Polaris is located at RA 2 h 41 m 39 s, Dec. In keeping with right ascension's time theme, hours are subdivided into minutes and seconds, and are even written out as minutes (m) and seconds (s). By convention, 0 h is used instead of 24 h. Declinations are labeled every 10° and the hours of right ascension are shown around the circle. This view shows the north celestial pole (NCP) and polar regions. That's a little more than one-half the width of the W-shaped constellation Cassiopeia. Because there are 24 hours in a day, each hour of right ascension measured along the equator equals 1/24th of a circle (360° divided by 24) or 15°. The sky can be treated as a clock, since it wheels by as Earth rotates, so the zero point of right ascension is called "0 h" for "zero hours." Unlike longitude, right ascension is measured in just one direction - east. Presently, it's located in the constellation of Pisces, the Fish. While we use a physical location on Earth as our reference for longitude, what reference do we use for right ascension? Where is the 0° mark in the sky separating east from west? Astronomers use the spot the Sun arrives at on the first day of spring, called the vernal equinox. The coordinates of the bright stars Betelgeuse and Sirius are also shown. Here's a slice of sky bisected by the celestial equator with hours of right ascension (RA) labeled along the top and declination (Dec.) at left. Since we're inside a sphere, it would continue around the backside of the Earth as well. Viewed from Earth's equator, the celestial equator begins at the eastern horizon, passes directly overhead and drops down to the western horizon. The equator, which marks the 0° latitude line, now circles the sky as the celestial equator, while the north and south celestial poles hover over either end of the planet's polar axes. If you could pump the ball up into a gigantic sphere centered on the Earth, you'd look up and see lines of latitude and longitude imprinted on the sky. Let's use our imaginations and picture the latitude–longitude grid on the planet as the surface of a flexible, transparent soccer ball.
There are no roads in the sky, so knowing an object's coordinates is crucial to finding it in your telescope. Declination corresponds to latitude and right ascension to longitude. Like cities, every object in the sky has two numbers that fix its location called right ascension and declination, more generally referred to as the object's celestial coordinates. Detroit, Michigan, is shown here.ĬC 3.0 / Wikipedia Right Ascension & Declination Every location, whether it be a city, airport, or even your own home or apartment building lies somewhere on the worldwide coordinate grid (below), its location fixed by two numbers.Įvery city on the planet has a unique latitude and longitude, its "spot" on the grid. A negative sign in front of the latitude indicates south and a positive sign north. Or Wonglepong, Australia, situated along that continent's east coast at –27.0° south, 153.2° east. Take Tuscaloosa, Alabama, for example, which is located at latitude +33.2° north, longitude 87.6° west. It defines the the zero (0°) longitude line just as the equator defines the 0° latitude line.Įach city has a unique latitude and longitude. The prime meridian is an imaginary line that runs through the Royal Observatory in Greenwich, United Kingdom, and extends to the North and South Poles. To find anyplace on the globe of Earth, you only need to know its latitude, the distance in degrees north or south of the equator, and longitude, the distance in degrees east or west of the prime meridian. Then I realized they were just numbers like latitude and longitude, except applied to the stars. When I got started in astronomy at age 11, they were a little scary. Sooner or later every novice skywatcher runs into the terms RA and Dec., short for right ascension and declination. The sky is draped in an invisible coordinate grid that will help you easily find any object you wish, the same as you would locate a city on a map.
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