Calculations, Observations, And Estimations Of Aristarchus Of Samos, Early Greek AstronomerSource: Encyclopedia Brittanica

Aristarchus of Samos

 

Greek astronomer

 

Actions

Written by

James Evans

Fact-checked by

The Editors of Encyclopaedia Britannica

Article History

 

Table of Contents

Moon, Earth, and Sun diagrammed in Aristarchus's On the Sizes and Distances of the Sun and Moon

 

Moon, Earth, and Sun diagrammed in Aristarchus's On the Sizes and Distances of the Sun and Moon Diagram of (from top to bottom) the Moon, Earth, and Sun in a 1572 edition of Aristarchus of Samos's On the Sizes and Distances of the Sun and Moon. Aristarchus also maintained that Earth rotates on its axis and revolves around the Sun.

(less)

Ask the Chatbot a Question

Aristarchus of Samos (born c. 310 BCE—died c. 230 BCE) was a Greek astronomer who maintained that Earth rotates on its axis and revolves around the Sun. On this ground, the Greek philosopher Cleanthes the Stoic declared in his Against Aristarchus that Aristarchus ought to be indicted for impiety “for putting into motion the hearth of the universe.”

 

Aristarchus’s work on the motion of Earth has not survived, but his ideas are known from references by the Greek mathematician Archimedes, the Greek biographer Plutarch, and the Greek philosopher Sextus Empiricus. Archimedes said in his Sand-Reckoner that Aristarchus had proposed a new theory which, if true, would make the universe vastly larger than was then believed. (This is because a moving Earth should produce a parallax, or annual shift, in the apparent positions of the fixed stars, unless the stars are very far away indeed.)

 

Quick Facts

Born: c. 310 BCE

Died: c. 230 BCE

Subjects Of Study: Earth heliocentrism rotation

See all related content

In the 16th century Aristarchus was an inspiration for Polish astronomer Nicolaus Copernicus’s work. In his manuscript of Six Books Concerning the Revolutions of the Heavenly Orbs (1543), Copernicus cited Aristarchus as an ancient authority who had espoused the motion of Earth. However, Copernicus later crossed out this reference, and Aristarchus’s theory was not mentioned in the published book.

 

View of the Andromeda Galaxy (Messier 31, M31).

Britannica Quiz

Astronomy and Space Quiz

Aristarchus’s only extant work is On the Sizes and Distances of the Sun and Moon, the oldest surviving geometric treatment of this problem. Aristarchus takes as premises that

 

(1) as observed during a lunar eclipse, the diameter of Earth’s shadow is twice the diameter of the Moon;

(2) the Moon and Sun are each 2 degrees in angular diameter; and

(3) at the time of quarter Moon, the angular distance between the Moon and the Sun is 87 degrees.

Using premise 3, Aristarchus showed that the Sun is between 18 and 20 times farther away from Earth than the Moon is. (The actual ratio is about 390.) Using this result and premises 1 and 2 in a clever geometric construction based on lunar eclipses, he obtained values for the sizes of the Sun and Moon. He found the Moon’s diameter to be between 0.32 and 0.40 times the diameter of Earth and the Sun’s diameter to be between 6.3 and 7.2 times the diameter of Earth. (The diameters of the Moon and the Sun compared with that of Earth are actually 0.27 and 109, respectively.)

 

In Aristarchus’s day the geometric method was considered more important than numerical measurements. His premise 1 is reasonably accurate. Premise 2 overestimates the Moon’s angular diameter by a factor of four, which is puzzling, since this is an easy measurement to make. (In a later publication, Aristarchus gave the angular size of the Moon as half a degree, which is about right, but he apparently did not modify his earlier work.) Premise 3 was probably not based on measurement but rather on an estimate; it is equivalent to assuming that the time from first quarter Moon to third quarter Moon is one day longer than the time from third quarter to first quarter. The true angle between Sun and Moon at the time of quarter Moon is less than 90 degrees by only 9 minutes of arc—a quantity impossible to measure in antiquity.

 

Later Greek astronomers, especially Hipparchus and Ptolemy, refined Aristarchus’s methods and arrived at very accurate values for the size and distance of the Moon. However, because of the influence of premise 3, all ancient results grossly underestimated the size and distance of the Sun. Aristarchus’s 19:1 ratio nevertheless remained more or less standard until the 17th century.

 

Are you a student?

Get a special academic rate on Britannica Premium.

Subscribe

James Evans

Science

Astronomy

heliocentrism

 

astronomy

 

Actions

Also known as: heliocentric system, heliocentric theory

Written and fact-checked by

The Editors of Encyclopaedia Britannica

Last Updated: Oct 7, 2024 • Article History

 

Table of Contents

Ask the Chatbot a Question

heliocentrism, a cosmological model in which the Sun is assumed to lie at or near a central point (e.g., of the solar system or of the universe) while the Earth and other bodies revolve around it. In the 5th century BC the Greek philosophers Philolaus and Hicetas speculated separately that the Earth was a sphere revolving daily around some mystical “central fire” that regulated the universe. Two centuries later, Aristarchus of Samos extended this idea by proposing that the Earth and other planets moved around a definite central object, which he believed to be the Sun.

 

The heliocentric, or Sun-centred, model of the solar system never gained wide support because its proponents could not explain why the relative positions of the stars seemed to remain the same despite the Earth’s changing viewpoints as it moved around the Sun. In the 2nd century AD, Claudius Ptolemy of Alexandria suggested that this discrepancy could be resolved if it were assumed that the Earth was fixed in position, with the Sun and other bodies revolving around it. As a result, Ptolemy’s geocentric (Earth-centred) system dominated scientific thought for some 1,400 years.

 

Key People: Nicolaus Copernicus Aristarchus of Samos

Related Topics: solar system Tychonic system Copernican system geocentric model

(Show more)

See all related content

In 1444 Nicholas of Cusa again argued for the rotation of the Earth and of other heavenly bodies, but it was not until the publication of Nicolaus Copernicus’s De revolutionibus orbium coelestium libri VI (“Six Books Concerning the Revolutions of the Heavenly Orbs”) in 1543 that heliocentrism began to be reestablished. Galileo Galilei’s support of this model resulted in his famous trial before the Inquisition in 1633. See also geocentric model; Ptolemaic system; Tychonic system.

Done