We are living in a modern age, satellites positioning systems have automated navigation of the seas, skies and continents. Time is measured precisely by using atomic clocks. Modern telescopes are controlled and targeted using modern automated computer drive systems. All these modern technologies today have far outpace the time keeping and the basic navigational of the past scientific instruments. Well, lets go back to the past and have a look at;
The Astrolabe – By Bruce Levell
“For about 500 years the astrolabe was probably the most sophisticated scientific instrument in the world. True to its etymology (from the Greek word for “star finder” or “star taker”), the astrolabe is a model of the sky, showing the positions of the Sun and main stars at any time of the day or night throughout the year. It can be used to:
- Navigate, since the positions of the stars at a known time reveal the observer’s location on the globe and the direction in which to travel;
- Calculate setting and rising times for the Sun, and hence prayer times or the length of the day, for and day of the year;
- Determine the likelihood of a new Moon being sighted on any particular day;
- Tell the time from the observed positions of the Sun or stars.
In addition, astrolabes were frequently used to cast horoscopes.
A planispheric astrolabe is based on a two dimensional map of the sky, rendered in most cases by stereographic projection. Such a projection can be imagined as the view of the sky that would be seen by an observer standing on the south pole and looking straight “down”, through the solid earth, at the northern sky. The observer would thus see a circular sky, centered on the north celestial pole.
Normally, astrolabes take the projection of the Tropic of Capricorn (the line of latitudes at 23.5 degrees south) as the edge of the circular map. Other key latitudes – the Equator and the Tropic of Cancer – would project onto this view as smaller concentric circles.
A real observer, of course, has no x-ray vision and only sees part of the sky at any one time – and part that he does see varies with his latitude and the time of the day. The markers of the astrolabe solved this problem by fitting each astrolabe with a series of removable plates (Arabic: safiha), each showing a polar grid and horizon for a small range of latitudes. By rotating the star map above these plates, one can simulate the rising and setting of stars against the fixed horizon and thus the sky view at a particular latitude. This “rete” (Arabic: ankubat) was typically fashioned as a beautiful frame that carried star names without obscuring the underlying grid lines.
The astrolabe was not primarily an observational instrument, although on its reverse side it was equipped with an alidade, or sighting rule (Arabic: alidada), along which a star’s altitude or bearing could be determined. Some astrolabes were modified for use at sea; they were made heavier or perforated so as to reduce the chance that instrument would be blown by strong winds on the deck of a ship.
The astrolabe was probably perfected by Arab astronomers, such as Muhammed Musa al Khwarizmi in the early 9th century, building on Greek understanding. The earliest surviving examples, however, date from the late 10th century. The astrolabe fell out of use in the 17th century in the West and somewhat later in the East, succumbing to more accurate observational tools and printed astronomical almanacs.
Astrolabes are still made today in Afghanistan and India for tourist market; they can even be found in Oman in Muttrah souq. But these are often crude imitations of the real thing, which was not simply an accurate scientific instrument but an object of great beauty. Perhaps the most beautiful surviving astrolabe is the spherical one signed “work of Musa year 885” (i.e. 1480-1 CE) pictured above.
They are different types of astrolabes. The spherical astrolabe (top) is generally acknowledged as the most beautiful of its kind. It dates back to 1480 CE. Conventional astrolabes (bottom left and right on above picture) were flat and consists of rotatable frames and removable, engraved plates.
Some pictures of Astrolabes: (click to view in full size)
More about Astrolabe:
- Astrolabe Wikipedia
- The Astrolabe
- Museum of the History of Science (large collection)
To Be Continued later 🙂