Cleomedes () was a Greek

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

who is known chiefly for his book ''On the Circular Motions of the Celestial Bodies'' (Κυκλικὴ θεωρία μετεώρων), also known as ''The Heavens'' ().

Placing his work chronologically

His birth and death dates are not known—historians have suggested that he wrote his work sometime between the mid-1st century BC and 400 AD. The earlier estimates rely on the fact that Cleomedes refers extensively in his writing to the work of mathematician and astronomer Posidonius of

Rhodes Rhodes (; ) is the largest of the Dodecanese islands of Greece and is their historical capital; it is the List of islands in the Mediterranean#By area, ninth largest island in the Mediterranean Sea. Administratively, the island forms a separ ...

(c. 135 – c. 51 BC), and yet seemingly not at all to the work of

Ptolemy Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...

(c. 100 – c. 170 AD). (Cleomedes also refers to

Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...

(384–322 BC), Pytheas of Massalia (310–306 BC), Aratus (c. 315/310 BC – 240 BC), Eratosthenes (276–195 BC), and

Hipparchus Hipparchus (; , ; BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hippar ...

(c. 190 – c. 120 BC). 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 AD (±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.

''On the Circular Motions of the Celestial Bodies''

The book for which Cleomedes is known is a fairly basic astronomy

textbook A textbook is a book containing a comprehensive compilation of content in a branch of study with the intention of explaining it. Textbooks are produced to meet the needs of educators, usually at educational institutions, but also of learners ( ...

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 The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

suggests a spherical

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

. 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.

Optics

As a disciple of Posidonius, Cleomedes noted some elementary qualitative properties of

refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...

, 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. Carl Benjamin Boyer, ''The Rainbow: From Myth to Mathematics'' (1959)

Legacy

Cleomedes is now memorialized by the crater Cleomedes in the northeastern portion of the visible Moon.

References

External links