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The ancient Greek astronomer Hipparchus noted the apsidal precession of the Moon's orbit (as the revolution of the Moon's apogee with a period of approximately 8.85 years); [4] it is corrected for in the Antikythera Mechanism (circa 80 BCE) (with the supposed value of 8.88 years per full cycle, correct to within 0.34% of current measurements). [5]
The apsides refer to the farthest (2) and nearest (3) points reached by an orbiting planetary body (2 and 3) with respect to a primary, or host, body (1). An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides / ˈ æ p s ɪ ˌ d iː z / AP-sih-deez) [1] [2] is the farthest or nearest point in the orbit of a planetary body about its primary body.
An object with an e of between 0 and 1 will have an elliptical orbit, with, for instance, an object with an e of 0.5 having a perihelion twice as close to the Sun as its aphelion. As an object's e approaches 1, its orbit will be more and more elongated before, and at e =1, the object's orbit will be parabolic and unbound to the Solar System (i ...
For illustration, the long axis of the planet Mercury is defined as the line through its successive positions of perihelion and aphelion. Over time, the long axis of most orbiting bodies rotates gradually, generally no more than a few degrees per complete revolution, because of gravitational perturbations from other bodies, oblateness in the ...
The Earth spends less time near perihelion and more time near aphelion. This means that the lengths of the seasons vary. [ 14 ] Perihelion currently occurs around 3 January, so the Earth's greater velocity shortens winter and autumn in the northern hemisphere, and summer and spring in the southern hemisphere.
The perihelion and aphelion distances are, respectively, 9.195 and 9.957 AU, on average. [6] [88] The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case). Astronomers use three different systems for specifying the rotation rate of ...
Even further out, Sedna has an extremely-high eccentricity of 0.855 due to its estimated aphelion of 937 AU and perihelion of about 76 AU, possibly under influence of unknown object(s). The eccentricity of Earth's orbit is currently about 0.016 7; its orbit is nearly circular.
For instance, the maximum angular speed of the Earth as measured from the Sun varies by a semitone (a ratio of 16:15), from mi to fa, between aphelion and perihelion. Venus only varies by a tiny 25:24 interval (called a diesis in musical terms). [7] Kepler explains the reason for the Earth's small harmonic range: