A7 Astronomical Coordinate Systems

The geographical coordinates of latitude (north-south) and longitude (east—west) are used to locate objects on the surface of our planet. Astronomers use similar angular coordinates to locate objects on the sky.

Imagine drawing a line across the heavens that is always directly above the earth's equator This is known as the celestial equator. The angle measured north and south from this celestial equator is called declination (directly equivalent to latitude on earth). The North Celestial Pole, located directly over the North Pole of the earth (just about where the Pole Star is found), is at +90 decimation.

The astronomical equivalent of terrestrial longitude, the coordinate measured east and west of Greenwich, England, is called right ascension, and can also be given as an angle around a circle, but is commonly measured as a time—24 h span the equator, equivalent to a full circle of 360 degrees. Right ascension, in hours, minutes, and seconds of time, is measured east of an agreed upon zero-point known as the First Point in Aries. Although there are technical complications associated with the precise definition of these coordinate systems due to the precessional wobble of the earth over a 26,000-year cycles, suffice it to say that right ascension and declination are the basic astronomical coordinates.

Another system of coordinates is based on the Galaxy itself. A line defining the central plane of the galaxy, a line that runs along the center of the Milky Way band of stars, is defined as the galactic equator. Galactic latitude (b) is measured in degrees and minutes of arc north or south of this equator, and galactic longitude {1) is measured in degrees and minutes along the galactic equator, using the direction of the center of the galaxy, in the constellation of Sagittarius (see Chapter 5), as the zero point.

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