Cleomedes (Greek : Κλεομήδης) was a Greek astronomer who is known chiefly for his book On the Circular Motions of the Celestial Bodies (Κυκλικὴ θεωρία μετεώρων), also known as The Heavens (Latin : Caelestia).
His birth and death dates are not known—historians have suggested that he wrote his work sometime between the mid-1st century BCE and 400 CE. The earlier estimates rely on the fact that Cleomedes refers extensively in his writing to the work of mathematician and astronomer Posidonius of Rhodes (135 BC-51 BCE), and yet seemingly not at all to the work of Ptolemy (85-165 CE). (Cleomedes also refers to Aristotle (384-322 BCE), Pytheas of Massalia (325 BCE), Aratus (310-240 BCE), Eratosthenes (276-195 BCE), and Hipparchus (190-120 BCE).). These conclusions have been challenged on the grounds that Cleomedes' work was in relatively elementary astronomy, and that reference to Ptolemy would not necessarily be expected. The 20th century mathematician Otto Neugebauer, however, looked closely at the astronomical observations made by Cleomedes, and concluded that a date of 371 CE (±50 years) better explains what is found there. Neugebauer's estimate has been challenged on the grounds that Cleomedes makes observational errors with enough frequency that there is difficulty in deciding which observations to trust for the purpose of dating his work.
The book for which Cleomedes is known is a fairly basic astronomy textbook in two volumes. His purpose in writing seems to have been as philosophical as it was scientific—he spends an extensive amount of time criticizing the scientific ideas of the Epicureans.
Cleomedes' book is valued primarily for preserving, apparently verbatim, much of Posidonius' writings on astronomy (none of Posidonius' books have survived to the modern day). Cleomedes is accurate in some of his remarks on lunar eclipses, especially his conjecture that the shadow on the Moon suggests a spherical Earth. He also remarks presciently that the absolute size of many stars may exceed that of the Sun (and that the Earth might appear as a very small star, if viewed from the surface of the Sun).
This book is the original source for the well-known story of how Eratosthenes measured the Earth's circumference. Many modern mathematicians and astronomers believe the description to be reasonable (and believe Eratosthenes' achievement to be one of the more impressive accomplishments of ancient astronomy).
Cleomedes deserves credit for the earliest clear statement of the apparent distance explanation of the Sun Illusion or Moon Illusion. He argued that the sun appeared farther away on the horizon than in the zenith, and therefore larger (since its angular size was constant). He attributed this explanation to Posidonius.
As a disciple of Posidonius, Cleomedes noted some elementary qualitative properties of refraction, such as the bending of a ray toward the perpendicular in passing from a less dense to a more dense medium, and suggested that due to atmospheric refraction, the Sun and its rainbow may be visible when the Sun is below the horizon.
Cleomedes is now memorialized by the crater Cleomedes in the northeastern portion of the visible Moon.
The astronomical unit is a unit of length, roughly the distance from Earth to the Sun and equal to about 150 million kilometres. The actual distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once each year. The AU was originally conceived as the average of Earth's aphelion and perihelion; however, since 2012 it has been defined as exactly 149597870700 m.
Hipparchus of Nicaea was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry but is most famous for his incidental discovery of precession of the equinoxes.
Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, cosmological, calendrical, and astrological beliefs and practices of prehistory: vestiges of these are still found in astrology, a discipline long interwoven with public and governmental astronomy. It was not completely separated in Europe during the Copernican Revolution starting in 1543. In some cultures, astronomical data was used for astrological prognostication. The study of astronomy has received financial and social support from many institutions, especially the Church, which was its largest source of support between the 12th century to the Enlightenment.
Claudius Ptolemy was a mathematician, astronomer, geographer and astrologer who wrote several scientific treatises, three of which were of importance to later Byzantine, Islamic and Western European science. The first is the astronomical treatise now known as the Almagest, although it was originally entitled the Mathematical Treatise and then known as The Great Treatise. The second is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world. The third is the astrological treatise in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day. This is sometimes known as the Apotelesmatiká (Ἀποτελεσματικά) but more commonly known as the Tetrábiblos from the Koine Greek (Τετράβιβλος) meaning "Four Books" or by the Latin Quadripartitum.
Eratosthenes of Cyrene was a Greek polymath: a mathematician, geographer, poet, astronomer, and music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. His work is comparable to what is now known as the study of geography, and he introduced some of the terminology still used today.
