Astronomers in the Middle Ages used the pointers of the seven star Great Bear
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All solar instruments, however, have one difficulty in common, and it cannot be overcome, even by the most careful adjustment. They are useless at night. In order to give K-K-Katie some better way of making dates than "When the Moon shines over the c-c-cowshed," astronomers in the Middle Ages used the pointers of the seven star Great Bear (Ursa Major, or the Big Dipper) like the hands of a clock.
Probably the custom originated far from astronomical circles, and was used to serve dubious purposes. We have Falstaff's word that "we who take purses go by the Moon and the seven stars, and not by Phoebus"; but even honest men had to be abroad sometimes after nightfall, and for their use a "nocturnal" was invented.
The nocturnal, like all these instruments except a sundial, requires a knowledge of the Sun's position in the ecliptic. The pointer attached to the movable time scale must be set to this position. Then the observer stands facing north, and arranges his face so that with one eye he can see the Pole Star through the hole in the center of the instrument. With the other eye he adjusts the long ruler until it is in line with the pointers of the Great Bear. The observation requires a squint, but a little practice makes the facial adjustment easy, and the effects are not permanently damaging. The weight of one end of the nocturnal keeps the instrument hanging vertical, and gives direction to the scale. The intersection of the long ruler with the time scale gives the reading. We have in our possession a photographic copy of a nocturnal, the original of which is in the Metropolitan Museum of Art in New York City. We have often tried our model, and with reasonable care we can determine the time to within ten minutes. Even K-K-Katie could not object to that much delay. Incidentally no member of our family has squintish eyes.
Both time and direction are calculated from the altitude of heavenly bodies; and a great many instruments to help the reckonings have been devised. By far the most famous of these devices was the "Astrolabe."
A great number of elementary American history books state on their first page, "Columbus was able to discover America because just before his time the astrolabe was introduced into Europe." "Reintroduced" would have been a more correct word, for, as one might expect, the original idea was probably due to a Greek. His name was Hipparchus, and he lived in Alexandria about 150 B. C. Since the word "astrolabe" means simply, "I take a star," the Greeks might have applied it to any instrument for astral observations; but the particular altimeter which Hipparchus invented has permanently acquired the name. It consists of a graduated round disk, surmounted by a handle and equipped with a ruler. When held by the handle the weight of the disc keeps the instrument vertical, and the appropriate lines in place. The ruler, known as an alidade, has sights for making observations and fiducial edge for reading the lines.
During the Dark Ages which followed the fall of Rome, the astrolabe was forgotten in western Europe; but Arabia and Persia-the countries which served as homes for lost scientists, instrument makers, and other heretics during the centuries--kept the design. They improved upon it; and in the thirteenth century, when Greek learning crept back to Europe by the roundabout path of the Mohammedans, they reintroduced the astrolabe to its original home. Immediately the navigators seized joyously upon the old invention, which was so new to them. Not a single journey of exploration during the fifteenth or sixteenth centuries was made without its aid. In one respect at least the sentence in American history books is perfectly correct. The astrolabe was a great help to Columbus when he so unwittingly discovered America.
It is hardly worthy of mention in a book on astronomy, but there was just one curious echo of the instrument in medieval Europe, long before Columbus, and nearly a hundred years before we find the device in practical use. The reference comes from a source so remote that we are tempted to speak of it as a curiosity if nothing more. Even the place it appears is unexpected; for it comes from the greatest of all medieval love stories--the history of Abélard and Héloïse. They gave the name "Astrolabe" to their infant son. That is so strange that we might almost believe they chose the word for its Greek meaning; but Greek was little known in France during the twelfth century, and the first faint sounds of Arabia were already coming across the seas. Perhaps they did take it from the Greek; but more probably they had seen one of the instruments, or heard tales of its wonders. Abélard was the finest philosopher of his day, and like most philosophers he may have been ahead of his time. We cannot tell. The instrument which Hipparchus designed was used only for taking altitudes; and instruments designed for that single use continued at least until the time of Columbus.
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