Ancient Greek Astronomers and Mathematicians
Apollonius
Apollonius was a great mathematician who discovered conic sections (curves obtained by cutting a cone with a plane). He also explained the retrograde motion of the planets using epicycles.
Meton
Meton discovered the Metonic cycle (19 years = 235 lunar months) to predict eclipses of the sun and moon, which recur every 19 years. The sun cycle coincides with the moon cycle every 19 years with few exceptions.
Eratosthenes
Eratosthenes discovered the Sieve of Eratosthenes to calculate prime numbers. He invented the armillary sphere, used to determine the position of the stars. Due to this, he discovered the ecliptic obliquity, i.e., the inclination of Earth’s axis. He was the first to measure the circumference of the Earth. Working in Alexandria, he measured the shadow cast by an obelisk (during the summer solstice), achieving accuracy with few errors. He was less successful with Earth-Moon-Sun measurements.
Hipparchus
Hipparchus developed trigonometry and created the first trigonometric tables. However, these tables were based on chords, not angles, making them less useful for modern trigonometry but suitable for double angles. He compiled the first star catalog, rating stars by their brightness (magnitude). Thanks to this catalog, he discovered the precession of the equinoxes, the Earth’s wobble on its axis. This discovery led to the distinction between the sidereal year (the time it takes for the sun to return to the same point relative to the stars) and the tropical year (the time it takes for the sun to pass through the same cardinal point on Earth). He improved measurements of the Earth-Moon distance and the Earth’s axis. He also made significant contributions to geography, inventing the system of meridians.
Julius Caesar and Claudius Ptolemy
Julius Caesar
Julius Caesar reformed the Roman calendar, which originally consisted of ten months, alternating 30 and 31 days, starting in March when battles were planned. When discrepancies in dates became apparent, two months were added. By Caesar’s time, there was a two-month lag. With the help of Sosigenes, Caesar implemented the Julian calendar. August has 31 days to match July. Most importantly, he introduced the leap year.
Claudius Ptolemy
Claudius Ptolemy brought classical astronomy to its peak, particularly in his optical work, which established the laws of reflection and refraction, essential for his trigonometry. He wrote a book on music called Harmonics, defining octaves. He also contributed to geography, defining locations by altitude and latitude, using a meridian in the Canary Islands as a reference. His most important discoveries were in astronomy. He wrote the Almagest (Great Treatise), which served as the primary astronomical text for centuries. He improved upon the measurement of the Earth’s radius and invented the astrolabe to measure the height of stars. Notably, he created the first catalog of constellations in the northern hemisphere. He advocated for a geocentric model, where planets revolved around the Earth on epicycles, not deferents.
Aristarchus
Aristarchus was the first to propose a heliocentric model. While lacking evidence that the sun was larger than the Earth, he explained the alternation of day and night by suggesting that the Earth rotates on its axis. He also explained the Earth’s axial tilt. However, his heliocentric model was not accepted because it could not explain the Earth’s motion or stellar parallax.