Aristarchus of Samos
-
Who was Aristarchus of Samos?
Aristarchus of Samos (c. 310–230 BCE) was a Greek astronomer who theorized that the Earth rotates on its axis and revolves around the Sun. Because of this, he was accused of impiety.
Although his work on the motion of the Earth is lost, his ideas are known through references made by Archimedes, Plutarch, and Sextus Empiricus. His only surviving work, On the Sizes and Distances of the Sun and Moon, is the oldest known geometric analysis of this problem. In the 16th century, Nicolaus Copernicus cited Aristarchus as an inspiration for his own work.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
When and where did Aristarchus live?
Aristarchus of Samos, a Greek astronomer, lived from approximately 310 to 230 BCE. He is known for proposing that the Earth rotates on its axis and revolves around the Sun. His ideas were later referenced by Nicolaus Copernicus in the 16th century. His only surviving work is On the Sizes and Distances of the Sun and Moon, which uses geometry to explore the sizes and distances of the Sun and Moon relative to Earth.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What is Aristarchus known for in astronomy?
Aristarchus of Samos (c. 310–230 BCE) was a Greek astronomer known for proposing that Earth rotates on its axis and revolves around the Sun. This heliocentric view was not widely accepted in his time, as it failed to explain why the positions of stars appeared constant despite Earth's movement.
Aristarchus's ideas are known through references by Archimedes, Plutarch, and Sextus Empiricus. Archimedes noted that Aristarchus's theory implied a much larger universe to account for the lack of observed parallax, or shift, in the positions of stars due to Earth's motion.
His only surviving work, On the Sizes and Distances of the Sun and Moon, uses geometry to estimate these celestial sizes and distances. While his measurements were not entirely accurate due to the limitations of available instruments, his methods influenced later astronomers.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What was Aristarchus’s model of the solar system?
Aristarchus of Samos, a Greek astronomer, theorized that the Earth rotates on its axis and revolves around the Sun, which is a heliocentric model. His ideas were known through references by Archimedes, Plutarch, and Sextus Empiricus because his work on the motion of the Earth did not survive. Although his heliocentric model did not gain wide support, Aristarchus inspired Nicolaus Copernicus in the 16th century.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How did Aristarchus estimate the distances and sizes of the Sun and Moon?
Aristarchus of Samos estimated the sizes and distances of the Sun and Moon using geometric methods. His calculations appear in On the Sizes and Distances of the Sun and Moon, the oldest surviving geometric work on this topic.
Aristarchus's method was valid, but his initial premises included questionable numerical values. He presumed that during a quarter Moon, the angle between the Sun and Moon, observed from Earth, was 87 degrees, and that the angular size of the Sun or Moon was 2 degrees. From this, he determined the Sun's distance to be 19 times the Moon's distance from Earth.
Using these values, Aristarchus calculated the Moon's diameter to be 0.32 to 0.4 times the Earth's diameter and the Sun's diameter to be 6.3 to 7.2 times the Earth's diameter.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why was Aristarchus's heliocentric theory not widely accepted in his time?
Aristarchus of Samos (c. 310–230 BCE) was a Greek astronomer who proposed that the Earth rotates on its axis and revolves around the Sun. This heliocentric model did not gain wide support at the time because its proponents could not explain why the stars' positions appeared constant despite Earth's changing viewpoint as it moved around the Sun.
In addition, the Greek philosopher Cleanthes the Stoic, in his Against Aristarchus, stated that Aristarchus ought to be indicted for impiety "for putting into motion the hearth of the universe".
Claudius Ptolemy suggested in the 2nd century CE that the discrepancy could be resolved if the Earth was fixed, with the Sun and other bodies revolving around it. As a result, Ptolemy’s geocentric (Earth-centered) system dominated scientific thought for about 1,400 years.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
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.)
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.
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
- (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.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
