Celestial Globe

At one time or another we have all seen a Globe of the Earth with lines of latitude [measuring an angle from the Equator to the poles] and longitude [measuring an angle east or west of the Prime Meridian running through Greenwich England]. Charlestown, as an example, is 71.4 degrees west of the Prime Meridian and 41.3 degrees north of the Equator.

Astronomers use several equivalent globes to find their way about the heavens. The horizon system globe uses the horizon as an equator and the zenith as the pole. The ecliptic system globe uses the plane of the Earth about the Sun as the equator with the Sun's north pole as the system's pole. The galactic coordinate system uses the disk of our galaxy as the equator and the our galaxy's north pole as the pole.

However, for at least four centuries, the most commonly used celestial globe is the projection of the Earth's own coordinate system onto the sky. It is simply called the Equatorial System. However the Earthly and celestial Equatorial Systems are rarely in synchronization. because both systems spin at slightly different rates. For a moment in September the slightly faster celestial globe overtakes the Earth globe and the two systems synchronize. They are completely out of step in March. The rest of the year they are somewhere in between.

Having the Earth and celestial globes spin at different rates is cumbersome but a necessary byproduct of the difference between the rate at which the Earth turns relative to the Sun and the stars. If a distant star is directly overhead at midnight, when the Earth turns 360°, that same star will be overhead at 11:56 PM the next night. If however you pick Sun (a very near star), when the Sun is due south at noon today, after the Earth turns 360° the Sun will not yet quite be due south. During this time the Earth has moved in its orbit almost a full degree. To place the Sun due south requires turning the Earth a bit more or almost 361°. This extra degree takes about 4 minutes. In effect, the celestial globe makes a full turn in 23 hours and 56 minutes, while the Earth globe requires 24 hours.

When a navigator wishes to sail to an unseen location, she must sail towards a specific latitude and longitude. For example, if she left England, hoping to arrive at Charlestown, she would have to sail towards 41°22.2' North and 71°20.1' West. When an astronomer wishes to look at a particular object in the sky, he turns his telescope to a particular celestial latitude [a declination] and celestial longitude [a right ascension]. For example if the astronomer wished to view Cor Coroli [also named for King Charles II] he would turn the telescope to 38°18.5' and 194°01.5' in the sky. However, as a slight complication, astronomers have always used clocks to measure longitudes and that same 194°01.5' would be specified in units of time as 12h56.1m. [194°01.5' is 194.025° . Multiplied by 4° per degree we get 776.1 minutes or 12h56.1m.

Today astronomers usually cheat a bit, letting computers keep track of all these annoying confusions and conversions. If you are a modern astronomer, you would move a mouse cursor to Cor Coroli on a digitized star map, double click and let your telescope move to the star. However the old coordinates are still with us in books and the settings circles ("protractors" used to position telescopes in the past) Today, only a few astronomers still guide their telescopes by hand to a destination in the sky using the three centuries old coordinate system. If you are interested, come to Frosty Drew Observatory and we'll show you our telescope's angle measuring "setting circles".

September is a month of transitions for astronomers. Summer constellations linger as the sky darkens earlier each night and the fall and winter constellations rise early and earlier. After the relatively few hours of hot humid nights during summer, the lengthening days are a welcome relief to astronomers. The spectacular Jovian and Saturn systems rise earlier allowing more and more youngsters to stay up long enough to see them.

Leslie Coleman
Leslie Coleman
Entry Date:
Sep 1, 2001
Published Under:
Leslie Coleman's Columns
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