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.
The development of astronomy by the Greek
and Hellenistic astronomers is considered,
by historians, to be a major phase in the
history of astronomy. Greek astronomy is characterized
from the start by seeking a rational, physical
explanation for celestial phenomena. Most
of the constellations of the northern hemisphere
derive from Greek astronomy, as are the names
of many stars, asteroids, and planets. It
was influenced by Egyptian and especially
Babylonian astronomy; in turn, it influenced
Indian, Arabic-Islamic and Western European
astronomy.
== Archaic Greek astronomy ==
References to identifiable stars and constellations
appear in the writings of Homer and Hesiod,
the earliest surviving examples of Greek literature.
In the oldest European texts, the Iliad and
the Odyssey, Homer has several astronomical
phenomena including solar eclipses. Eclipses
that can even permit the dating of these events
as the place is known and the calculation
of the time is possible, especially if other
celestial phenomena are described at the same
time.
In the Iliad and the Odyssey, Homer refers
to the following celestial objects:
the constellation Boötes
the star cluster Hyades
the constellation Orion
the star cluster Pleiades
Sirius, the Dog Star
the constellation Ursa Major
Hesiod, who wrote in the early 7th century
BC, adds the star Arcturus to this list in
his poetic calendar Works and Days. Though
neither Homer nor Hesiod set out to write
a scientific work, they hint at a rudimentary
cosmology of a flat earth surrounded by an
"Ocean River." Some stars rise and set (disappear
into the ocean, from the viewpoint of the
Greeks); others are ever-visible. At certain
times of the year, certain stars will rise
or set at sunrise or sunset.
Speculation about the cosmos was common in
Pre-Socratic philosophy in the 6th and 5th
centuries BC. Anaximander (c. 610 BC–c.
546 BC) described a cyclical earth suspended
in the center of the cosmos, surrounded by
rings of fire. Philolaus (c. 480 BC–c. 405
BC) the Pythagorean described a cosmos with
the stars, planets, Sun, Moon, Earth, and
a counter-Earth (Antichthon)—ten bodies
in all—circling an unseen central fire.
Such reports show that Greeks of the 6th and
5th centuries BC were aware of the planets
and speculated about the structure of the
cosmos.
Also, a more detailed description about the
cosmos, Stars, Sun, Moon and the Earth can
be found in the Orphism, which dates back
to the end of the 5th century BC, and it is
probably even older. Within the lyrics of
the Orphic poems we can find remarkable information
such as that the Earth is round, it has an
axis and it moves around it in one day, it
has three climate zones and that the Sun magnetizes
the Stars and planets.
=== The Planets in Early Greek Astronomy ===
The name "planet" comes from the Greek term
πλανήτης (planētēs), meaning "wanderer",
as ancient astronomers noted how certain lights
moved across the sky in relation to the other
stars. Five planets can be seen with the naked
eye: Mercury, Venus, Mars, Jupiter, and Saturn,
the Greek names being Hermes, Aphrodite, Ares,
Zeus and Cronus. Sometimes the luminaries,
the Sun and Moon, are added to the list of
naked eye planets to make a total of seven.
Since the planets disappear from time to time
when they approach the Sun, careful attention
is required to identify all five. Observations
of Venus are not straightforward. Early Greeks
thought that the evening and morning appearances
of Venus represented two different objects,
calling it Hesperus ("evening star") when
it appeared in the western evening sky and
Phosphorus ("light-bringer") when it appeared
in the eastern morning sky. They eventually
came to recognize that both objects were the
same planet. Pythagoras is given credit for
this realization.
== Eudoxan astronomy ==
In classical Greece, astronomy was a branch
of mathematics; astronomers sought to create
geometrical models that could imitate the
appearances of celestial motions. This tradition
began with the Pythagoreans, who placed astronomy
among the four mathematical arts (along with
arithmetic, geometry, and music). The study
of number comprising the four arts was later
called the Quadrivium.
Although he was not a creative mathematician,
Plato (427–347 BC) included the quadrivium
as the basis for philosophical education in
the Republic. He encouraged a younger mathematician,
Eudoxus of Cnidus (c. 410 BC–c. 347 BC),
to develop a system of Greek astronomy. According
to a modern historian of science, David Lindberg:
"In their work we find (1) a shift from stellar
to planetary concerns, (2) the creation of
a geometrical model, the "two-sphere model,"
for the representation of stellar and planetary
phenomena, and (3) the establishment of criteria
governing theories designed to account for
planetary observations".
