Page:Popular Science Monthly Volume 64.djvu/328

324 A page from Ptolemy's note-book may be transcribed. He was seeking the position of the bright star Regulus: "In the second year of Antoninus, the ninth day of Pharmauthi, the sun being near setting, the last division of Taurus being on the meridian (that is 5½ equinoctial hours after noon) the moon was in three degrees of Pisces by her distance from the sun (which was 92° 8'); and half an hour after, the sun having set, and the quarter of Gemini on the meridian, Regulus appeared, by the other circle of the astrolabe, 57½ degrees to the eastward of the moon in longitude." The position of the sun was known from the day of the year by the solar tables; the moon, at 5½ hours, was 92° 8' east of the sun; the moon's motion in half an hour was also known from the tables, and hence her position at 6 hours was determined; Regulus was at that time 57½ degrees east of the moon, and its place was thus fixed with respect to the sun. A modern note-book would give the year and day, and would record that Regulus crossed the meridian at a certain hour, minute, second and decimal of a second by the clock. The correction of the clock would be given as so many seconds and hundredths of a second. The sum of the clock-time and the correction is the position of the star. In Ptolemy's case it was known to half a degree (two minutes of time). A modern observation gives it with an error not above one tenth of a second; that is with an accuracy about 1,200 times greater.

Ptolemy's Almagest is, in essence, 'modern' in respect of the fact that its theories are designed to give quantitative results and are presented as general bases for special calculations. With a certain set of observations as data the desired results could be worked out in numbers. Tables for calculating future positions of the planets were also given and in Ptolemy's time the actual positions were fairly well represented by the predictions. As time went on, more accurate observations with better instruments, were made. The observed places of the planets did not agree with the predictions. The ingenuity of his disciples in the middle ages was taxed to improve the theory, and the tables of Ptolemy were supplanted in turn by the Hakemite tables of Ibn Yunus (about A. D. 1000), the Toledan tables of Arzachel (1080), the Alphonsine tables of Alphonso the Wise (1252) and others. Finally, in the first half of the sixteenth century it became evident that Ptolemy's theory was itself gravely at fault. It was the fortune of Copernicus to open a new way to scientific thought—to lay down a new theory of the world.

The details of the long history thus sketched out are only interesting to astronomers. We are here concerned with the main outlines alone.