Monday, July 27, 2015

Armillary Sphere


Armillary Sphere – Model of Objects in Celestial Sphere

An armillary sphere is a model of objects in the celestial sphere comprising of a spherical framework of rings that is centered on Earth representing lines of celestial longitude and latitude together with other astronomically important features like the ecliptic.

The armillary sphere has a number of graduated brass rings which represent the chief celestial circle, demonstrating the basic principles of astronomical motion as well as the horology – the science of measurement. The word armillary comes from the Greek term, `armilla’, which means bracelets or rings. Armillary sphere consists of skeletal celestial sphere – the basket, within a vertical ring – the meridian ring, wherein the basket has two colure rings which represents the celestial poles and one of each, the two tropics.

A band marked with 360 degrees intervals, circles the basket, representing the equator while the final component of the basket is the ecliptic ring that divides into the twelve signs of the zodiac, each of which is divided into thirty units or days. Towards the center of the basket is a globe that represents the Earth. Armillary sphere was the early astronomical device to represent the great circles of the heavens which was included in the most intricate instruments – the horizon, meridian, Equator tropics, polar circles and ecliptic hoop.

Skeleton Celestial Globe with Circles

The sphere is a skeleton celestial globe with circles that are divided into degrees for angular measurement. Such models in the 17th and 18th centuries were either suspended or rested on a stand or affixed to a handle, were utilized in showing the difference between the Ptolemaic theory of a central Earth and the Copernican theory of a central Sun.

Earliest known complete armillary sphere having nine circles is said to have been the meteoroskopion of the Alexandrine Greeks though earlier and simple forms of ring instruments were also used. Ptolemy in the almagest reckons to at least three. Hipparchus it is stated used a sphere of four rings while in Ptolemy’s instruments, the astrolabon, there were diametrically disposed tubes on the graduated circles and the instruments were kept vertical by a plumb line.

Similar instruments were also used by the Arabs, with diametric sight rules or alidades and probably were made and used in the 12th century by Moors in Spain that were the examples of later European armillary spheres.

Became a Complicate Instrument for Instruction/Calculation

The first armillary sphere was most likely invented in the 3rd century BC by Eratosthenes, the Greek astronomer and around 130 AD, Ptolemy built a model to support him in making observation of the path of the main astronomical bodies in the night sky.

 Towards early 1500, Copernicus had defeated the accepted geocentric premise and the spheres were built with the sun in the center. Over a period of time it became very complicated instruments for instruction as well as calculation of the movement of celestial objects and events. The armillary sphere can be utilized to –
  • Determining the time of sunrise and sunset
  • Clarify the meaning of the Spring and Vernal Equinox
  • In determining the position of a star and explain the meaning of right ascension and declination
  • In demonstrating the meaning of the Tropics – turning point, at the Summer and Winter Solstice
  • In locating the Polar Star
Armillary Sphere – An Old Tool to Represent the Heavens
An armillary sphere is an old tool which is presumed to represent the heavens and was models of what the scientists thought the heavens would look like and how they were supposed to move. It is also known as a spherical astrolabe, the armilla, or the armil and is related to the astrolabe which was used as a navigation tool in determining the position of the Sun, and stars as well as used for navigation by sailors.

The armillary sphere was invented hundreds of years ago and the identity of its invention has been debated. Some claim that its invention is a Eratosthenes while others believe that the Chinese or other Greek scholars invented it. Irrespective of its inventor, the armillary sphere seems to be the oldest astronomical instrument in the world and besides being used in the Greek world; it was also used throughout the whole of Asia as well as the Islamic Empire.

These devices were utilized as models and teaching tools and the models were used in showing the difference between the Ptolemaic as well as Copernican theories of the Solar System. While in the Copernican theory, the Sun is said to be the center of our Solar System, the Earth is the center of the Solar System in the Ptolemaic theory. The Ptolemaic theory was still accepted when armillary spheres were first invented.

 It was only after the armillary spheres were invented that Copernicus came up with his theory of the Sun being the center of the Solar System and it was not accepted till centuries later. Armillary Sphere links the concept of time, history and the changing universe in one element and this timeless symbology is still relevant today since man tends to continue searching the heavens for knowledge on how we tend to relate to the universe.

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