Astronomy with the Deaf World #7 Midnight sun, polar days and nights, Noctilucent cloud

What are noctilucent cloud? What is a midnight sun? And when do we say it's polar day or night? Jakub Malik explains in Episode 7 of Astronomy with the Deaf World.

Hi. Welcome to another episode of Astronomy with the Deaf World. Today I will tell you about several phenomena, that are interconnected, and some of them we can observe in Poland in June and July. And these phenomena are: “polar day” and “polar night”; “Midnight sun” (white nights) and “Noctilucent cloud” (silver clouds).

[Automatic translation. You have found an error – please let us know].

A normal night

Let’s start with what we know well and we observe every day in Poland. Imagine that you are standing by the Polish seaside in the summer, and you are looking in front of you, at the sea, that is, you are facing north. As you look a little to the left, that’s where in the evening you will see the sunset. And a few hours later, as you you look to your right, that’s where the sunrise will be. Simple and obvious, right? That is, for several hours of the night we can’t see the sun. The sun is hiding behind the horizon. In the animation it looks like this: [see in video 1:53]. We understand this animation, right?

Polar day and night

But closer to the poles, the situation is already different. How? Take a look at this picture [2:19]. The sun is not hiding behind the horizon. It is visible all night long! That is, the sun is getting lower, but does not disappear behind the horizon. So it is shining all the time. Admittedly, it is not a strong light, but it doesn’t go dark. On the animation it looks like this. And such a situation, when the Sun is seen in the sky for at least 24 hours, is called a “polar day.” And just as we have a “polar day”, That is, we see the Sun in the sky for 24 hours or more, the same way we have a “polar night” — is a phenomenon when there is no Sun for at least 24 hours. Both the polar day and night can last as long as half a year – depending on the location from which they are observed. When the North Pole The semiannual polar night is underway, then at the other pole, the south pole, there is also a semiannual polar day going on. And then the situation reverses.

The brighter part of the Earth is the one illuminated by the Sun. The darker part is where it’s nighttime. The Earth is rotating very fast here, because in this animation we see in acceleration the Earth’s semi-annual rotation around its axis. But look at the poles. One of them is in shadow all the time, that is, there is a night there, and the other is illuminated, so it’s daytime there. This is because the angle Earth’s tilt is changing. The yellow line helps to see what the looks like the changing angle of tilt. The earth “tilts” for six months and then “returns”.

Longest and shortest day in Poland

In Poland, neither day nor night polar nights do not occur, of course. The longest night in Poland is on December 21 or 22 and lasts more than 16 hours on average. We call it the “winter solstice”. In contrast, the longest day falls around June 20-21 and also lasts more than 16 hours. It is called the “summer solstice”.

White astronomical nights

Poland, on the other hand, has a different phenomenon. Let’s look at the third animation [5:48]. The sun is setting and fully disappears behind the horizon. However, it hides shallow enough, That in the north we can see its slight glow throughout the night — meaning twilight. In turn, before the sun rises, we see its dawn. Twilight and dawn are similar phenomena. Here in Poland, during this period, perfect darkness does not occur. In Poland, we can observe this in June and July, when the nights are the shortest. Such a phenomenon is called a “white night” –. That is, we do not see the sun, but we see its glow. This lingering glow, here in Poland, is small, even hardly noticeable. But for astronomers it has It is already of great significance, because it makes it difficult for them to observe stars which are located in the north. It’s simply too bright there. This is why astronomers this time are called “white astronomical nights”. And such white nights – astronomical – occur also in Poland precisely in June and July.

Silver clouds

We are still left with an explanation, what are silver clouds? Let’s take a look at the illustrations [7:53]. Silver clouds are those silver-blue ones, very high clouds. Normal clouds are at heights of up to 13-15 kilometers. And these form in the polar regions, At an altitude of 80-100 km, and thus even at the Kármán line. What is this line? You will find out in the episode On the boundaries of space. Silver clouds are thus the the highest clouds on Earth.

Silver clouds are formed in late spring and in summer. They have a silvery-blue glow even when the sky is dark. They were first observed in 1885. This was two years after the Krakatau volcano eruption. Why is this information about the volcano is important? Silver clouds are made up made of tiny ice crystals, formed around microscopic dust particles that drift through the air. This dust comes precisely from volcanic eruptions, But also from meteoroids, (i.e., rock crumbs), Which fall into the Earth’s atmosphere and burn up in it.

Why do we only see them in the summer? In the summer months at this altitude there is more water vapor than during the rest of the year. Water molecules adhere to the dust and freeze. The temperature at this altitude can be as low as minus 130 degrees Celsius! Clouds can suddenly appear and suddenly disappear. We can’t forecast them.

In Poland, we can also observe silver clouds, even with the naked eye. The further north we are, the greater chance we have of spotting them. They appear in the period from about mid-May to about mid-August. Appear about an hour after sunset Sun on the northwestern horizon Or 2.5 hours before sunrise on the northeastern horizon.

Why do we see silver clouds?

But actually, why do we see these clouds even then, when it’s already night and we shouldn’t be able to see anything anymore? Let’s take a look at the illustration [11:25].

The human who is observing the silver clouds, is already in the Earth’s shadow, which means it’s nighttime. After the parva, we see sunlight falling on the illuminated part of the Earth. And the silver clouds? The sunlight reflects off them and is visible by our human, even though it is already standing at night. They are so high that the sun still illuminates them.

Thank you for watching this episode. I wish you a successful sky observation. You may also see silver clouds. If so, take a picture and show it off. See you in the next Astronomy with the Deaf World.

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