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2018-04-13 Thread graham stapleton via sundial 
Diese Nachricht wurde eingewickelt um DMARC-kompatibel zu sein. Die
eigentliche Nachricht steht dadurch in einem Anhang.

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(apologies for an off-subject posting - but who else is likely to know?)
Does anybody know of any kind of group, forum, or organisation that studies 
calendars (either as a time system or tangible objects) in terms of their 
structure, mathematics, history, culture etc. It seems unlikely that there is, 
but it would be good to know.

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Re: Analemma intersection

2018-04-13 Thread Ale de la Puente 
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  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 
>
> *Sent:* Thursday, April 12, 2018 3:46 AM
>
> *To:* Sundial List 
>
> *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
> --
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-- 
...
*Ale de la Puente*

www.aledelapuente.org

skype: aledelapuente
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