Role As Pole Star
Further information: Pole starBecause in the current era α UMi lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole—the north celestial pole—Polaris stands almost motionless in the sky, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation and for astrometry. The moving of Polaris towards, and in the future away from, the celestial pole, is due to the precession of the equinoxes. The celestial pole will move away from α UMi after the 21st century, passing close by Gamma Cephei by about the 41st century. Historically, the celestial pole was close to Thuban around 2500 BC., and during Classical Antiquity, it was closer to Kochab (β UMi) than to α UMi. It was about the same angular distance from either β UMi than to α UMi by the end of Late Antiquity. The Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. However, as one of the brighter stars close to the celestial pole, Polaris was used for navigation at least from Late Antiquity, and described as αει φανης "always visible" by Stobaeus (5th century). α UMi could reasonably be described as stella polaris from about the High Middle Ages.
In more recent history it was referenced in Nathaniel Bowditch's 1802 book, The American Practical Navigator, where it is listed as one of the navigational stars. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is slightly displaced to East or West, and to bearing must be corrected using tables or a rough rule of thumb. The best approximate was made using the leading edge of the "Big Dipper" asterism in the constellation Ursa Major as a point of reference. The leading edge (defined by the stars Dubhe and Merak) was referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.
Read more about this topic: Polaris
Famous quotes containing the words role as, role, pole and/or star:
“Recent studies that have investigated maternal satisfaction have found this to be a better prediction of mother-child interaction than work status alone. More important for the overall quality of interaction with their children than simply whether the mother works or not, these studies suggest, is how satisfied the mother is with her role as worker or homemaker. Satisfied women are consistently more warm, involved, playful, stimulating and effective with their children than unsatisfied women.”
—Alison Clarke-Stewart (20th century)
“Always and everywhere children take an active role in the construction and acquisition of learning and understanding. To learn is a satisfying experience, but also, as the psychologist Nelson Goodman tells us, to understand is to experience desire, drama, and conquest.”
—Carolyn Edwards (20th century)
“Not because Socrates has said it, but because it is really in my nature, and perhaps a little more than it should be, I look upon all humans as my fellow-citizens, and would embrace a Pole as I would a Frenchman, subordinating this national tie to the common and universal one.”
—Michel de Montaigne (1533–1592)
“A brush had left a crooked stroke
Of what was either cloud or smoke
From north to south across the blue;
A piercing little star was through.”
—Robert Frost (1874–1963)