Aristarchus of Samos was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the center of the known universe with the Earth revolving around it. He was influenced by Philolaus of Croton, but Aristarchus identified the "central fire" with the Sun, and he put the other planets in their correct order of distance around the Sun. Like Anaxagoras before him, he suspected that the stars were just other bodies like the Sun, albeit farther away from Earth. His astronomical ideas were often rejected in favor of the geocentric theories of Aristotle and Ptolemy. Nicolaus Copernicus attributed the heliocentric theory to Aristarchus.
In astronomy, the geocentric model is a superseded description of the Universe with Earth at the center. Under the geocentric model, the Sun, Moon, stars, and planets all orbited Earth. The geocentric model was the predominant description of the cosmos in many ancient civilizations, such as those of Aristotle in Classical Greece and Ptolemy in Roman Egypt.
Posidonius "of Apameia" or "of Rhodes", was a Greek Stoic philosopher, politician, astronomer, geographer, historian, mathematician, and teacher native to Apamea, Syria.
Conon of Samos was a Greek astronomer and mathematician. He is primarily remembered for naming the constellation Coma Berenices.
The Almagest is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy. One of the most influential scientific texts of all time, it canonized a geocentric model of the Universe that was accepted for more than 1200 years from its origin in Hellenistic Alexandria, in the medieval Byzantine and Islamic worlds, and in Western Europe through the Middle Ages and early Renaissance until Copernicus. It is also a key source of information about ancient Greek astronomy.
The earliest documented mention of the spherical Earth concept dates from around the 5th century BC, when it was mentioned by ancient Greek philosophers. In the 3rd century BC, Hellenistic astronomy established the roughly spherical shape of the Earth as a physical fact and calculated the Earth's circumference. This knowledge was gradually adopted throughout the Old World during Late Antiquity and the Middle Ages. A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's circumnavigation (1519–1522).
Geodesy (/dʒiːˈɒdɨsi/), also named geodetics, is the scientific discipline that deals with the measurement and representation of the Earth. The history of geodesy began in pre-scientific antiquity and blossomed during the Age of Enlightenment.
Eratosthenes crater is a relatively deep lunar impact crater that lies on the boundary between the Mare Imbrium and Sinus Aestuum mare regions. It forms the western terminus of the Montes Apenninus mountain range. It is named after ancient Greek astronomer Eratosthenes of Cyrene, who estimated the circumference of the Earth, and the distance from the Earth to the Sun.
Geminus of Rhodes, was a Greek astronomer and mathematician, who flourished in the 1st century BC. An astronomy work of his, the Introduction to the Phenomena, still survives; it was intended as an introductory astronomy book for students. He also wrote a work on mathematics, of which only fragments quoted by later authors survive.
Lunar theory attempts to account for the motions of the Moon. There are many small variations in the Moon's motion, and many attempts have been made to account for them. After centuries of being problematic, lunar motion is now modeled to a very high degree of accuracy.
Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman, and Late Antiquity eras. It is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the language of scholarship throughout the Hellenistic world following the conquests of Alexander. This phase of Greek astronomy is also known as Hellenistic astronomy, while the pre-Hellenistic phase is known as Classical Greek astronomy. During the Hellenistic and Roman periods, much of the Greek and non-Greek astronomers working in the Greek tradition studied at the Musaeum and the Library of Alexandria in Ptolemaic Egypt.
Babylonian astronomy was the study or recording of celestial objects during early history Mesopotamia. These records can be found on Sumerian clay tablets, inscribed in cuneiform, dated to around 1000 BCE.
Copernican heliocentrism is the name given to the astronomical model developed by Nicolaus Copernicus and published in 1543. This model positioned the Sun near the center of the Universe, motionless, with Earth and the other planets orbiting around it in circular paths, modified by epicycles, and at uniform speeds. The Copernican model displaced the geocentric model of Ptolemy that had prevailed for centuries, which had placed Earth at the center of the Universe. Copernican heliocentrism is often regarded as the launching point to modern astronomy and the Scientific Revolution.
Egyptian astronomy begins in prehistoric times, in the Predynastic Period. In the 5th millennium BCE, the stone circles at Nabta Playa may have made use of astronomical alignments. By the time the historical Dynastic Period began in the 3rd millennium BCE, the 365-day period of the Egyptian calendar was already in use, and the observation of stars was important in determining the annual flooding of the Nile.
Earth's circumference is the distance around the Earth. Measured around the poles, the circumference is 40,007.863 km (24,859.734 mi). Measured around the equator, it is 40,075.017 km (24,901.461 mi).