Pole star

A pole star is a special kind of star that appears very close to the axis of rotation of a planet or other celestial body. This means that, from that planet, the pole star seems to stay almost still in the sky while the other stars move around it. For Earth, having a pole star is very useful because it helps people find direction.

Right now, Earth has two pole stars. In the north, the bright star Polaris, also called Alpha Ursae Minoris, is our main pole star. It is easy to see and has been very important for navigation over many years. In the south, there is another star called Polaris Australis, or Sigma Octantis, but it is much dimmer and harder to see.

Long ago, between about 1700 BC and 300 AD, Earth’s northern pole star was not Polaris but two stars called Kochab and Pherkad. These stars were twin "pole stars," but they were not as close to the exact north point in the sky as Polaris is today.

History

In classical antiquity, Beta Ursae Minoris (Kochab) was closer to the celestial north pole than Alpha Ursae Minoris (Polaris). The Phoenicians used the whole constellation of Ursa Minor, known as Cynosura, to find the northern direction. Polaris was later called aeiphanes, meaning "always above the horizon," and was known as scip-steorra, or "ship-star," in Anglo-Saxon England because it helped with navigation. The name stella polaris was created during the Renaissance, even though Polaris was not exactly at the north pole.

Precession of the equinoxes

The North Star, a special star that appears straight above Earth's North Pole, changes over time due to a movement called the precession of the equinoxes. This slow wobble in Earth's rotation takes about 26,000 years to complete. Because of this, different stars become the North Star at different times.

Today, Polaris is our North Star, located very close to the north celestial pole. In the past, other stars like Thuban in the constellation Draco served as the North Star. In the future, stars in constellations like Cepheus, Cygnus, Lyra, and Hercules will take turns being the North Star. Sometimes, no single star will be close enough to serve as a clear North Star, and other times, stars will only give an approximate guide to north.

Southern pole star (South Star)

Currently, Earth does not have a bright star like Polaris to guide us from the South Pole. The closest star is Sigma Octantis, but it is quite dim and hard to see. This star is a yellow giant located 294 light years from Earth.

The Southern Cross constellation helps point toward where a southern pole star would be. Over thousands of years, different stars will move closer to the south celestial pole. In the far future, Sirius might become a southern guide star.

Other planets

Pole stars of other planets are stars that appear closest to the point where the planet's axis of rotation meets the sky. These stars must be bright enough to see without a telescope, usually brighter than magnitude 6. Since each planet spins at a different angle, they each have their own unique pole stars. For more information, see Poles of astronomical bodies.

In religion and mythology

Polaris, the North Pole Star, has been important in many cultures and religions. In medieval times, it was called stella maris, meaning "star of the sea," because it helped guide sailors. It was also linked to Marian veneration, with Our Lady, Star of the Sea being a title for the Virgin Mary.

In Mandaean cosmology, the Pole Star is seen as a sign of the World of Light, and people face north when they pray. In Hinduism, the Pole Star is known as Dhruva, named after a devoted follower of the god Vishnu who became the Pole Star.