Polar Star

The projection of the north pole precession route in the fixed sky of the J2000.0 period for the time interval of 48000 a. c. to 52000 d. c.

The pole star is called the star visible to the naked eye that is located in the celestial vault closest to the axis of rotation of the Earth or celestial pole; although by convention, the term polar star refers to the star closest to the north pole. Due to the precession of the equinoxes, the celestial poles move relative to the stars around the ecliptic pole and, consequently, the pole star in each hemisphere is not the same over the years.

Currently

Currently, the polar star in the northern hemisphere is α Ursae Minoris, which, located at the end of the tail of Ursa Minor, is also known as Polaris or Cinosura, because it is closest to the pole, from which it is less distant of a degree. It will still get closer, and in 2100 it will not be more than 28' away from it. From then on, the pole will move away from it, and it will not be the pole star again until some 25,780 years later.^{[citation needed]}

In the past

4800 years ago, the closest star to the north celestial pole, that is, the polar star of that distant epoch, was Thuban (α Draconis), of magnitude 3.6. It was just 10' from the celestial pole (the current pole star is 50' away). Thuban was famous in China and Egypt, as ancient Chinese astronomers inscribed it in their annals from the time of Emperor Huang Di, who reigned in 2700 BC. C.^{[citation required]}

The celestial pole then shifted between α Ursae Minoris and α Draconis. At that time, the Chiron sphere was built, the oldest known, corresponding to the time of the expedition of the Argonauts, 1200 BC. C. From then on, the pole moved towards what is currently the polar star.^{[citation needed]}

At the beginning of our era, there was no bright star to indicate the location of the pole. In the play Julius Caesar, by William Shakespeare, the main character says:

"But I am constant as the polar star that has no paragon in terms of stability in the firmament."

It is clear that Shakespeare's verses are an anachronism, since in his time, Polaris was the polar star, but not in the time of Julius Caesar, when the north pole did not point to any star, since it was more than of 12th of her.

About the year 800, it passed close to a small double star in the constellation of Camelopardalis, the giraffe.^{[citation needed]}

In the future

The current pole star, at magnitude 2, is one of the brightest found in the path that the pole travels, and for this reason it has carried the title for more than a thousand years. It will be able to keep it until about the year 3500, when the trajectory of the pole will pass close to a third magnitude star called Errai or Alrai (γ Cephei). In the year 6000, it will be between two stars of the third magnitude, Alfirk (β Cephei) and ι Cephei; around the year 7400, it will be close to the bright first-magnitude star, Sadr (γ Cygni), and around the year 13,600 the polar star will be the brightest in the northern summer sky, Vega (α Lyrae), which will retain this primacy for three thousand years, at least. This will be the pole star of future generations, as it was fourteen thousand years ago, in the ice age.^{[citation needed]}

Astronomical navigation

Since Polaris is practically at the north celestial pole, it appears in the center of the circular paths that other stars seem to describe due to the effect of Earth's rotation.^{[citation needed]}

This characteristic makes the observation of the local coordinates of the pole star independent of time, with the height above the horizon expressed in degrees and minutes being the latitude of the observer. To achieve the accuracy necessary for navigation, this observation must be corrected as the pole star is not exactly at the north celestial pole, currently it differs by about 40' of a degree (2022).^{[ citation required]}

The privileged position in the celestial vault of the pole star makes it an ally of the navigator, who with a simple observation can verify course and determine latitude.^{[citation required]}

At the South Pole

σ Octantis is to the naked eye the closest star to the south celestial pole, although it is not very helpful due to its low apparent luminosity. The constellation of the Southern Cross points approximately to that point. At the equator it is possible to see Polaris and the Southern Cross.^{[citation needed]}

Other planets

Each planet has a different polar star according to its axis of rotation.

• Alfa Pictoris in the south and Ómicron Draconis in the north of Mercury.
• Fi Draconis, the closest in the south of Venus.
• Delta Doradus, south of the moon.
• Kappa Velorum at 2° south and Sadr and Deneb—the two brightest stars of Cygnus—in the north of Mars.
• Delta Octantis in the south of Saturn.
• Eta Ophiuchi in the north and 15 Orionis in the south of Uranus.
• The north of Neptune is between Gamma and Delta Cygni. Gamma Velorum is in the south.
• The north of Pluto points to the constellation Delphinus.

The North Star and The Beatles

On February 5, 2008, at 00:00 UTC, NASA broadcast The Beatles' song "Across The Universe" in the direction of the polar star. The transmission was made using a 70m antenna at DSN's Madrid Deep Space Communication Complex, located on the outskirts of Madrid, Spain. And it was done with an "X band" transmitter, giving the antenna 18 kW.^{[citation needed]}

The goal was to celebrate the 40th anniversary of the song, the 45th anniversary of the Deep Space Network (DSN), and the 50th anniversary of NASA. The idea was conceived by Beatles historian Martin Lewis, who invited all fans of the band to play the song as if it were going to be sent to a distant star. This is the second time a piece of music has been intentionally transmitted into outer space (the first being the interstellar message: "1st Theremin Concert to Aliens"), endorsed by Paul McCartney, Yoko Ono, and Apple Records. ^{[citation required]}