Page:Popular Science Monthly Volume 64.djvu/325

Rh we now know, twenty thousand times as far from us as Saturn, and it is this fact—which was not finally established till 1837—that explains why the miniature apparent orbits of the stars were not seen by the Greeks or by their successors. They were too minute to be discoverable.

All the important writings of Hipparchus, who lived in the second century B. C., are lost, and the doctrines of this 'most truth-loving and labor-loving man' are known to us only through Ptolemy, his expositor and ardent admirer. Hipparchus was an indefatigable observer, a mathematician of tact and insight, an astronomer of original and profound genius. By his own observations, made at Rhodes (188–127 B. C.), he fixed the positions (the celestial longitudes and latitudes) of the principal fixed stars. Comparing their present places with their past positions as determined by Timocharis and Aristillus, he discovered that backward motion of the equinoctial points which causes the epoch of the sun's passage through the equinox to recur earlier and earlier each year—the precession of the equinoxes—and fixed its probable annual amount. Comparing his own determinations of the date of the vernal equinoctial passage of the sun with those of Aristarchus, he determined the length of the year with accuracy. It is by systematic comparisons of the sort that many of his discoveries were made.

It is very noteworthy that he gives not only his results, but likewise an estimate of their probable errors. His observations of the time of the sun's arriving at a solstice might be erroneous, he says, by about three fourths of a day; at an equinox by about one fourth. Comparisons made in this systematic fashion, and estimates of error of this sort, we are apt to think of as 'modern.' Certainly they are not characteristic of observational astronomy till the eighteenth century, two thousand years after Hipparchus showed the way. Some of his most important researches related to measures of time. What was the length of the year? Were all years of the same length? His observations showed him no difference between one year and another. It is interesting to note how he formulates his conclusions. He does not say that all years are, without doubt, of one and the same length; he asserts simply that the differences, if any, must be very small, so small that his observations are not delicate enough to detect them.

In the year 134 B. C. a new star suddenly appeared in Scorpio, and Hipparchus began the formation of a catalogue of stars visible to him. With such a conspectus of the present state of the sky no new appearances could subsequently occur without detection. His catalogue gave the position and magnitude (brightness) of 1,080 stars for the epoch 128 B. C., and arranged them in the constellation figures that have