polar Star

First of all, by its position on the celestial sphere, which is distant from the North Pole of the world – the point on the celestial sphere, where the imaginary axis of the Earth’s rotation intersects with the imaginary sphere – by a very small amount. The position of the North Star in the first approximation coincides with this point.

Somehow there is a lot of abstraction that requires special imagination – isn’t it?

In fact, everything is simple. If the line connecting the poles of the globe – north and south – is extended in a north direction, this line will point with a high degree of accuracy to the North Star.

Looking at this star from any point in the northern hemisphere of the Earth, we will always see where the Earth’s axis of rotation is directed.

But, in the southern hemisphere of the planet, the Pole Star, alas, is not visible. Neither winter nor summer. Never.

Therefore, its use for maritime navigation is limited.

But it was the navigation application that made this star so popular. After all, navigation began precisely in the northern hemisphere.

It was very easy for ship captains and navigators to find this star in the sky, lower the plumb line down from it, and thus find the north point, and therefore all directions in the open sea (east, south, west), where there are no other landmarks anymore. Only the stars.

But the stars change their position with respect to the horizon over time – they rise, set – during the night they sometimes move across the entire sky. You cannot navigate a ship over a star that itself is still moving somewhere, even if this movement is illusory and caused by the rotation of our planet.

And only the North Star always shines over the point of the north.

For an observer at the North Pole, the North Star will be exactly above his head – at its zenith. The farther south the observer is, the closer to the horizon the North Star will be for him. At the equator, the North Star will be visible exactly on the line of the mathematical horizon. The influence of the atmosphere and the roundness of the Earth will allow it to be seen just above the open horizon in the desert, steppe or in the sea. But if we are happy with an accuracy of ± 1 °, then the height of the North Star above the horizon, expressed in degrees, will tell us the latitude of our location. Agree, this is very convenient for maritime navigation. I went out on deck at night, measured the height of the North Star with a sea sextant, and that’s it – you already know the latitude of your ship’s location.

Special tables and refraction corrections will make it possible to determine the latitude of the observation site from the Polar Star many times more accurately – no longer with an accuracy of a degree, but with an accuracy of one or two arc minutes, which corresponds to a couple of kilometers. And this is quite enough to find the desired island, even a very small one.

Alas, the North Star will not help us in determining the longitude. There are other methods for this. But this article is not about nautical navigation.

How did it happen that this star was so close to the North Pole of the world on the celestial sphere? How long has she been there?

This is more a game and a will of chance. For example, there is no star of comparable brightness near the south pole of the world. In the oceans and seas of the southern hemisphere before the beginning of the GPS era, navigation was somewhat more difficult.

And even in the northern hemisphere, but a couple of thousand years ago, no one drove ships on the North Star. She did not even have such a name then.

Polar alpha Ursa Minor is called relatively recently. Only in the Renaissance it began to be called so – with the light hand of the Dutch cartographer Gemma Frisus, who in 1547 described it in one of his works as “stella illa quae polaris dicitur”. And it stuck to her. Although in those years it was 4 times farther from the pole than it is now.

And earlier, the Alpha Ursa Minor was called differently by different peoples. The Greeks called her “Kinosura” – “dog’s tail”, and saw around her not a bear at all, but a dog – the very dog ​​that followed her mistress – the nymph Callisto, turned by a jealous Hero into a Bear, but saved by Zeus and ascended to heaven.

The ancient Celts called her “Ship-star” – “Scip-steorra” – they already knew something about the future use of this star.

And the ancient Arabs, who gave names to almost all the visible stars in the sky, called the Polar “Al-Judei”, which translates simply as “Father”. A large number of Arabic star names are used in astronomy to this day. But the ancient name of the North Star turned out to be supplanted and practically forgotten.

Be that as it may, but in ancient times, from where all the astrononyms of the North Star have come down to us, this luminary was not used for navigation, orientation in the terrain and related matters. It was far from the pole.

