The astrolabe: what it is and how it works –

To find out what an astrolabe is and how it works, we have to go back in time. We are facing an ancient instrument, used in astrology, to determine the height of the stars, as well as their position on the sky. The word comes from the Greek and literally means star finder. It has been traditionally used by scientists and navigators to orient themselves at sea and locate stars in the sky.

Throughout history there are many instruments and devices that have been developed and used. You have to think that then they lacked the knowledge that there is nowso being able to develop such an instrument was quite a feat.

Today you have access to any type of information with a couple of clicks and a computer connected to the Internet. But think of how many people would be alive when the astrolabe was invented and they didn’t even know about this instrument. That is why in About History we like to analyze this type of scientific developments and how they have affected what we know today.

In addition to knowing everything about this instrument, we are also going to analyze what types of astrolabes were developed and what use was given to each of them, do not miss it!

what is the astrolabe

Technically, we could define the astrolabe, as a seeker of stars, following that Hellenic tradition and other previous civilizations, fascinated by what was seen in the sky. Over time, it began to be used as a representation of the celestial sphere.

Classical astrolabes were made of brass and were between 15 and 20 centimeters in diameter, although there was a great diversity of sizes, both larger and smaller.

The main body of an astrolabe consists of a mater, a disk with holes in the center. In addition, we have a ring that marks the degrees of latitude, including time, in its most modern European version. In the central part, there is the tympanum, which is engraved with circles of altitude and height.

Likewise, there is also the web or spider, which consists of a disc cut to observe the eardrum below it, with the points representing the number of stars.

Right, on the spider we have the needle or index, which points to the star, indicating the position of the stars. For its part, the alidade, serves to, holding it vertically, making it roll, determine how far away a sighted star is.

Historically, its operation has been quite complex, since many of its users have traditionally needed manuals of hundreds of pages to understand it. But the objective has always been to determine the position of the stars in the sky, observing their movement.

In addition, it has traditionally been the navigation instrument par excellence, to obtain information on the time and latitude in which the sailors were.

How does the astrolabe work?

The astrolabe becomes a projection of a celestial sphere, with a graduated circumference, with a needle, with a crosshair that rotates around it. The purpose is to measure the angular height, measured in degrees of arc, above objects on the horizon. Normally, the star is focused, by the straw of the object, and another person has to read the string number on the scale of the astrolabe.

Another of its functions is to measure latitude. To do this, we have to recognize a star in the sky, as well as its declination, which is obtained through some tables (Sirius, from the constellation Can Major at -16 degrees, Aldebaran in the constellation Taurus, at 17 degrees, Antares, in the constellation Scorpio at -26 degrees and Rige, in the constellation Centaurus at -61 degrees) and we need a compass and our own astrolabe.

To measure the latitude of a place, we have a simple mathematical formula, which varies if we are in the northern or southern hemisphere. In the first case, it consists of adding the average height of the star and the declination of the star, subtracting 90 degrees. In the second case, you have to add the average height of the star and its declination.

history of the astrolabe

The origins of the astrolabe are in classical Greece. Apollonius, in 225 BC, was the first person to study its projection. It was basically a conic section encoder.

Hipparchus, born in Asia Minor, now Turkey, made considerable progress in the projection of the astrolabe, and did a lot of work on it, in Rhodes, around 180 BC.

Claudio Tolomeo, in his work El Planisfera, wrote extensively about this projection, already in the second century of our era, perfecting the geometry of the solar system, to apply it to this type of objects later.

Muslim Spain was a pioneer in Europe in the development of astrolabes. There were Latin words right next to the original Arabic terminology. This is reflected in the Arabic influence in the names of some stars. This is the case of zenith, nadir, azimuth, etc.

After the Reconquest of Toledo, by the Catholic Monarchs, the astrolabe began to spread throughout Europe.

In Europe the astrolabe had tremendous importance, especially in the Middle Ages and later in the Renaissance. We are talking about the fifteenth and sixteenth centuries, when astronomy was an important part of any quality education. Knowing how to use the astrolabe correctly used to be synonymous with having had a good education.

