angular measure The angle between the line from the observer to object 1 and the line from the observer to object 2. For example: Shortly after sunrise, you see that the sun is only a few degrees above the horizon, while at noon it is elevated to nearly 90 degrees (or directly overhead). You are the observer, the sun is Object 1, and the horizon is Object 2. Angular units are degrees, arcminutes, and arcseconds. See also linear measure.
arcminute A unit of angular measure. 1 degree = 60 arcminutes. 1 arcminute = the angular diameter of a penny at 69 meters (75 yards). The angular diameter of the moon as seen from earth is 30 arcminutes.
arcsecond A unit of angular measure. 1 degree = 60 arcminutes. 1 arcsecond = the angular diameter of a penny at 4.12 km (2-1/2 miles).
astronomical unit, AU The average distance between the Sun and the Earth. 1 AU is about 93 million miles, or 150 million km, or 8.5 light minutes.
celestial coordinates Coordinate system used by astronomers to specify the angular position of an object on the sky, as seen from the Earth. Right ascension (RA), analogous to terrestrial longitude, measures east-west locations. Declination (dec), analogous to terrestrial latitude, measures north-south locations, where positive declination means the object is north of the celestial equator and negative declination means the object is south of the celestial equator. The celestial equator is the projection of the Earth's equator into the sky. The celestial north pole is the projection of the Earth's north pole into the sky. In this aeon, the celestial north pole is near the star Polaris, at the end of the handle of the Little Dipper.
degree of arc A unit of angular measure. One degree is 1/360'th of the way around a circle. The angular diameter of a penny held at arm's length is about 1-2 degrees (depending on how long your arms are!) The angular diameter of the moon as seen from earth is 1/2 degree.
degree of temperature A unit of measure related to heat. Scientists measure degrees in Kelvin (K), which can be used directly in thermodynamic computations. Other common units are degrees Celcius (C) or degrees Farenheit (F). The following conversions hold:
galactic coordinates Coordinate system related to the plane of the galaxy. Usually abreviated as U, V, and W. +U is toward the galactic center, +V is in the direction of galactic rotation, and +W is towards the North Galactic Pole.
light minute The distance that light can travel in one minute, about 18 million km, or 11 million miles, or about 1/8 of an AU.
light year, LY The distance that light can travel in a year's time. This is a definable quantity because the speed of light in a vacuum is constant, about 300,000 km per second. 1 LY is about 0.3 parsecs, or 63240 AU, or 9.5 trillion km, or 5.9 trillion miles.
linear measure Distances on Earth are in units of kilometers or miles. Distances between planets or in the vicinity of a star are in Astronomical Units (AU) or light minutes. Distances between stars or between galaxies are in units of light years or parsecs. See also angular measure.
parallax The apparent motion of a star caused by the motion of the Earth as it orbits the Sun. Usually given in units of arcseconds. Once a star's parallax has been measured, it's distance is known: distance = 1/parallax, where parallax is given in arcseconds and distance is given in parsecs.
parsec, PC The distance from the sun to a star which has an annual parallax of 1 arcsecond. Proxima Centauri, the closest star to the Sun, is at a distance of 1.3 pc. 1 PC is 206,265 AU, or 3.26 light years, or about 31 trillion km, or about 19 trillion miles.
proper motion The component of a star's space motion across the sky. Usually given in units of angular speed (arcseconds per year) in a given direction relative to celestial North (see celestial coordinates. If star's parallax has been measured, then its distance is known and the angular speed can be converted to a linear speed (km per second).
radial velocity The component of a star's space motion along the observer's line of sight. Usually given in units of km/second. By convention, a positive value indicates the motion is away from the Sun and a negative value indicates the motion is towards the Sun.
space motion The actual motion of a star through space. Space motion is determined by measuring both the proper motion and the radial velocity and converting to the galactic coordinate frame.