Without doubt and second to none from the ancient times, Hipparchus of Rhodes is the unquestionable father of modern
astronomy. Although very little remains of his work due to the destructive fire at the Library of Alexandria, many
astronomers and historians of consequent generations fortunately described in detail the work of Hipparchus, particularly
Ptolemeus, thus allowing his exhaustive work to survive and, where possible, to be improved upon. It is somewhat
ironic that his sole work to survive, "Περί των Αράτου και Ευδόξου φαινομένων εξηγήσεις, βιβλία τρία" (ie.
"Commentary on Aratus and Eudoxus"), also happens to be the least significant of his seventeen published works.
Perhaps the critical event that set Hipparchus on a revolution was his observation in 134 BC of a supernova of a star which was not even visible the previous night. It is at this point that he realized that stars are not eternal but some are born whereas other die. To this end, he started on a systematic and methodical but massive project of cataloguing the stars of the sky. This cataloguing effort not only involved the precise location of each star but included a measure of its brightness as well so as to enable future astronomers to not only identify the movement of stars but also the change in their brightness! To this end, around 127 BC, he produced his catalog of the 1039 brightest stars and which formed the foundation, for example, for Ptolemy and many others in consequent generations.
Using a device of his own creation, the astrolab, he not only developed stereographic projection as a means to accurately describe the location and measurement of the stars but he deduced the third key motion involving earth, namely, the precession of the equinoxes (the other two being its orbit around the sun and its axis of rotation). Furthermore, he derived the length of the year to be 365.236667 days (modern estimates are for 365.242217 days or greater than Hipparchus' estimate by 6.5 minutes); he derived the length of the four seasons (92.5 days for each of spring and summer; 88.125 days for fall; and 90.125 days for winter) which are virtually identical to modern-day estimates; he computed the tilt in the earth's axis of rotation relative to its orbital plane to be 23° 51' and not 23° 28' as computed earlier by Eratosthenes (modern-day estimates are for 23° 48'); he computed the length of a synodic month (29 days, 11 hours, 44 minutes and 3.33 seconds) with great accuracy; he computed the distance between the earth and the moon to be 59 earth-radii (actual is 60.40) and the diameter of the moon to be approximately 0.33 that of the earth (actual is 0.27); he computed the diameter of the earth at its widest point to within 40 km of the modern-day value; he computed the annual rate of precession of the equinoxes to be 46" (modern-day estimate is 50.26"); and, for example, used various measurements to establish the fact that earth is generally spherical in nature.
His work on stereographic projection alluded to above also had serious implications and developments for the fields of geography and cartography, for it provided the foundation for these disciplines. Similarly, his work on chords was the first documented attempt at trigonometry and is often credited for having invented trigonometry. Furthermore, his studies of weather patterns through observation enabled him to provide forecasts which impressed his peers.
Hipparchus of Rhodes has been honored with the naming of a crater near the center of the moon and just below the lunar equator (see Rukl: 44) after him as well as a crater on the Martian surface. Lying just southeast of Sinus Medii, crater Hipparchus (5.1° S, 5.2° E), measuring 138-km in diameter, is characterized with a very flat floor and countless craterlets and small hills. A pair of nearly parallel rilles near the northwest quadrant as well as an eroded subcrater to the south are perhaps the most recognized formations. Crater Hipparchus is best observed at first quarter or just before third quarter with high-power images on this site available shortly.
For a thorough discussion on the life and ingenious work of Hipparchus of Rhodes, the reader is referred to: