Talk:Analemma
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Is the analemma the intersection of a sphere and a cone? -phma
The last link notes, "incorrectly claims Mercury and Venus lack analemmae". The link titled 1 also makes this claim, and states why. If the author of this comment thinks that they do, what would they look like?
The external link to the article by Brian Tung (currently dated and copyrighted 2002), is I think, important, because at the end it contains a link to a C program for the analemma or equation of time, which uses a more accurate formula than many use. Particularly, it is more complex than simply working out the Equation of Time due to the two components (eccentricity and obliquity) on their own, independantly of each other, then adding the two results. The movement of the Sun eastwards among the stars due to the orbital motion of the Earth, is itself uneven due to eccentricity of the orbit; this needs to be taken into account when working out the component of the Equation of Time due to the obliquity. As Brian Tung states in the last paragraph of his page, the formula which many use (working out the two effects independantly then adding them linearly) works reasonably well for small eccentricities and obliquities, but becomes noteably inaccurate for extreme orbits and inclinations. -Roo60 13:54, 2 Apr 2005 (UTC)
It is not correct that the analemma would be a straight line if the orbit were circular but the axis tilted as it is. It is true that if the axial tilt were only slight, then the analemma would be almost a straight north-to-south line; but with very great axial tilts, the figure-of-eight feature would be very marked indeed.
As is explained in the article Equation of time, when the sun is crossing the equator (at the time of the equinoxes), then it is climbing/lowering at a 23.44° angle, which means that for every 1.09° of actual motion along the celestial sphere, it only moves eastwards in Right ascension by 1°. Conversely, at the solstices, the sun is cutting across meridians, meaning that for every 1° of motion along the plane of the ecliptic, its Right ascension increases by 1.09°. Thus, the Equation of time follows two cycles per year. - Roo60 20:05, 2005 Apr 7 (UTC)