toward its center. Therefore, it comes to the observer's eye or camera both from the sides and directly in a straight line from the Sun, painting a larger picture.
How will this change as red dust from the tail of Planet X increases in the atmosphere? It is obvious that any light penetrating the atmosphere will increasingly shift to the red region of the light spectrum. Dust looks red because it mainly reflects light rays from the red region of the spectrum, while absorbing light rays from other parts of the spectrum. So what will be the effect, given that the sunlight that reaches the Earth will increasingly belong to the red region of the light spectrum? Of course, red auroras have been observed in N America recently in part due to the gravity dance between Earth and Planet X. Will there be other distortions?
As an attentive observer noted, at sunset the Sun seems larger than usual. If the light of the red spectrum after leaving the Sun is deflected towards the Earth, what will the increased amount of red dust in the Earth's atmosphere do with these rays of light coming from the Sun towards the Earth? One can expect their additional deflection towards the gravitational center of the Earth, with an even larger apparent size of the Sun at sunrise and sunset. The sizes of all planetary objects can be distorted. The moon can appear larger and thus closer, sometimes disturbing observers. The authorities will not have any explanation for this, and they will, as usual, fall silent without offering anything. NASA and experts will be even more embarrassed, and more anxious people will start scouring the internet for answers, as red dust is mentioned in doomsday prophecies and cannot be hidden.
