ASTROLABE: ORIGINS, HISTORY, TYPES, PARTS, USE - CULTURE VOCABULARY - 2025

The astrolabe is a measuring instrument whose best known use is to be able to calculate the altitude of a celestial body (moons, planets or stars) above the horizon and thus identify time and latitude locally. This object has been used throughout history by astronomers and navigators.

Measurements with the astrolabe are made using angles. In this case, calculating altitude refers to identifying the elevation of a celestial body above the horizontal surface. For example, calculating the elevation of a star using the sea level as a reference, a resource frequently used by sailors.

Planispheric Astrolabe

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Among other functions, the use of the astrolabe as an instrument of Muslims has been associated to determine prayer times and even identify the point of orientation towards Mecca. The Islamic versions had an additional data set for these purposes.

The effectiveness of an astrolabe is closely linked to its construction. Since ancient times it depended heavily on artisans for complexity and artistic detail. The instrument consists of several discs recorded with stereographic projections (projections of a sphere on a plane) and also has a kind of referential template that identifies the brightest or most visible celestial bodies.

Due to the variety of uses, there are different types of astrolabe, but in its most basic and general aspect, an astrolabe can be defined as an ancient instrument attributed to the first sciences, which allows to calculate time and in turn serve for observational measurements.

origins

The invention of the astrolabe dates back to Ancient Greece, however, the period of most evolution for this instrument manifests itself in the Middle Ages. During this time, new tools were added, increasing its uses and complexities.

The authorship of the astrolabe is not well defined. It is presumed that it was invented by Hipparchus of Nicea, but it is also attributed to Apollonius of Perge and many other important figures in history.

The references for the construction of the astrolabe have been the descriptions that have been made of this object through the centuries. One of the first important characters to describe the artifact was the astronomer Claudius Ptolemy, preceded in the 12th century by the Englishman Geoffrey Chaucer, whose texts inspired several of the best astrolabs of the time.

Due to its importance to Islamic culture, the astrolabe received many modifications and attributes by astronomers and mathematicians practicing the religion. Thus, the artifact was introduced into European territory during the 12th century, at which time the Iberian Peninsula was known as Al-Andalus and was under Muslim rule.

It was during the Middle Ages and the Renaissance that it reached its peak. The use of the astrolabe was a basic principle in education just like the teaching of astronomy. Most were made in Portugal and the favorite materials of the time were brass, wood or steel.

This instrument was one of the most popular for sailors until about the 13th century. Later, more appropriate instruments for navigation such as the sextant began to emerge. The astrolabe could be imprecise for navigators, partly because of the unevenness of the sea surface. For these reasons it was eventually replaced.

Types

There are only three known types of astrolabe. Designs vary in the dimensions in which the celestial sphere is projected and its uses.

Spherical astrolabe

It has a three-dimensional quality. It is a spherical object surrounded by a skeleton called a "rete" that functions as a map. This guide contains various circles and points that indicate the most relevant celestial bodies and specifically the passage of the sun. The only known specimen of a spherical astrolabe is in the Museum of the History of Science in England and dates back to 1480 AD.

Linear astrolabe

Devised by the mathematician and astronomer Sharaf al-Din, it is one of the least practical designs and of which no historical specimens have been preserved. This version of the instrument proposed the use of a graduated ruler with which the celestial sphere and the horizon were projected onto a line.

Planispheric astrolabe

The planispheric astrolabe is the most widely used design. It contains the projection of the celestial sphere on the flat surfaces of the disks that are contained in the instrument. Like the spherical astrolabe, it contains a skeleton with reference data on the brightest celestial bodies.

Parts of a planispheric astrolabe

To understand how this instrument works, it is important to know its composition. Especially in the case of the planispheric astrolabe, which is the one that has been used the most in history.

The base of the instrument is a circular container called “mater / madre”, which houses discs known as “tympas / eardrums” inside. These discs are recorded with latitudes. Above the eardrums is the “rete” or the “spider”, which is a kind of map of the brightest celestial bodies. A ruler for measurements is also included. Both the spider and the ruler are items that can be rotated.

