Want to stay on top of all the space news? Follow @universetoday on Twitter
Armillary sphere with astrononomical clock Credit: Chris Bainbridge
A celestial globe is a physical model of what was called by ancient astronomers the celestial sphere. These astronomers Ptolemy among them thought that the universe only included the sun, moon, earth, and the stars visible in the night sky. For this reason the assumed that the universe was a sphere with the Earth as the center.
This assumption created certain assumptions about the motions of celestial bodies. For example it was this assumption that created the concept of sidereal time. Just like the sun seems to rise in the east and set in the west, it was believed that all celestial bodies did the same following the same 24 hour pattern as the Sun. There were of course some motions that did not match this assumption. To correct this astronomers proposed that the Earth while at the center of the universe, actually traveled on an elliptical orbit called the Grand Elliptic. This altering of the center of observation from Earth was supposed to explain this phenomenon.
As you can see this was an astronomy based on terracentric universe as opposed to the Copernican model. However many of the ideas based of this model still had some merit. Using what they knew they created a model of the celestial sphere which could be used like a globe to find the location of stars and make rough calculations. These models were called by several names. They were also known as astrolabes, armilaries, and of course celestial globes. An interesting fact is that they were also one of the world’s first computers allowing astronomers to make calculations of future celestial events such as eclipses.
The use of other instruments such as the sextant were also based on the concept of the celestial sphere. The instruments and technologies not only help expand mankind’s understanding of the heavens, but also lead to the Age of Exploration during which humanity discovered how to cross oceans using the stars to navigate and eventually circumnavigating the globe.
What is most interesting about the concept of the celestial sphere is that several cultures other than the Greeks and European knew about it. The Chinese had their own version and created some of their own celestial models. The Arabs, Aztecs, and Indians were other peoples who observed the stars and came up with similar conclusions.
If you enjoyed this article there are others on Universe Today that you will be sure to love. There is a great article about Armillaries. There is also an article about astronomical clocks.
You can also find out more online. There is a site with pictures and descriptions of a Chinese celestial globe. There is also an explanation on the Cornell University website.
You can also listen to Astronomy Cast. Episode 157 talks about constellations.
Sources:
http://en.wikipedia.org/wiki/Globe#Celestial
http://www.hps.cam.ac.uk/starry/celglobe.html
http://sln.fi.edu/learn/sci-tech/celestial-globe/celestial-globe.php?cts=space
Comments on this entry are closed.