Sunday, July 3, 2011

Earth at Aphelion on the 4th of July

MICA, the US Naval Observatory's Multiyear Interactive Computer Almanac, indicates that the Earth will be at aphelion, the furthest distance from the Sun, on 4 July 2011 at about 14:54 UT.

The terms perihelion and aphelion were first used in connection with the epicycle discription of the motion of the Sun, Moon and planets referring to points in their orbits so these concepts may be over 2000 years old. Ptolemy uses the terms "perigee" and "apogee" since his epicycle orbits were centered on the Earth. The composite motion of epicycle and deferent is somewhat elliptical and are a good approximation to the motion of the Sun and Moon so it is not surprising that there are near and far points in these "orbits." Ptolemy gives Gemini 5° 30' or 65° 30' for the position of the Sun's apogee.

Supplemental: When comparing Kepler's elliptical orbits with Ptolemy's epicycles it might be fairer to think in terms of the relative motion of the Sun and the Earth. According to Archimedes, Aristarchus postulated that the Earth circled the Sun circa 300 BC. Ptolemy may have been aware of this since he had access to Archimedes (see Almagest p. 44). It is probably best to separate observations from theory as much as possibly to avoid contaminating them with errors unnecessarily. Especially when the observations are solely Earth based.

No comments: