Polaris

Polaris or North Star, star nearest the north celestial pole (see equatorial coordinate system). It is in the constellation Ursa Minor (see Ursa Major and Ursa Minor; Bayer designation Alpha Ursae Minoris) and marks the end of the handle of the Little Dipper. Polaris's location less than 1° from the pole (1992 position R.A. 2h23.3m, Dec. +89°14') makes it a very important navigational star even though it is only of second magnitude; it always marks due north from an observer. Polaris can be located by following the line upward from the two stars (the Pointers) at the right end of the bowl of the Big Dipper or, if the Big Dipper is not visible, by following the line through the left side of the square in Pegasus through the end star in Cassiopeia. The star is a Cepheid variable and oscillates in brightness roughly every four days. Because of the precession of the equinoxes, Polaris will not remain the polestar indefinitely; in 2300 B.C. the polestar was in the constellation Draco, and by A.D. 12,000 the star Vega in the constellation Lyra will be the polestar.

azimuth

Azimuth, in astronomy, one coordinate in the altazimuth coordinate system. It is the angular distance of a body measured westward along the celestial horizon from the observer's south point.

prime meridian

Prime meridian, meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the Greenwich meridian. Universal time, the standard basis for determining time throughout the world, is civil time measured at the prime meridian.

altitude

Altitude, vertical distance of an object above some datum plane, such as mean sea level or a reference point on the earth's surface. It is usually measured by the reduction in atmospheric pressure with height, as shown on a barometer or altimeter. In surveying and astronomy, it is the vertical angle of an observed point, such as a star or planet, above the horizon plane. The altitude of a feature of the earth's surface is usually called its elevation. Recent spacecraft instrumentation has also measured vertical distances on the earth and other planets, determining the height of planetary features by means of radar and optical imaging.

In astronomy, altitude is the angular distance of a heavenly body above the astronomical horizon as determined by the angle which a line drawn from the eye of the observer to the heavenly body makes with the plane of the horizon. The reading of the apparent altitude, as determined by a telescope attached to a graduated circle, must be corrected for refraction by the atmosphere and for certain other effects to ascertain the true altitude. The altitude of the north celestial pole, which is approximately that of the star Polaris, is equal to the observer's latitude.

Solar System

Solar System, the Sun and everything that orbits the Sun, including the nine planets and their satellites; the asteroids and comets; and interplanetary dust and gas. The term may also refer to a group of celestial bodies orbiting another star (see Extrasolar Planets). In this article, solar system refers to the system that includes Earth and the Sun.

The dimensions of the solar system are specified in terms of the mean distance from Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto, orbits about 39 AU from the Sun. Estimates for the boundary where the Sun’s magnetic field ends and interstellar space begins—called the heliopause—range from 86 to 100 AU.

The most distant known planetoid orbiting the Sun is Sedna, whose discovery was reported in March 2004. A planetoid is an object that is too small to be a planet. At the farthest point in its orbit, Sedna is about 900 AU from the Sun. Comets known as long-period comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more.

The solar system was the only planetary system known to exist around a star similar to the Sun until 1995, when astronomers discovered a planet about 0.6 times the mass of Jupiter orbiting the star 51 Pegasi. Jupiter is the most massive planet in our solar system. Soon after, astronomers found a planet about 8.1 times the mass of Jupiter orbiting the star 70 Virginis, and a planet about 3.5 times the mass of Jupiter orbiting the star 47 Ursa Majoris. Since then, astronomers have found planets and disks of dust in the process of forming planets around many other stars. Most astronomers think it likely that solar systems of some sort are numerous throughout the universe. See Astronomy; Galaxy; Star.

articles:

• The Sun and the Solar Wind
• The Major Planets
• Other Orbiting Bodies
• Movements of the Planets and Their Sattelites