An astrolabe is an ancient tool used in solving problems that involve time and the position of the Sun and stars. Astrolabes can be used in timekeeping, surveying, geography, and astronomy to name a few disciplines. One of its most well-known uses is navigation. Using an astrolabe, you can determine how the sky looked at a certain point in time at a specific place. Since it really is a visible map of the sky, it has proven extremely helpful in astronomical equations.

The astrolabe was invented sometime around 200 BC, and the Greek astronomer Hipparchus is often credited with its invention. A number of Greek scholars wrote in-depth treatises and texts on the astrolabe. Eventually, the tool was introduced to scholars in the Islamic world. They soon started using the instrument, mainly for navigation, and wrote many texts on the instrument themselves.  Texts were also written on the subject in India, showing the extent to which this tool was used around the world.

The astrolabe is constructed of a hollow disk that is known as the “mater.” The mater can hold several flat plates that are known as “tympans” or “climates.” Each tympan is made for a specific latitude. The mater is  marked indicating hours, degrees, or both measurements. The rete is the actual map of the ecliptic plane and has several pointers to indicate the brightest stars. You can think of the rete as a star chart. Often, different scales are engraved on the back of the mater to help in calculations. The engravings differed, and some of them included trigonometric scales and a calendar to convert between the day of the month and the position of the Sun according to the astrolabe. The alidade is attached to the back of the astrolabe. The alidade is used to take a star’s altitude.

The first universal astrolabe was invented by the Islamic scholar Abu Ishaq Ibrahim al-Zarqali. Unlike its predecessors, this astrolabe could be used at any location around the world instead of only at a specific latitude.

There are a number of astrolabe collections around the world, and you can still purchase astrolabes from a variety of locations. A later variation of the astrolabe is the spherical astrolabe, which looks like a sphere surrounded by a number of rings. The spherical astrolabe was also used in astronomy. The astrolabe is a predecessor of the sundial, which is still common today as an ornament in many gardens.

Universe Today has a more in-depth article on the armillary sphere and one on ancient astronomy.

For more information on astrolabes, you may want to check out astrolabes and the mariner’s astrolabe.

Astronomy Cast has an episode on choosing and using a telescope.


Armillary Sphere

Armillary sphere with astrononomical clock Credit: Chris Bainbridge


Despite the fact that the term “armillary sphere” sounds like a high-tech weapon or something from a science fiction movie, it is neither. An armillary sphere is an old tool that is supposed to represent the heavens. They were models of what scientists thought the heavens looked like and how they were suppose to have moved. The armillary sphere is also known as the spherical astrolabe, the armilla, or the armil. The armillary sphere is related to the astrolabe, which was a navigation tool used for determining the position of the Sun and stars and used by sailors for navigating.

The armillary sphere was invented hundreds of years ago. The identity of who created the sphere has been debated. Some credit its invention to a Greek named Eratosthenos. Others have said that the Chinese or other Greek scholars invented it. Regardless of its inventor, the armillary sphere is one of the oldest astronomical instruments in the world. In addition to its being used in the Greek world, the armillary sphere was also used throughout Asia and the Islamic Empire.

These devices were used as teaching tools and models. The models were used to show the difference between the Ptolemaic and Copernican theories of the Solar System. In the Copernican theory, the Sun is the center of our Solar System, while the Earth is the center of the Solar System according to the Ptolemaic theory. When armillary spheres were first invented, the Ptolemaic theory was still the accepted view. It was soon after armillary spheres were invented that Copernicus set forth his theory of the Sun as the center of the Solar System, although it was not widely accepted until centuries later.

The armillary sphere looks like a sphere circled by a ring and set upon a base. Armillary spheres were made with different numbers of circles arranged at various angles. Spheres with both four and nine circles have been known to exist – as well as ones with different numbers. These rings would then be adjusted in order to trace the path of the stars.

The armillary sphere also turns up in the Portugal flag, originally as a symbol for the country’s colony Brazil. The armillary sphere was widely used for navigating at sea, and exploration was heavily promoted by the Portugese royalty. In the early 1800’s, the sign was removed from the national flag when Brazil gained its independence. However, it was replaced in 1911 after Portugal became a Republic. You can still purchase armillary spheres today, although some of them are extremely expensive, especially if they are antiques.

Universe Today has articles on Solar System projects  and parallax.

If you are looking for further information, check out how to build an armillary sphere and astrolabes.

Astronomy Cast has an episode on telescopes.

Source: Wikipedia