The Danish Tycho Brahe created an astrolabe with a radius of 3 meters, which gained a lot in precision. Even Isaac Newton studied them to end up creating the sextants, including a binoculars and a set of mirrors.

The main manufacturing centers were in what we know today as Germany, in Augsburg and Nuremberg, as well as in France and northern Belgium, specifically in Leuven.

Initially, astrolabes were made of brass, which is how we have traditionally known them. But, the first cardboard figures appear, with the arrival of the printing press.

Universal astrolabes also arrived later, although they did not last long, since they were both high cost and complicated to handle.

The classic circular astrolabe, derived at the time, in the form of a quadrant, called ‘quadrans Novus’, created in the area of ​​Montpellier, in France, and developed in the area of ​​the Ottoman Empire, even until the beginning of the 20th century.

The astrolabe began to decline, at least in Europe, from the seventeenth century, with the invention of more accurate measuring instruments, both in astronomy and navigation, as is the case with telescopes. In the Arab world, the astrolabe continued to be used normally, until well into the 20th century.

Without a doubt, astrolabes have been decisive in improving cartography and calculating navigation time. Scientific discoveries, as well as the arrival in America of Columbus, or other types of advances in the Middle Ages and in the Modern Age, cannot be understood without astrolabes and compasses, which served as orientation for navigators in the seas, very far from the coasts.

types of astrolabes

Along the history Different types of astrolabes have been created that were adapted to the needs of each moment. In addition, the constant discovery of new techniques and materials to make instruments was very useful for them to evolve by leaps and bounds.

Within the types of astrolabes we can find:

  • The planispheric astrolabe.
  • Also the universal astrolabe.
  • Finally, the marine astrolabe.

We are going to see how all these different types of astrolabes are alike and how they are different and how they developed from one to the other. You will see how all of them have a great influence on the technology that we know today and that It has greatly simplified the study of the stars and everything that this entails.

planispheric astrolabe

The planispheric astrolabe was created with the intention of being able to analyze the stars at a single latitude. The data and the different planes of the instrument were adjusted for the latitude in which the scientist was located. If later you wanted to analyze another area, you had to adjust everything again and start from scratch.

It is the simplest instrument of all, and also the one with the most limited utility. Over time, it was replaced by other more developed devices. And that made the job easier.

universal astrolabe

The universal astrolabe is the logical evolution of the planispheric. It was not only valid to know the location of the stars at a specific latitude, but also allowed to know this information from all latitudes at the same time.

The breadth of its uses and its information comes hand in hand with the difficulty of its understanding. It is the most complex device and many scientists took a long time to get the hang of it and start reading it freely. Of course, once this device was controlled, the information it provided was much and very valuable.

With the universal astrolabe you can know more data in addition to the location of the stars. You could read the time on it, know the time of the calendar, the seasons, etc. It is a very complete and complex instrument.

As a result of the development of the universal astrolabe, many other devices and instruments have been created that have been used to analyze the sky. Today you can have information about the stars, the planets, your latitude or any other data with a simple mobile application. But to get to what we have today, it has been necessary to go through times in which scientists have been able to create instruments like this.

sailor astrolabe

The astrolabe, as we mentioned in the introduction to this article, was used not only for those who studied the sky, but also for sailors. Thanks to this device they could orient themselves better. Keep in mind that they didn’t have as much information and state-of-the-art technology as we have now. Getting lost at sea was more common than you think, and from land it was difficult to locate the different ships.

Seeing the potential of this tool, it was decided to develop an adapted version that is the marine astrolabe or astrolabe of the sea. It was useful to know the latitude in which a ship was located thanks to the study of the sky. In the end is a primitive version of navigation systems that we know today.

The problem with this instrument is that it was not easy to use. Sailors who chose to use it on their voyages need to take some classes first to help them understand the astrolabe and how to apply it to navigation. In order to know what latitude they were at, they had to use the height of the meridian of a known star with respect to the horizon. This data was applied, through mathematical operations, to know the exact place of the ship on Earth and where it was going.

Planispheric or universal astrolabes are not considered an exact apparatus. It is a much simpler version that presented fewer complications in its use and less information. It could be simplified as an instrument used to measure angles between the ship and the stars.

Today this device is…