The astrolabe is a type of scientific instrument used many years ago that was used to calculate time and for observational purposes . A widely elaborated variety of this instrument, the planisphere astrolabe , enabled astronomers to calculate the position of the sun and stars with respect to both the horizon and the meridian. It provided them with a flat image of the celestial sphere and the main circles , namely those representing the ecliptic., the celestial equator and the tropics of Cancer and Capricorn. Due to such characteristics, the spherical planar astrolabe can be considered as a kind of rudimentary analog computer.


What is the astrolabe?

The astrolabe is an instrument that was used in astronomy to be able to check the location of the different stars , in addition, it worked to indicate the height and position of the stars in the sky.



Astrolabs date back to the 6th century , and were apparently used extensively since the Middle Ages in Europe and the Islamic world. Towards the middle of the 15th century, astrolabs were adopted by sailors and used in star-based navigation. The so-called sailor’s astrolabe was later replaced by sextants.


The principles of the astrolabe projection were known before 150 BC, and true astrolabs were made before 400 AD The astrolabe was highly developed in the Islamic world in 800 and was introduced to Europe from Islamic Spain at the beginning of the century XII. It was the most popular astronomical instrument until about 1650, when it was replaced by more specialized and precise instruments. Astrolabs are still prized for their unique abilities and value for astronomy education .

Who Invented the Astrolabe?

Diogenes Laertius , Berosius the Chaldean, and Archimedes are considered the creators of the astrolabe, although Delambre credits Hipparchus with the invention of the flat astrolabe . Some historians believe that Ptolemy was the inventor of the flat astrolabe. The oldest in the world was built by Nastulus around the year 927 and is kept in the Kuwait National Museum.

What is it for?

The functions of the astrolabe are the following:

  • It is used to measure the angular height and the measure in degrees of arc, on the objects on the horizon.
  • It works to be able to measure latitude .
  • It allows to determine the position of the stars on the celestial vault.
  • It was used by navigators, astronomers and scientists to locate the stars and observe their movement .
  • It worked to be able to determine the  local time from the latitude or, vice versa, to find the latitude knowing the time.


The typical planisphere astrolabe used by medieval astronomers measured 3 to 18 inches and was made of metal , brass, or iron . It had several main parts: a base plate known as a mater with a network of lines representing celestial coordinates; an open pattern disk or rete with a ” map ” of the stars, including the aforementioned circles, which revolved on the mater around a central pin corresponding to the north celestial pole; and a straight ruler or the alidade, which was used to observe objects in the sky.

The alidade made it possible to use the astrolabe for surveying applications , for example determining the height of a mountain. Most astrolabs also had one or more plates better known as climates that were etched with coordinate lines for different latitudes and were placed between the mater and the rete.

How does it work?

The astrolabe is a projection of a celestial sphere , with a graduated circumference and a needle , with a crosshair that rotates around it. Normally, you focus on the star, by the straw of the object, and someone else has to read the string number on the astrolabe scale.

To measure the latitude of a place, a simple mathematical formula is used , which varies depending on the hemisphere. In the Northern Hemisphere , you add the mean height of the star and the declination of the star and then subtract 90 degrees. In the southern hemisphere , add the mean height of the star and its declination.

How to use?

To be able to measure we must focus on a star or planet in the straw and read the number of the string of the astrolabe scale, this will be the height of the object. After the height of the star is calculated, the following formula is used:

  • Latitude of the place = (measured height of the star) + (declination of the star) – 90 °, this formula for the southern hemisphere.
  • Latitude of the place = 90 ° – (height of the star) + (declination of the star), for the northern hemisphere.

How to make an astrolabe?

The simplest way to make an astrolabe is as follows:


  • protractor , especially if it has a small hole in the center of the straight edge.
  • sorbet for plastic soda or straw.
  • Strong adhesive tape.
  • Fishing rope or twine.
  • A fishing sinker.


  • Joined with adhesive tape the straw along the straight edge of the protractor . This will act as a star and planet observation guide .
  • Tie the fishing sinker to one end of the rope.
  • Pass the other end of the rope through the hole in the protractor and tie a knot to hold it in place.
  • Hold the protractor, curved down, and look at the star through the straw.
  • Press the rope firmly against the side of the protractor and lower it carefully, paying attention to the degree that the rope crosses.
  • Take measurements of several stars every half hour to see how they rotate the sky.

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