Roger (and others),

A slight correction concerning the motion of Earth's perihelion with respect to the seasons. Owing to precession, the equinoxes and solstices drift slowly westward along the ecliptic in a cycle of about 26,000 years. But at the same time perturbations by the other planets cause the Earth's perihelion point to drift slowly eastward along the ecliptic. The net effect is that the perihelion migrates all the way around the ecliptic (with respect to the seasons) in about 21,000 years. Bernard M. Oliver wrote a classic article about the changing shape of the analemma for Sky & Telescope (July 1972, pages 20-22). He gave A.D. 1246 as the year when perihelion and the winter solstice coincided. Among the other effects he noted, in A.D. 6489 the two lobes of the analemma will be essentially equal in size and perihelion will coincide with the vernal equinox. (Full disclosure: I remember that article well, because one of its diagrams was the very first one I prepared after joining the magazine staff!) Roger S. From: sundial [mailto:sundial-boun...@uni-koeln.de] On Behalf Of Roger Bailey Sent: Thursday, April 12, 2018 6:13 PM To: Dan-George Uza; Sundial List Subject: Re: Analemma intersection Hi Dan, To me the value of the EQT at the intersection is an indication of the asymmetry of the analemma caused by the difference between the solstice and perihelion dates. The tilt of the earths axis is one parameter that defines the analemma. This is shown at the extremes, the summer and winter solstices. The eccentricity of the orbit is the other parameter that defines the analemma. This is indicated by the perihelion. If the date of the perihelion is the same as the solstice, I would expect the curve would be symmetrical and the EQT at the intersection would be equal to zero. Perihelion was 2 Jan 2018 and the winter solstice was 21 Dec 2018. This 12 day difference defines the offset of the intersection of the analemma loops. When was the perihelion on the winter solstice? The perihelion changes in a cycle of 25,800 years. So 12 days gives 12/365.25x25,800 or 878 years ago. In 1140 AD I would expect a symmetrical analemma. Of course there is more to this than this simple approximation of orbital dynamics. What was the actual date when the perihelion and solstice were the same? I offer this as quick answer to the question on the significance of the analemma curve intersection. Regards, Roger Bailey Walking Shadow Designs From: Dan-George Uza <mailto:cerculdest...@gmail.com> Sent: Thursday, April 12, 2018 3:46 AM To: Sundial List <mailto:sundial@uni-koeln.de> Subject: Analemma intersection Hello, Tomorrow the Sun will have reached the point of intersection in the analemma 8-curve. How do you compute the exact time of intersection (i.e. when both the hour angle and the solar declination match for two days)? And does it have any special significance? Dan _____ --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial _____ <https://www.avast.com/antivirus> Image removed by sender. Avast logo This email has been checked for viruses by Avast antivirus software. www.avast.com <https://www.avast.com/antivirus>

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