Theories of Planetary Motion

Observations and Theories of Planetary Motion

Observations and Theories of the Motion of Saturn

Observations and Theories of the Motion of Jupiter

Observations and Theories of the Motion of Mars

Observations and Theories of the Motion of Venus

Observations and Theories of the Motion of Mercury

Babylonian Planetary Observations in Ptolemy’s Almagest

Ptolemy’s Almagest mentions the details of three planetary observations that were probably derived from observations made in Babylon. They are dated according to the Babylonian calendar and the positions of the planets are given in Babylonian angular measures with respect to Babylonian ‘Normal Stars’.

  1. In the 67th year of the Chaldaean calendar, early morning of 5 Apellaios (504th year from Nabonassar, 27/28 Thoth of the Egyptian calendar = 18/19 November 245 BC), the planet Mercury was observed about ½ cubit [1.2º] above the star beta Scorpii (Almagest IX 7).
  2. In the 75th year of the Chaldaean calendar, early morning of 14 Dios (512th year from Nabonassar, 9/10 Thoth of the Egyptian calendar = 29/30 October 237 BC), the planet Mercury was observed at maximum western elongation about ½ cubit [1.2º] above the star alpha Librae (Almagest IX 7) – the close proximity of the planet Mars to alpha Librae on that date was apparently not noted in Ptolemy’s source.
  3. In the 82nd year of the Chaldaean calendar, evening of 5 Xantihikos (519th year from Nabonassar, 14 Tybi of the Egyptian calendar = 1 March 229 BC), the planet Saturn was observed about 2 digits [0.2º] below the star gamma Virginis (Almagest XI 7).

Babylonian ‛Telescopes’ and the Phases of Venus and Mars

A few cuneiform texts describe the planets Venus and Mars as having ‘horns’. Some have claimed that these texts refer to the phases of these planets and that Mesopotamian astronomers must have discovered this with the aid of primitive telescopes. Others have argued that Venus and other deities were often depicted with horns resembling the lunar crescent to emphasize their celestial nature.

A naked-eye detection of the phases of Mars can be ruled out on account of the small disk of the planet (only 25 seconds of arc at its nearest distance to the Earth) and the fact that its maximum phase angle never exceeds about 45º, implying an illuminated disk than is always more than 85% of the total disk.

In the case of Venus, the circumstances are more favourable. Near its inferior conjunction with the Sun, the apparent disk of Venus can measure up to 68 seconds of arc when its horns will be extremely slender. Under favourable atmospheric conditions, it appears to be possible for observers with an acute eyesight to detect the crescent form of Venus with the naked eye.

The American theologian and social reformer Theodore Parker (1810-1860) claimed that, as a 12-year old child, he observed the phases of Venus without an optical aid. According to the French astronomer Camille Flammarion, the phases of Venus were seen by several observers in France in May 1868 and on the island of Réunion (Indian Ocean) in July 1883 although at those times Venus was far from inferior conjunction with the Sun. Several more claims were made during the second half of the 19th century and the first half of the 20th century.

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