The two-sphere model is a geocentric model
that divides the cosmos into two regions,
a spherical Earth, central and motionless
(the sublunary sphere) and a spherical heavenly
realm centered on the Earth, which may contain
multiple rotating spheres made of aether.
Plato's main books on cosmology are the Timaeus
and the Republic. In them he described the
two-sphere model and said there were eight
circles or spheres carrying the seven planets
and the fixed stars.
According to the "Myth of Er" in the Republic,
the cosmos is the Spindle of Necessity, attended
by Sirens and spun by the three daughters
of the Goddess Necessity known collectively
as the Moirai or Fates.
According to a story reported by Simplicius
of Cilicia (6th century), Plato posed a question
for the Greek mathematicians of his day: "By
the assumption of what uniform and orderly
motions can the apparent motions of the planets
be accounted for?" (quoted in Lloyd 1970,
p. 84). Plato proposed that the seemingly
chaotic wandering motions of the planets could
be explained by combinations of uniform circular
motions centered on a spherical Earth, apparently
a novel idea in the 4th century.
Eudoxus rose to the challenge by assigning
to each planet a set of concentric spheres.
By tilting the axes of the spheres, and by
assigning each a different period of revolution,
he was able to approximate the celestial "appearances."
Thus, he was the first to attempt a mathematical
description of the motions of the planets.
A general idea of the content of On Speeds,
his book on the planets, can be gleaned from
Aristotle's Metaphysics XII, 8, and a commentary
by Simplicius on De caelo, another work by
Aristotle. Since all his own works are lost,
our knowledge of Eudoxus is obtained from
secondary sources. Aratus's poem on astronomy
is based on a work of Eudoxus, and possibly
also Theodosius of Bithynia's Sphaerics. They
give us an indication of his work in spherical
astronomy as well as planetary motions.
Callippus, a Greek astronomer of the 4th century,
added seven spheres to Eudoxus' original 27
(in addition to the planetary spheres, Eudoxus
included a sphere for the fixed stars). Aristotle
described both systems, but insisted on adding
"unrolling" spheres between each set of spheres
to cancel the motions of the outer set. Aristotle
was concerned about the physical nature of
the system; without unrollers, the outer motions
would be transferred to the inner planets.
== Hellenistic astronomy ==
=== 
Planetary models and observational astronomy
===
The Eudoxan system had several critical flaws.
One was its inability to predict motions exactly.
Callippus' work may have been an attempt to
correct this flaw. A related problem is the
inability of his models to explain why planets
appear to change speed. A third flaw is its
inability to explain changes in the brightness
of planets as seen from Earth. Because the
spheres are concentric, planets will always
remain at the same distance from Earth. This
problem was pointed out in Antiquity by Autolycus
of Pitane (c. 310 BC).
Apollonius of Perga (c. 262 BC–c. 190 BC)
responded by introducing two new mechanisms
that allowed a planet to vary its distance
and speed: the eccentric deferent and the
deferent and epicycle. The deferent is a circle
carrying the planet around the Earth. (The
word deferent comes from the Greek fero φέρω
"to carry"and Latin ferro, ferre, meaning
"to carry.") An eccentric deferent is slightly
off-center from Earth. In a deferent and epicycle
model, the deferent carries a small circle,
the epicycle, which carries the planet. The
deferent-and-epicycle model can mimic the
eccentric model, as shown by Apollonius' theorem.
It can also explain retrogradation, which
happens when planets appear to reverse their
motion through the zodiac for a short time.
Modern historians of astronomy have determined
that Eudoxus' models could only have approximated
retrogradation crudely for some planets, and
not at all for others.
In the 2nd century BC, Hipparchus, aware of
the extraordinary accuracy with which Babylonian
astronomers could predict the planets' motions,
insisted that Greek astronomers achieve similar
levels of accuracy. Somehow he had access
to Babylonian observations or predictions,
and used them to create better geometrical
models. For the Sun, he used a simple eccentric
model, based on observations of the equinoxes,
which explained both changes in the speed
of the Sun and differences in the lengths
of the seasons. For the Moon, he used a deferent
and epicycle model. He could not create accurate
models for the remaining planets, and criticized
other Greek astronomers for creating inaccurate
models.