At the time of Archimedes and Pythagoras, the role of the “polar star” was played by another star – beta Ursa Minor with the Arabic name “Kokhab” (“Kokhab-el-shemali” is the full name of this star), which translates as “Star of the North”. And even earlier – in the era of the construction of the pyramids – relevant in this sense – the closest to the North Pole of the World – was the Alpha Dragon – a star named “Tuban”.

Of course, there was no Dragon on the star maps at that time.

As we can now understand, the North Pole of the world is slowly drifting across the star map, moving from star to star, and making a full circle in 26 thousand years. Now it is approaching the Pole Star, and the closest to it in 2102 – in 80 years. The minimum distance of the Polar from the North Pole of the world will be 27 arc minutes – slightly less than the lunar disk. Now it’s almost a degree.

Over time, the Pole Star will cease to be “polar” and will give way to the scale of Cepheus – in two thousand years. And 10,000 years later, the dazzling beauty Vega – the alpha of the constellation Lyra – will become the “polar” star for the inhabitants of the northern hemisphere of the Earth.

What else interesting can you say about the North Star?

This star, as already mentioned here, heads a small constellation – Ursa Minor – being the alpha and the brightest star in it. For the sake of fairness, it must be said that the beta of the Ursa Minor – the already familiar Kohab – is only a few hundredths of a stellar magnitude weaker than the polar one, and there will be no difference for the eye.

The brightness of the Polar Star is comparable to the brightness of the stars of the Big Dipper’s bucket, with the help of which they usually find the Polar Star. If we extend the segment connecting the extreme stars of the bucket five times, we will see a not very bright star – only 2nd magnitude – and this will just be Polar.

For most people, it does not stand out in the sky except for its position. Astronomers have certainly found a number of features and unique properties in this star.

Polaris is a multiple star. In an amateur telescope, it is possible (although difficult) to see a faint (8th magnitude) satellite star – Polaris B. Much closer to the main star is another component of this system – Polaris Ab – it is accessible only to the Hubble telescope, or comparable to it vigilance. At some distance from this “trinity” two more stars are slowly floating side by side in the same direction. Astronomers have not yet decided on their status – whether these stars are connected with Polar in a single stable system, or are they members of an unstable open mini-cluster, which will eventually scatter.

Astronomers have identified a number of stars of constant luminosity around the North Star – the North Polar Series are stars of the photometric standard. Their constant height above the horizon allows them to serve as a reference for determining the brightness of other stars, since the degree of absorption of their light by the atmosphere is the same if the sky is clear and transparent, because the height above the horizon does not change.

There was a time when Polar was one of them and served as a model for a stable star.

It is now known that Polaris pulsates changing its brightness according to the same law as most Cepheids – variable stars of the Cephei delta type. But Polyarnaya showed an oddity that makes her a unique Cepheid. Firstly, the amplitude of the brightness change at Polyarnaya is very small – hundredths of a stellar magnitude – therefore it was considered stable when there were no accurate photometers. Plus, and so insignificant, the amplitude of the change in the brightness of the Polar Star decreased by 4 times during its study, and the overall brightness of the star increased – over the past century, the Polar Star became brighter in about 0.2m, which is a lot for astronomy.

It is still unknown what this evolution of the Pole Star, taking place before our eyes, will lead to.

The distance to the North Star is great. According to various estimates, it ranges from 300 to 500 light years (according to the latest data, 447 light years, and it is shrinking). The large scatter is due to the fact that it is not possible to accurately measure the distance by the parallax method for such distant stars. But in any case, Polyarnaya is several times farther than the stars of the Big Dipper’s bucket, with which it visually has the same brightness. This means that its real luminosity is significantly higher than the luminosity of the bucket stars.

The North Star belongs to the class of supergiants. It has a white-yellow hue – somewhat close to the sun, but in terms of luminosity it exceeds the Sun by one and a half thousand times, and by mass – six times.

Such stars do not live very long, and in a few million years Polaris will explode into a supernova. But until that time, she has a chance several more times to again become a guiding star for the inhabitants of planet Earth.

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At the end of the article, a musical bonus is for those who have mastered the entire text. A play dedicated to the nymph Callisto, mentioned in the article, whose dog carries in its tail the North Star, which is so important to us.