Maths Museum Museum
    Persian Celestial Globe Maths Museum
Persian Celestial Globe
© Museum of the History of Science, Oxford
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This mathematical instrument is called a celestial globe. 'Celestial' means 'of the sky' or 'heavenly', so this globe is basically a map of the sky at night, or a star globe.

Actually it is more than just a map of the stars at night. It is also a map of the sky during the day because it shows the position of the sun. We know that the position of the sun in the sky changes every day. It starts off low in the east, moves round to the south climbing all the time, and then falls to the west where it sets. This globe can model this movement.

But this is not all. As well as the change in the sun's position during the day, this globe also shows how the sun's position changes over a whole year. If we could dim the sun during the day so that the sky went black and the stars became visible, just like in the eclipse in 1999, we would see that the sun seems to move through the background pattern of the stars. In spring it is near the stars called Aries (the Ram), in summer it has moved nearer to Gemini (the Twins), and in winter it is in the constellation called Capricorn (the Goat). The constellations that the sun moves through during the year are called the Zodiac. This is where the star signs used in astrology come from. If your star sign is Pisces (the Fishes) then it means that the sun was near the constellation Pisces when you were born.

Globes which are maps of the earth, like the ones you see in geography classrooms, are now much more popular than star globes. Two or three hundred years ago it was very common for people to have two globes: one celestial globe and one globe of the earth (called a terrestrial globe). However, this celestial globe was made nearly 700 years ago. Surprisingly, when this globe was made, globes of the earth hadn't even been invented.

Celestial globes came a long time before terrestrial globes because it was much more important for certain reasons for people to known about astronomy and the movements of the sun and the stars than about geography. This globe was made in Persia, the country now called Iran. Persia was (and Iran still is) an Islamic country where people were Muslims rather than Christians. Muslim countries such as Persia were extremely important places for the study of mathematics. In fact, in the Middle Ages and the Renaissance, people in Muslim countries in the Middle East knew far more about mathematics than people in Christian countries in Europe. They learnt a lot by reading old texts that came from the ancient Greeks and did lots of research into the mathematics of astronomy. Much of what we now know about mathematics first came from Islamic countries.

Mathematics and astronomy are important in Islamic countries for religious reasons. It is the movements of the sun and the moon that determine when Muslims should say prayers and when they should start fasting. For example, Muslims have to say 'Salat az-zuhr', the noonday prayer just after the sun has reached its highest point in the sky. Ramadan, the month of fasting, starts and ends as soon as the first crescent of the moon can be seen after a particular new moon. If you are good at mathematics then you stand a better chance of predicting when and where these things will happen.

This globe was probably used for both astronomy and astrology, either by a sultan, or by an important mathematical teacher in a caliph's palace, or by a priest in a large mosque. Astrology means study of the universe in terms of human activities and characteristics, whereas Astronomy is purely scientific study. At that time there was very little difference between Astrology and Astronomy.

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