Hipparchus also compiled a star catalogue.
According to Pliny the Elder, he observed
a nova (new star). So that later generations
could tell whether other stars came to be,
perished, moved, or changed in brightness,
he recorded the position and brightness of
the stars. Ptolemy mentioned the catalogue
in connection with Hipparchus' discovery of
precession. (Precession of the equinoxes is
a slow motion of the place of the equinoxes
through the zodiac, caused by the shifting
of the Earth's axis). Hipparchus thought it
was caused by the motion of the sphere of
fixed stars.
=== Heliocentrism and cosmic scales ===
In the 3rd century BC, Aristarchus of Samos
proposed an alternate cosmology (arrangement
of the universe): a heliocentric model of
the solar system, placing the Sun, not the
Earth, at the center of the known universe
(hence he is sometimes known as the "Greek
Copernicus"). His astronomical ideas were
not well-received, however, and only a few
brief references to them are preserved. We
know the name of one follower of Aristarchus:
Seleucus of Seleucia.
Aristarchus also wrote a book On the Sizes
and Distances of the Sun and Moon, which is
his only work to have survived. In this work,
he calculated the sizes of the Sun and Moon,
as well as their distances from the Earth
in Earth radii. Shortly afterwards, Eratosthenes
calculated the size of the Earth, providing
a value for the Earth radii which could be
plugged into Aristarchus' calculations. Hipparchus
wrote another book On the Sizes and Distances
of the Sun and Moon, which has not survived.
Both Aristarchus and Hipparchus drastically
underestimated the distance of the Sun from
the Earth.
== Astronomy in the Greco-Roman and Late Antique
eras ==
Hipparchus is considered to have been among
the most important Greek astronomers, because
he introduced the concept of exact prediction
into astronomy. He was also the last innovative
astronomer before Claudius Ptolemy, a mathematician
who worked at Alexandria in Roman Egypt in
the 2nd century. Ptolemy's works on astronomy
and astrology include the Almagest, the Planetary
Hypotheses, and the Tetrabiblos, as well as
the Handy Tables, the Canobic Inscription,
and other minor works.
=== Ptolemaic astronomy ===
The Almagest is one of the most influential
books in the history of Western astronomy.
In this book, Ptolemy explained how to predict
the behavior of the planets, as Hipparchus
could not, with the introduction of a new
mathematical tool, the equant. The Almagest
gave a comprehensive treatment of astronomy,
incorporating theorems, models, and observations
from many previous mathematicians. This fact
may explain its survival, in contrast to more
specialized works that were neglected and
lost. Ptolemy placed the planets in the order
that would remain standard until it was displaced
by the heliocentric system and the Tychonic
system:
Moon
Mercury
Venus
Sun
Mars
Jupiter
Saturn
Fixed starsThe extent of Ptolemy's reliance
on the work of other mathematicians, in particular
his use of Hipparchus' star catalogue, has
been debated since the 19th century. A controversial
claim was made by Robert R. Newton in the
1970s. in The Crime of Claudius Ptolemy, he
argued that Ptolemy faked his observations
and falsely claimed the catalogue of Hipparchus
as his own work. Newton's theories have not
been adopted by most historians of astronomy.
A few mathematicians of Late Antiquity wrote
commentaries on the Almagest, including Pappus
of Alexandria as well as Theon of Alexandria
and his daughter Hypatia. Ptolemaic astronomy
became standard in medieval western European
and Islamic astronomy until it was displaced
by Maraghan, heliocentric and Tychonic systems
by the 16th century. However, recently discovered
manuscripts reveal that Greek astrologers
of Antiquity continued using pre-Ptolemaic
methods for their calculations (Aaboe, 2001).
== Influence on Indian astronomy ==
Hellenistic astronomy is known to have been
practiced near India in the Greco-Bactrian
city of Ai-Khanoum from the 3rd century BC.
