Filters:
People: John Churchill, 1st Duke of Marlborough

Ptolemy of Alexandria, a Greek astronomer and …

Years: 135 - 135

Ptolemy of Alexandria, a Greek astronomer and geographer about whose life little is known, makes astronomical observations from Alexandria (where he probably spends most of his life) beginning in about 127.

Ptolemy in about 135 records the brightness and positions of one thousand and twenty-two stars in a magnificent astronomical synthesis, the Almagest ("The Greatest Compilation"—probably the earliest of his works).

Compiling the astronomical knowledge of the past with his own research, he presents in detail his mathematical theory of the motions of the Sun, Moon, and planets.

In each of the book’s thirteen chapters, Ptolemy explains a specific aspect of his geocentric world system.

Accepting the solar theory of his predecessor Hipparchus, Ptolemy improves on the lunar theory by accounting for the Moon’s chief irregularity, called the equation of the center, which allows for the prediction of eclipses.

He also discovers and corrects another irregularity, evection, at other points of the Moon's orbit by employing an epicycle on a movable eccentric deferent, whose center revolves around the Earth.

Developing a further refinement known as prosneusis, Ptolemy is able to predict the place of the Moon within 10 1/6 feet, or 1/6 degrees, of arc in the sky.

In his most original contribution, Ptolemy presents details for the motions of each of the planets.

Because the apparent motions of the planets in the sky are clearly not circular, Ptolemy bases his complicated explanatory geometric model on circular motions of planets around points going in larger circles within the framework of the basic Earth-centered system supplied by Aristotle.

Ptolemy’s epicycle system (though eventually—fourteen centuries later—proved wrong) predicts, with relatively good accuracy, the apparent positions of the planets over several years.

Ptolemy also introduces a kind of trigonometry in terms of a table of chords that was equivalent to a sine table.

The Almagest, which contains formulas for the sines and cosines of the sums and differences of angles, also provides the rudiments of spherical trigonometry.

In the chapters on planetary motion, Ptolemy introduces the concept of uniform planetary motion around a point eccentric to the Earth, known as the equant point.

Related Events

Filter results