The front part of the astrolabe also contains various etchings on the edges and the various circles and lines that make up the spider. There are a variety of data that indicate things like the 24-hour divisions for the days, others that indicate the different zodiacal constellations and the respective tropics next to the equator, located in the center of the instrument.

On the back of the astrolabe it is customary to see more engravings with different gradations or time conversion scales. This information varies depending on the craftsman or manufacturer. In this back part there is also the "alidade".

This last piece contains the viewers with which it is possible to measure the altitude of the celestial bodies that are used as a reference. Generally, the rear part is the one that provides the necessary data that must be obtained during the observation to be able to make a reading in the front part.

The "throne" is another important piece for the use of the astrolabe. It is a ring through which the thumb is inserted and allows the user to hold the astrolabe so that it is completely perpendicular to the ground.

How do you use an astrolabe?

Measurement of the latitude of a celestial body with respect to sea level.

Image by OpenClipart-Vectors from Pixabay

The uses that can be given to the astrolabe are extensive, however, one of its main qualities is to help identify latitude. Knowing this information through the instrument was very important for sailors and explorers. Sailors found latitude by knowing the altitude of a celestial body. During the day they used the sun as a reference and at night the stars.

The alidade is the piece that allowed the first step to be carried out. The two holes at each end were aligned so that the chosen celestial body could be viewed through both holes.

Once the position is determined, the user looks for the angle on the graduated circle that is usually on the back of the astrolabe. This data allows the latitude to be located on the front of the instrument with the help of the star map inscribed in the spider and the other recorded data.

The astrolabe also allows the user to obtain data such as the time, the point of the year in which it is located or to locate and analyze the movement of the stars. However, there is a huge amount of data that can be obtained with it. During the 10th century, the Persian astronomer al-Sufi spoke of the thousand uses of the astrolabe that could be used in different branches of science.

In order to use the astrolabe in a much broader way, it is also necessary to have knowledge of astronomy. Since its inception, this has been a very important instrument for the study of the stars.

Astrolabs have been the predecessors of instruments such as the sextant or the astronomical clock.

The astrolabe through history

The birth of the astrolabe dates from the 1st and 2nd centuries. C., being used from its beginnings in Ancient Greece as an observation instrument for astronomy. Its use then extended into the Byzantine period.

The oldest treatise in existence was written by John Philoponus, a 6th century Alexandrian philologist. During the 8th century, brass began to be spoken of as the main building material in a treatise by the Mesopotamian Bishop Severus Sebokht.

During the Middle Ages, the astrolabe took on great relevance. The instrument begins to penetrate other areas such as Islam. Many Muslim astronomers added new features for religious uses. This era also marks the increasing use of the astrolabe as a device for navigation.

The Middle Ages witnessed the introduction of the astrolabe to Europe. Some versions made of the instrument were also born, such as the spherical astrolabe and the "balesilha", a much simpler astrolabe oriented only to the calculation of latitude.

The end of the popular use of the astrolabe would culminate with the Middle Ages and the development of new instruments for navigation. However, it was an object of great relevance for the discoveries of the different civilizations in history.

References

1. The Editors of Encyclopaedia Britannica (2019). Astrolabe. Encyclopaedia Britannica, inc. Recovered from britannica.com
2. Wikipedia the free encyclopedia. Astrolabe. Recovered from en.wikipedia.org
3. The Mariners' Museum & Park. Mariner's Astrolabe. Recovered from exploration.marinersmuseum.org
4. Museo Galileo - Institute and Museum of the History of Science. Astrolabe components. Recovered from catalogue.museogalileo.it
5. Meech K (2000). Astrolabe History. Institute for Astronomy, University of Hawai Retrieved from ifa.hawaii.edu
6. Mathematical Institute, Utrecht University. The Astrolabe: Description, History and Bibliography. Recovered from staff.science.uu.nl
7. History of Science Museum. Spherical astrolabe. Masters of the Universe. Recovered from hsm.ox.ac.uk
8. Hayton D (2016). A Spherical Astrolabe. Recovered from dhayton.haverford.edu