The origins of the astrolabe were in classical Greece (astro-star and labe(labium) the one who search) and thus astrolabe is the one who search for stars. Appollonius (225 BC) was the great codifier of conic sections, probably studied the astrolabe projection. The most influential individual on the theory of the astrolabe projection was Hipparchus who was born in Nicaea in Asia Minor about 180 BC but studied and worked on the island of Rhodes. Hipparchus, who also discovered the precession of the equinoxes and was influential in the development of trigonometry, redefined and formalized the projection as a method for solving complex astronomical problems without spherical trigonometry and probably proved its main characteristics. Hipparchus did not invent the astrolabe but he did refine the projection theory.

The first major writer on the projection was the famous Claudius Ptolomy (150 AC) who wrote extensively on it, in his work known as the Planisphaerium. There are tantalizing hints in Ptolomy's writing that he may have had and instrument that could justifiable be called an astrolabe. Ptolomy also refined the fundamental geometry of the Earth-Sun system that is used to design astrolabes.

The Astrolabe in Europe

The earliest astrolabes used in Europe were imported from Moslem Spain with Latin words engraved alongside the original Arabic. It is likely that these imported astrolabes influenced European use of Arabic star names. By the end of the 12th century there were at least a half dozen competent astrolabe treatises in Latin, and there were hundreds available only a century later. Europeans makers extended the plate engravings to include astrological information and adopted the various timekeeping variations used in that time.

The astrolabe was widely used in Europe in the late Middle ages and Renaissance, peaking in popularity in the 15th and 16th centuries, and was one of the basic astronomical education tools. Knowledge of astronomy was considered to be fundamental in education ans skill in the use of the astrolabe was a sign of proper breeding and education. Their primary use was, however, astrological. Geoffry Chaucer thought it was important for his son to understand how to use an astrolabe, and his 1391 treatise on the astrolabe demonstrates a high level of astronomical knowledge.

Astrolabe manufacturing was centered in Augsburg and Nuremberg in Germany in the 15th century with some production in France. In the sixteenth century, the best instruments came from Louvain in Belgium. By the middle of the seventeenth century astrolabes were made all over Europe. George Hartmann in Nuremberg founded a particularly interesting workshop in about 1525. He used a early form of mass production to produce his high quality instruments. Brass astrolabes were quite expensive, and only the wealthy could afford a good one. Paper astrolabes (as the Antiquus one) became available as printing developes, and many were surely made, although few survive.

Several interesting astrolabe variations known as Universal astrolabes which make a single instrument usable in all latitudes were invented in the 15th and 16th centuries, but due to their high cost and complex operation, never gained the popularity of the planispheric type. These instruments projected the celestial sphere on the equinoctial colure and lacked the intuitive appeal of the planispheric type.

A Derivate of the circular astrolabe is reduced to a quadrant was described in 1288 by Profiat Tibbon de Montpelier. Few examples of astrolabe quadrants, commonly called the quadrant novus (new quadrant) survive, but many treatises on its construction and use were published . A form of astrolabe quadrant was quite popular in the Ottoman Empire until the early XXth century. Several quadrants using the stereographic projection were introduced in the 17th century. The most popular was devised by Edmund Gunter (1581-1626) in 1618. Gunter's quadrant was quite easy to use in comparison to the older quadrants novus and become widely used.

The Parts of an Astrolabe

Classical astrolabes were usually made of brass and were commonly about sis or eight inches (15 to 20 cm.) in diameter, although much larger and smaller ones were made.

Most astrolabe problems were solved using the front of the instrument. The front of an astrolabe has two types of parts: represent time scales and Stereographic projection of the sky as seen from a specific latitude. The rotating parts simulate the daily rotation of the sky.

Mater and Plate

The main body of a typical astrolabe would consist of a brass disc that is hollowed out in the center (fig.1).

The ring around the edge of the disk (the limb) was marked in degrees and on many European astrolabes, into 24 hours with noon at the top and midnight at the botton for telling time.

In the hollowed center section of the disk is the plate (also called tympanum) (fig.2) for the local latitude engraved with circles of altitude and azimuth for a certain latitude.

Rete

Over the plate is fitted a disk (the rete, Latin for net) (fig.1) also made of brass, that was mostly cut away (pierced) so you could see the plate under it.

