The Night Sky Zone by Kevin Manning
On a clear, moonless night, the average person can see between 3 and 4 thousand stars across the sky. It's as if we're inside of some great celestial (sky) sphere upon which the stars appear to be attached. Of course, stars are actually at different distances from us in a 3-dimensional understanding. In science we like to use models to illustrate abstract concepts. A model of the celestial sphere has a ball representing the earth centered inside of a larger ball with stars painted in patterns we call constellations. The International Astronomical Union officially designated a total of 88 constellations in the entire celestial sphere in 1930. If we took our model and crushed it into a flat, two dimensional plane, then we would have created a planisphere. A circular cardboard cutout looking like a star map freely rotating on a central pivot and connected to a square cardboard piece can be set to a time and day of the month while facing a certain direction. This device helps in identifying bright stars and constellations in the night sky, and is commonly called a starfinder.
Recognizing stars and constellations is a good start, but locating much smaller, specific areas of the sky requires a detailed set of star charts. Just like there are maps helping us find our way in unknown geographic locations on the surface of the earth, we could create maps that indicate relative positions of objects in the night sky as well. Lines representing streets and roads going north-south and east-west on our earth map can be turned into lines representing celestial coordinates on our sky map. The fictitious lines of latitude and longitude crisscrossing our earth map can be thrown up into the sky directly overhead. These celestial lines of latitude north and south of the equator are now called lines of declination. They measure north of the celestial equator from 0 degrees to +90 degrees for the north celestial pole and south from 0 degrees to -90 degrees for the south celestial pole. Each degree can be further subdivided into 60 equal minutes (not time) of arc (part of the circle formed by the complete line) and each minute divided into 60 equal seconds (again, not time) of arc. The Earth rotates on its polar axis through fifteen minutes of arc in every minute of sidereal time (the motion of the stars due to the rotation of the earth). One minute of arc at the Earth's celestial equator is approximately equal to one nautical mile on the earth. The celestial lines of longitude east and west of the prime meridian, a line representing 0 hours or the beginning of a new day over Greenwich, England, are now called right ascension. They are called that because if an observer is facing north, then stars will appear to ascend (rise) toward their right (east). They measure hour angles increasing westward from 1 to 24 (0 and 24 are the same line) and are subdivided further into minutes and seconds of actual time.
Using this celestial coordinate system along with maps called star charts; we can pinpoint the location in the sky of any celestial object of interest. Equatorial mountings for telescopes can incorporate setting circles to manually align the instrument or use computers and motors to locate and track celestial objects. Sextants and astrolabes are used in celestial navigation to compare a star's known declination with its angle above the north-south horizon.
On a clear, moonless night, the average person can see between 3 and 4 thousand stars across the sky. It's as if we're inside of some great celestial (sky) sphere upon which the stars appear to be attached. Of course, stars are actually at different distances from us in a 3-dimensional understanding. In science we like to use models to illustrate abstract concepts. A model of the celestial sphere has a ball representing the earth centered inside of a larger ball with stars painted in patterns we call constellations. The International Astronomical Union officially designated a total of 88 constellations in the entire celestial sphere in 1930. If we took our model and crushed it into a flat, two dimensional plane, then we would have created a planisphere. A circular cardboard cutout looking like a star map freely rotating on a central pivot and connected to a square cardboard piece can be set to a time and day of the month while facing a certain direction. This device helps in identifying bright stars and constellations in the night sky, and is commonly called a starfinder.
Recognizing stars and constellations is a good start, but locating much smaller, specific areas of the sky requires a detailed set of star charts. Just like there are maps helping us find our way in unknown geographic locations on the surface of the earth, we could create maps that indicate relative positions of objects in the night sky as well. Lines representing streets and roads going north-south and east-west on our earth map can be turned into lines representing celestial coordinates on our sky map. The fictitious lines of latitude and longitude crisscrossing our earth map can be thrown up into the sky directly overhead. These celestial lines of latitude north and south of the equator are now called lines of declination. They measure north of the celestial equator from 0 degrees to +90 degrees for the north celestial pole and south from 0 degrees to -90 degrees for the south celestial pole. Each degree can be further subdivided into 60 equal minutes (not time) of arc (part of the circle formed by the complete line) and each minute divided into 60 equal seconds (again, not time) of arc. The Earth rotates on its polar axis through fifteen minutes of arc in every minute of sidereal time (the motion of the stars due to the rotation of the earth). One minute of arc at the Earth's celestial equator is approximately equal to one nautical mile on the earth. The celestial lines of longitude east and west of the prime meridian, a line representing 0 hours or the beginning of a new day over Greenwich, England, are now called right ascension. They are called that because if an observer is facing north, then stars will appear to ascend (rise) toward their right (east). They measure hour angles increasing westward from 1 to 24 (0 and 24 are the same line) and are subdivided further into minutes and seconds of actual time.
Using this celestial coordinate system along with maps called star charts; we can pinpoint the location in the sky of any celestial object of interest. Equatorial mountings for telescopes can incorporate setting circles to manually align the instrument or use computers and motors to locate and track celestial objects. Sextants and astrolabes are used in celestial navigation to compare a star's known declination with its angle above the north-south horizon.