Sky Coordinates

Mike Swanson

Besides the constellations, we also refer to other imaginary boundaries in the sky.  The horizon is the line where the land meets the sky.  The zenith is the point directly overhead.  The meridian is the line running from the northern horizon, up through the north celestial pole, overhead through the zenith, then down to the southern horizon.  Thus it splits the sky into eastern and western halves.  The celestial equator is a line that runs from east to west, directly above the Earth's equator.  And finally, the ecliptic is a wavy line traveling north, then south of the celestial equator.  The ecliptic is significant as the Sun, the Moon, and all the planets travel through our sky near to this line.

Similar to longitude and latitude used to pinpoint locations on the Earth, we use right ascension (RA) and declination (Dec) to pinpoint locations in the sky.  Lines of right ascension run from the north celestial pole to the south celestial pole, similar to longitude on the Earth.  Thus they meet or converge at the celestial poles.  Lines of declination run east to west, parallel to one another, just like latitude.  

We measure right ascension in hours, minutes and seconds.  RA starts at 0h00m00s then goes clockwise around the north celestial pole until we come to 23h59m59s just before where we started.  Thus there are 24 hours of right ascension.  Declination is measured in degrees ( ), arcminutes ( ' ), and arcseconds ( " ).  The declination of the celestial equator (right above the Earth's equator) is 000'00" (0 degrees), the declination of the north celestial pole is 90 and the declination of the south celestial pole is -90.  From this system we can give the coordinates for any object in the sky.  For example, the coordinates for Rigel, a bright star in the constellation Orion, are RA 05h14m30s, Dec -0812'06".

The line of right ascension directly above us at the meridian is known as local sidereal time (LST).  Every hour, local sidereal time changes about one hour.  In other words, if local sidereal time is currently 18h RA, in one hour LST will be 19h RA.  Naturally this corresponds to the fact that the Earth rotates once every 24 hours.  Sidereal rate is the rate that objects move across the sky - approximately one hour of right ascension for every hour of time here on Earth. Since the 360 degrees of the circle divided by 24 hours yields 15, this rate of motion corresponds to 15 degrees at the celestial equator.

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