A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Patrick Goold
Date: 2011 Feb 10, 12:53 -0500
As for Ptolemy:
The equation of time results mainly from two different superposed astronomical causes (explained below), each causing a different non-uniformity in the apparent daily motion of the Sun relative to the stars, and contributing a part of the effect:
- the obliquity of the ecliptic (the plane of the Earth's annual orbital motion around the Sun), which is inclined by about 23.44 degrees relative to the plane of the Earth's equator; and
The irregular daily movement of the Sun was known by the Babylonians, and Ptolemy has a whole chapter in the Almagest devoted to its calculation (Book III, chapter 9). However he did not consider the effect relevant for most calculations as the correction was negligible for the slow-moving luminaries. His calculation did not correctly account for both of the two major annual variations as they are now known, and he only applied it for the fastest-moving luminary, the moon.
My question is why, if the maximum variation is 14 min 13 seconds, has the astro website I have been using to tell me the time of meridian passage been telling me LAN is 12:19 here in Portsmouth? Are they just wrong or am I misinterpreting what they are telling me?
The asymmetry is I think the result of an elliptical orbit around the Sun (Kepler discovery ?).
I believe Ptolemy caught this one first. He had circular orbits with the center of the orbit displaced in order to get this in a geocentric model.
Dr. Patrick Goold
Department of Philosophy
Virginia Wesleyan College
Norfolk, VA 23502
757 455 3357
Charles Olson: "Love the World -- and stay inside it."