Pointers represented a number of fixed stars. A circle showing the projection of the sun‘s annual path in the sky (the ecliptic) was included on the rete. The ecliptic circle was always divided into degree sections representing the signs of the zodiac. The rete is assumed to rotate in one sideral day to simulate the daily rotation of the stars in the sky.

Rule

On top of the rete was a clock-type hand called the rule. Not all astrolabes had a rule depending on the intended use of the instrument. The rule and the rete were held in position by a pin through the center of the instrument and could rotate over the plate.

Back

The back of the instrument was engraved with a wide variety of scales depending on where and when the astrolabe was made. (fig. 3).

All astrolabes including scales for measuring angles and scales for determining the sun’s longitude for any date. Almost all European astrolabes, and many Islamic ones has a scale for solving simple trigonometry problems called the shadow square. European astrolabes often had a scale for converting between unequal (planetary) hours and equal hours.

The back of every astrolabe included an alidade for measuring the altitude of celestial objects.

Uses of the Astrolabe

In the 10th century Abd-al-rahma B. Umar al-sufi wrote a detailed treatise on the astrolabe consisting of 386 chapters in which he described 1000 uses for the astrolabe.

Al-Sufi perhaps overstated the flexibility of the astrolabe, but astrolabes can be used to solve many astronomical problems that would otherwise require rather sophisticated mathematics. They were used to tell the time of sunrise and sunset and, thus the length of the day, to locate celestial objects in the sky, as a handy reference of celestial positions and, as astrology was a deeply embedded element of the cultures that uses astrolabes to determine aspects of horoscopes. Islamic prayer times are astronomically determine the required times. Modern astrolabes can be used to solve astronomy problems involving sideral time and can be used with modern civil time.

Following are two examples of astrolabe uses. (our Antiquus astrolabe “postal card” is made for latitude 41º).

Finding the time of day

The time of day is found in the following steps:

1- The altitude of the Sun or a bright star is determined using the back of the instrument. Keep the astrolabe parallel to the ground. The astrolabe is oriented so the Sun or star is lined up with the back of the astrolabe.

2- The alidade is rotate until the Sun’s shadow or the star itself is visible through the sights on the alidade (in postal card astrolabe, put first them is vertical position). The altitude is noted from the altitude scale on the back of the instrument. The Sun’s position on the ecliptic is found by setting the alidade on the date and reading the Sun’s longitude on the zodiac scale.

3- On the front of the astrolabe, the rule is rotated until it crosses the ecliptic at the Sun’s current longitude. The point where the rule crosses the ecliptic is the Sun’s current position.

4- The rete and rule rotated together until the Sun or star pointer is at the measured altitude.

5- The rule points to the apparent solar time on the limb. Apparent solar time is the time as shown on a sundial and is different for each latitude. In modern use, apparent solar time must be corrected to zone time by compensating for the equation of time and the different in longitude from the Greenwich meridian.

It should also be noted that in the middle ages the time of day was usually expressed as the part of the day or night that have passed. That is sunrise was the beginning of the 1st hour of the day, noon was the end of the 6th hour and sunset was the end of the 12th hour of the day and the beginning of the 1st hour of the night. The length of the hour changed during the year with the change on latitude. An hour was longer in the summer that in the winter. These hours are called “unequal hours” and many astrolabes had curves on the plate for determining the unequal hour of the day or night . The use of unequal hours for civil time keeping gradually declined as reliable clocks and watches became available in the 17th and 18th centuries although their use continued in parts of the world well into the 19th century. The use of unequal hours is not as awkward as it might sound. The unequal hour of the day is the percent of the day that has passed. The convention is quite ease to get used to, and is quite valuable for some environments.

Finding the time of celestial event

The time of a celestial event such as sunrise sunset or the culmination of a star is found by setting the astrolabe to the circumstances of the event and reading the time.

Determine the Sun’s position on the ecliptic.

Se the rule to that position on the ecliptic on the front of the astrolabe.

Rotate the rete and rule together until the desired event is in position. For example to find the time of sunrise, rotate the rete and rule until their intersection is right on the easter horizon.

Read the time from the rule’s position on the limb.

The length of the day can be found by finding the time of sunrise and sunset and calculating the difference. Similarly the time until sunrise and sunset can be found as the different with the current time.

If you are in another different latitude for which the astrolabe is made (41º) the obtained results will be erroneous. Despite it you will have learned to handle an instrument that was denominated, with all reason, the mathematical jewel of Middle Age.