Various sun-dials, including an equatorial
sundial adjusted to the latitude of Ujjain
have been found in archaeological excavations
there. Numerous interactions with the Mauryan
Empire, and the later expansion of the Indo-Greeks
into India suggest that some transmission
may have happened during that period.Several
Greco-Roman astrological treatises are also
known to have been imported into India during
the first few centuries of our era. The Yavanajataka
("Sayings of the Greeks") was translated from
Greek to Sanskrit by Yavanesvara during the
2nd century, under the patronage of the Western
Satrap Saka king Rudradaman I. Rudradaman's
capital at Ujjain "became the Greenwich of
Indian astronomers and the Arin of the Arabic
and Latin astronomical treatises; for it was
he and his successors who encouraged the introduction
of Greek horoscopy and astronomy into India."Later
in the 6th century, the Romaka Siddhanta ("Doctrine
of the Romans"), and the Paulisa Siddhanta
(sometimes attributed as the "Doctrine of
Paul" or in general the Doctrine of Paulisa
muni) were considered as two of the five main
astrological treatises, which were compiled
by Varahamihira in his Pañca-siddhāntikā
("Five Treatises"). Varahamihira wrote in
the Brihat-Samhita: "For, the Greeks are foreigners.
This science is well established among them.
Although they are revered as sages, how much
more so is a twice-born person who knows the
astral science."
== 
Sources for Greek astronomy ==
Many Greek astronomical texts are known only
by name, and perhaps by a description or quotations.
Some elementary works have survived because
they were largely non-mathematical and suitable
for use in schools. Books in this class include
the Phaenomena of Euclid and two works by
Autolycus of Pitane. Three important textbooks,
written shortly before Ptolemy's time, were
written by Cleomedes, Geminus, and Theon of
Smyrna. Books by Roman authors like Pliny
the Elder and Vitruvius contain some information
on Greek astronomy. The most important primary
source is the Almagest, since Ptolemy refers
to the work of many of his predecessors (Evans
1998, p. 24).
== Famous astronomers of antiquity ==
In addition to the authors named in the article,
the following list of people who worked on
mathematical astronomy or cosmology may be
of interest.
Anaxagoras
Archimedes
Archytas
Aristaeus
Aristarchus
Aristyllus
Conon of Samos
Democritus
Empedocles
Heraclides Ponticus
Hicetas
Hippocrates of Chios
Macrobius
Martianus Capella
Menelaus of Alexandria (Menelaus theorem)
Meton of Athens
Parmenides
Porphyry
Posidonius
Proclus
Thales
Theodosius of Bithynia
== See also ==
Antikythera mechanism
Greek mathematics
History of astronomy
Babylonian influence on Greek astronomy
== Notes ==
== 
References ==
Aaboe, Asger H. (2001). Episodes from the
Early History of Astronomy. New York: Springer.
ISBN 978-0-387-95136-2.
Dreyer, John L. E. (1953). A History of Astronomy
from Thales to Kepler (2nd ed.). New York:
Dover Publications. ISBN 978-0-486-60079-6.
Evans, James (1998). The History and Practice
of Ancient Astronomy. New York: Oxford University
Press. ISBN 978-0-19-509539-5.
Heath, Thomas L. (1913). Aristarchus of Samos.
Oxford: Clarendon Press.
Lindberg, David C. (2010). The Beginnings
of Western Science: The European Scientific
Tradition in Philosophical, Religious, and
Institutional Context, 600 B.C. to A.D. 1450
(2 ed.). Chicago: University of Chicago Press.
ISBN 978-0-226-48204-0.
Lloyd, Geoffrey E. R. (1970). Early Greek
Science: Thales to Aristotle. New York: W.
W. Norton & Co.
Neugebauer, Otto E. (1975). A History of Ancient
Mathematical Astronomy. Berlin: Springer.
ISBN 978-0-387-06995-1.
Newton, Robert R. (1977). The Crime of Claudius
Ptolemy. Baltimore: Johns Hopkins University
Press. ISBN 978-0-8018-1990-2.
Pedersen, Olaf (1993). Early Physics and Astronomy:
A Historical Introduction (2nd ed.). Cambridge:
Cambridge University Press. ISBN 978-0-521-40340-5.
Revello, Manuela (2013). "Sole, luna ed eclissi
in Omero", in TECHNAI 4. Pisa-Roma: Fabrizio
Serra editore. pp. 13–32.
Toomer, Gerald J. (1998). Ptolemy's Almagest.
Princeton: Princeton University Press. ISBN
978-0-691-00260-6.
== External links ==
Almagest Planetary Model Animations
MacTutor History of Mathematics Archive
