The Way to the Stars: Build Your Own Astrolabe
What is it?
Astrolabes are an ancient astronomical instrument. They were first used in ancient Greece, were extensively developed in the medieval Islamic world and became the key astronomical instrument of the western middle ages.
When mapping the heavens astronomers assume that the stars seen in the night sky are all at an equal distance from the earth, existing on the inside of an enormous sphere that has the earth at its centre. By using this model, they can create the two-dimensional representation of this celestial sphere seen on star charts and astrolabes, the starry equivalent of a map of the earth.
The front of the astrolabe shows a map of the night sky in the form of a rotating, net-like, rete. The rete was also sometimes described by medieval writers with its Arabic name, alhanthabuth or alancabut, meaning spider.
The spikes on the rete indicate the positions of individual stars. Sirius, also known as the Dog Star, is often represented by the head of the dog. The rete also shows the path of the sun against the background stars; this path is known as the ecliptic, and the constellations through which the sun passes on the ecliptic are the well-known signs of the Zodiac.
Beneath the rete sits a plate inscribed with a projection of sky above an observer at a given latitude: the plate could be removed and replaced with others calibrated for different latitudes if necessary. The plate and the rete nest inside the body (or womb) of the astrolabe, called the mater, which is inscribed on its outer edge, or limb, with scales of degrees and hours. Fixed to the centre of the astrolabe is a rotating rule used for taking readings.
On the back of the astrolabe is a rotating bar called the alidade or label, which is used to measure the altitude above the horizon of celestial bodies. This side of the instrument is divided into degrees for taking altitude measurements, and is also engraved with a calendar and divisions of the zodiac.
By using the data from both sides of the instrument and the measured positions of objects in the sky, the user of the astrolabe can calculate many facts about their position in time and space, including the hour of the day, the date, and their position on the earth’s surface.
Find out more about astrolabes and astronomy in the medieval period by investigating some of the manuscripts held in St John's College Library:
You will need:
- Flatpack Astrolabe Kit (pdf)
- A printer
- Thin card
- Acetate transparencies or tracing paper
- Permanent marker (to write on transparency)
- Split-pin paper fastener
- Ribbon or thin string
What to do:
- Download this pdf file: Flatpack Astrolabe Kit.*
- Print page 1 on paper for your reference in the future. Print pages 2, 3 and 5 on thin card. Print page 4 on acetate transparency, tracing paper, or some other transparent or translucent material.
- Carefully cut out the parts from each page.
- Glue together the two sides of the mother, with a sheet of thicker card between them if possible. You can write your name on the front of the mother in the space indicated.
- Mark constellations and stars on your rete using these instructions.
- Make a hole through the centre of all the pieces, in the place indicated by the small circle.
- Assemble the astrolabe with the label (the pointer with no markings) on the back of the mother, and the rete and then the rule (the pointer with a scale on it) on the front of the mother. Hold the astrolabe together with a split-pin paper fastener.
- You should be able to rotate each of the label, rule and rete independently from each other.
- Make a small hole in the top of the mother. Thread a small piece of ribbon through the hole, and tie the ends to make a loop. Thread another small piece of ribbon through this loop and tie it again into a loop. Now you can hang the astrolabe from your thumb.
*This astrolabe is calibrated for latitude 52° North. It will work across northern Europe and much of the United States of America. If you would like an astrolabe kit calibrated to a different latitude please download the file appropriate to your latitude from this page.
This astrolabe kit was designed and produced by Dominic Ford and Katie Birkwood, based on S. Eisner, 'Building Chaucer's astrolabe', Journal of the British Astronomical Association 86 (1975-1976), pp. 18-29, 125-132 and 219-227. The diagrams were produced using PyXPlot.
You can use your astrolabe to find out what will be visible in the sky throughout any night. The instructions below explain how to find out what you can see on 20 March at 7.30pm GMT (19.30 on the 24-hour clock). You can easily adapt the instructions to apply to any date and time.
The time must always be set according to Greenwich Mean Time (GMT, also know as Coordinated Universal Time, UTC). For astrolabe users in the UK, this means subtracting one hour from the clock time during British Summer Time periods (from the end of March to the end of October). For astrolabe users in other parts of the world, you will need to adjust your calculations accordingly.
Set the date
To set up your astrolabe for a particular date, you do not use the calendar date. Instead you use the position of the sun in the Zodiac.
To find out the position of the sun in the Zodiac on any day, use the back of the astrolabe. Find the date in question in the calendar on the back. Move the label round until it lines up with this date. Then read along the line of the label to see the position in the Zodiac.
In this example, we see that 20 March is equal to 29 Pisces.
Find the Date on the Rete
Remember the place in the Zodiac that you have just found. Turn your astrolabe over, and find this place in the Zodiac marked on the rete.
Rotate the rule on the front of the astrolabe until it rests on this place in the Zodiac.
Set the Time
In this example, we are going to find out which stars are above the horizon at 7.30pm on 20 March. The astrolabe uses the 24-hour clock, so we use 19.30 instead of 7.30pm.
Find the location of 19.30 on the rim of the front of the astrolabe. It is half-way between 19 and 20. Hold the rule and rete together, and rotate them as one so that the rule points to 19.30 and also still lines up with 29 Pisces on the Zodiac.
You have now set up your astrolabe.
What can I see?
The astrolabe now shows you which stars will be above the horizon at 19.30 on 20 March. The horizon is marked by a line on the plate, the part of the astrolabe beneath the rete. Everything inside this line will be above the horizon: this area is shaded in blue on the diagram.
The plate shows you where in the sky to see the stars. The concentric rings labelled with numbers tell you how high in the sky to look. The central point of these rings is the zenith, the point directly overhead, at an elevation of 90°.
The compass points along the edge of the horizon line tell you in what direction to look.
The constellations you see in the sky will be mirror images of the constellations shown on the rete. This is because the rete is drawn as if the stars are all located a fixed distance from the earth on an enormous sphere. The map on the rete is drawn from the perspective of an observer placed outside that sphere. Whilst the stars are not actually arranged this way (they are all at different distances from the Earth), the earth is so small in comparison to the distances to the stars that this method of projection creates an accurate image.
The Whipple Museum of the History and Philosophy of Science
Oxford Museum of the History of Science
The British Museum
- astrolabes are some of the highlights from the British Museum collections
- In collaboration with the BBC: A History of the World in 100 Objects (object number 62)
National Maritime Museum
Institute and Museum of the History of Science, Florence
- Introduction to the astrolabe and its varieties; astronomical, and terrestrial uses of the astrolabe
Geoffrey Chaucher, Treatise on the Astrolabe
- Online edition taken from The Works of Geoffrey Chaucer, ed. by F.N. Robinson, 2nd edn (Boston: Houghton Mifflin, 1957)