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Posidonius calculated the Earth's circumference by reference to the position of the star Canopus.As explained by Cleomedes, Posidonius observed Canopus on but never above the horizon at Rhodes, while at Alexandria he saw it ascend as far as 7 + 1 ⁄ 2 degrees above the horizon (the meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes).
Eratosthenes (c. 276 – c. 194/195 BC), a Greek mathematician who calculated the circumference of the Earth and also the distance from the Earth to the Sun. Hipparchus (c. 190 – c. 120 BC), a Greek mathematician who measured the radii of the Sun and the Moon as well as their distances from the Earth. On the Sizes and Distances
Hipparchus (c. 190 – c. 120 BC), a Greek mathematician who measured the radii of the Sun and the Moon as well as their distances from the Earth. Posidonius ( c. 135 – c. 51 BC ), a Greek astronomer and mathematician who calculated the circumference of the Earth.
On Sizes and Distances (of the Sun and Moon) (Greek: Περὶ μεγεθῶν καὶ ἀποστημάτων [ἡλίου καὶ σελήνης], romanized: Peri megethon kai apostematon) is a text by the ancient Greek astronomer Hipparchus (c. 190 – c. 120 BC) in which approximations are made for the radii of the Sun and the Moon as well as their distances from the Earth.
He measured the Earth's circumference by reference to the position of the star Canopus. His measure of 240,000 stadia translates to 24,000 miles (39,000 km), close to the actual circumference of 24,901 miles (40,074 km). [11] He was informed in his approach by Eratosthenes, who a century earlier used the elevation of the Sun at different latitudes.
In about 90 BC, Posidonius estimated the distance from the Earth to the Sun (see astronomical unit) to be 9,893 times the Earth's radius. This was still too small by half. In measuring the size of the Sun, however, he reached a figure larger and more accurate than those proposed by other Greek astronomers and Aristarchus of Samos. [39]
Aristarchus of Samos (/ ˌ æ r ə ˈ s t ɑːr k ə s /; Ancient Greek: Ἀρίσταρχος ὁ Σάμιος, Aristarkhos ho Samios; c. 310 – c. 230 BC) was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the center of the universe, with the Earth revolving around the Sun once a year and rotating about its axis once a day.
A parallel later ancient measurement of the size of the Earth was made by another Greek scholar, Posidonius (c. 135 – 51 BC), using a similar method as Eratosthenes. Instead of observing the Sun, he noted that the star Canopus was hidden from view in most parts of Greece but that it just grazed the horizon at Rhodes.