IN STARRY SKIES

Evening Post, Volume 137, Issue CXI, 10 June 1926, Page 16

IN STARRY SKIES

HIPPARCHUS

A GIANT AMONG THE ANXTZNTS

(By "Omega Centauri.")

Thffl achievemeuts of Hipparchns aro absolutely astounding. The more they are considered the greater is the marvel felt to be that any man, with only crude instruments such as ho possessed, could have inado his discoveries. We justly honour many of his predecessors, but he undoubtedly transcends them all. His astronomical genius was unrivalled amongst the ancients, and his work is comparable in value to that of Newton. He was the first to recognise the necessity of a long course of careful observations to set the science on a firm basis. He made numbers of these observations himself with astonishing skill, and he compared ther.. with those that had been handed down. The differences in some cases were traced to their source and this led to his greatest discoveries. To begin with, he solved tho problem that Plato had set to astronomers, to represent the motions of the sun, the moon, and thi planets entirely by uniform circular movements. This had been partly accomplished before, but he improved and perfected the method so thathc was able to prepare tables from which the positions of sun, moon, and planets could be found with tolerable accuracy at any time. The 3un was the simplest body to deal with. It had no retrograde motion like the planets, and was free from many of the complications whi ' render the moon so difficult to follow. But even the sun does not advance uniformly with reference to the stars, so Hipparchus concluded that the earth could not possibly be exactly in one centre of its orbit. It must appear to move fastest when nearest to the earth, and slowest when at the other extremity of the same diameter. Hipparchus determined the position and epoch oi perigee, the point where the sun and earth approach most closely. He also determined the eccentricity or distance of the earth from the centre of the sun's orbit. Now that planetary motions are recognised to be elliptic, the eccentricity is generally thought of as a pure number, namely, the ratio of the distances of any point on the curve from the focus and directrix of the conic. If, however, we take the semi-major-axis as'unity the distance between the focus and the centre of the ellipse is represented by exactly the same number. So the quantity that Hipparchus found was really the eccentricity of the ellipse in which the earth moves round the sun. The problem of the moon was much more difficult to deal with. The path of the moon is inclined to the ecliptic, and the points in which the two intersect is not by any means fixed. Ho soon found that these points or nodes were obviously in motion. Trom tho observations of six eclipses occurring in two groups of three separated by an interval of 180 years, Hipparchus determined the orbit of the moon with such accuracy that future eclipses could be predicted from his tables. He found the length of the lunar month to be 29 days 12 hours 44 minutes 2* seconds, which differs from the accepted value by less than one second. His estimates of the distances of the sun and moon wore more accurate than any previous ones. The two intervals of time which are of most interest and importance to man are the mean solar day and the tropical year. The former determines the alternations of light and darkness, the latter, which is the period from one spring equinox to the next, fixes the succession of the seasons. The tropical year is shorter than the sidereal year by about J) minutes. By comparing ono. of his own observations of the summer solstice with one made by Aristarchus 140 years before, he found that tho value 365 i days was too long. He made a correction of seven minutes, which was some four minutes too small, but the error was probably chiefly due to some inaccuracy of the observation of Aristarchus, for the observations of Hipparchus compared with modern ones make his value only 12 seconds f/rcater ■ than the truth. The length Is now known to a thousandth of a seclnd. He found that the seasons are unequal in length. He discovered that there were 941 days from the Vernal Equinox to the summer solstice, and 92* from the summer solstice to the autumnal equinox. So for 187 days the sun is north of the Equator. This leaves only 178 days for the sun to traverse the rest of its orbit. He accounted for this inequality by supposing the earth to be placed eccentrically within the sun s orbit, and ho determined the value ot this eccentricity. The obliquity of the ecliptic had been measured by Eratosthenes, and his results were verified by Hipparchus. About 370 B.C. Eudoxus wrote a full description of the principal constellations to be seen in the heavens. A hundred years later Aratus 'wrote a poem based on this description. This poem, in the original Greek, has survived, and has been translated into English, both in prose and verse. It is particularly interesting on account of the part it played in leading Hipparchus towards one of his greatest discoveries. He found that several statements in the poem did not agree with his own observations. It is now known that they were not even true at the time when Aratus wrote them, but that they were true when the constellations were named some 2000 years earlier. Hipparchus studied the question closely and his researches ultimately led him to discover the Precession of the Equinoxes. Another source of inspiration in this work was the appearance of an unknown object in the heavens in 134 B.C. It has generally been taken to be a "Nova or new star, but Dr. J. K. Eotheringham,' in a paper read to the Koyal Astronomical Society in 1919, gives good reasons for believing that it was a comet, which was also recorded by the Chinese. It must have been a very striking object, for it was compared with the sun in splendour and is said to have taken four hours to rise and set. Whatever it was, Hipparchus realised the importance of recording, the positions of all the heavenly bodies so as to detect with greater certainty any change which might occur. Ho- made a catalogue, the first that had ever been attempted. It included 1080 stars, whose positions he had determined. These were divided into six classes, according to their apparent brightness, the most brilliant of all being described as of the first magnitude, and the faintest seen with tho naked eye being of tho sixth. This historic catalogue has not been handed down in its original form, but fortunately Ptolemy reproduced the greater part of it. Of all the numerous works of Hipparchus, only a single one has como down to us. This is a commentary on the Phaenomena of Aratus and Eudoxus referred to above. This was republished at Florence in 1567. As Newton did long afterwards, Hipparchus found that the mathematics of his day was unable to deal with the problems he had to solvo, so he boldly invented new methods. We owe to

him both plane and spherical trigonometry, and the preparation of the first trigonometrical tables. He explained how tho position of places on the earth could be determined by latitude and longitude. The latter co-ordinate was the more difficult to fix. He used eclipses of tho moon to show the difference in the local times of places on the same parallel of latitude. He represented the positions of heavenly bodies on a planisphere. By 125 B.C. a comparison of the longitudes foun. for stars in his catalogue with those determined by Aristyllus and Timocharis 150 to 170 years before, showed )im that the Equator must bo slipping back round the ecliptic about 48 seconds of arc per year. This movement caused the annual arrival of the sun at either equinox to be a little earlier than it would otherwise have been, hence the term Precession of the Equinoxes. The actual rate was determined by Hipparchus within 5 per cent, of the truth. This motion is so slow that a vast period of about 26,000 years is required for one complete circuit, and that it was discovered and measured without instrument;) of precision, but with such remarkable accuracy, marks Hipparchus as one of tho greatest astronomers of all time. He was great in so many ways. He was a marvellous mathematician, a most skilful and patient obbserver, a keen theorist and philosopher, and a successful inventor of instruments. He found astronomy a maze of wild theories unsupported by observation or sound reasoning. He laid a sure roundation of accurate observation for others to build on. He invented the mathematical processes essential to the development of astronomy. He calculated the tables required for astronomical work. He improved the instrumental equipment then in use. He elaborated a geometrical thr-. -y to account for the strange apparent ".otions of sun, moon, and planets, a theory which was hardly improved until Copernicus changed the centre of the system from the earth to the sun, and Kepler and Newton replaced the geometrical by a dynamical explanation. Unfortunately, Hipparchus had no immediate successor. No advance was made for 300 years, and very little for another 1-00 years after that. The destruction of the works of Hipparchus was an irreparable loss to the world, and probably the greatest of all Ptolemy's contributions to science was to hand down some of the discoveries of Hipparchus.