Hello all, I subscribe to this Sundial List because I am working on an art project installation of a Sundial and a "Full Moondial" (however weird this sounds). I am an artist working on time, space and coincidences. I am writing you now because of this last question and answers entries. I find them inspiring and heartwarming. Usually, I know no answer to all entries, I just learn from you all. But today I can answer the question of Dan: *Does it have any special significance? * Yes, it does, since it arises the doubt of its significance. It definitely makes me start a new art project about it. Thank you Dan, and thank you all for this Sundial List. I will share with you my results. All the best and warm regards, Ale
On Thu, Apr 12, 2018 at 6:55 PM, Roger W. Sinnott <roger.sinn...@verizon.net > wrote: > 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 <cerculdest...@gmail.com> > > *Sent:* Thursday, April 12, 2018 3:46 AM > > *To:* Sundial List <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 > > > ------------------------------ > > [image: Image removed by sender. Avast logo] > <https://www.avast.com/antivirus> > > This email has been checked for viruses by Avast antivirus software. > www.avast.com <https://www.avast.com/antivirus> > > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > -- ... *Ale de la Puente* www.aledelapuente.org skype: aledelapuente T: + 52 55 55 54 0895 M: + 52 1 55 54 34 8548 Twitter: @aledelapuente instagram @aledelapuente
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