Talk:Analemma

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In the second paragraph, the following sentences don't make sense: "The mean position appears to revolve around the Earth once every mean solar day, because of the Earth's rotation. This daily revolution is not considered to be averaged out to get the mean. The mean position of the Sun is therefore at the same place in the sky at the same time every day, but not at other times."

I suggest deleting them. Instead I would insert "at a fixed time" in the preceding sentence after "an observer at a fixed point on the Earth" — Preceding unsigned comment added by 137.108.145.24 (talk) 2015-01-30T14:44:24‎

I did not write the above, but I agree with it. The first 2 paragraphs are very unsatisfactory and should certainly be deleted. The last sentence of the 2nd paragraph is the only correct thing there. The reference to a mean position on the celestial sphere is entirely wrong. Whoever wrote this has a very confused notion of what the analemma is. I have therefore deleted the first 2 paragraphs, and have written an alternative. I am not a professional astronomer, so what I have written may need to be revised, but I am confident that this new lead is better than what was there before. g4oep — Preceding unsigned comment added by 77.96.58.212 (talk) 2015-05-17T02:22:27‎

Would someone like to enlarge on this : "The figure-eight form arises from the relationship between the direction of the earth's axis, and the line passing through the perihelion and aphelion of the earth's elliptical orbit around the sun." It would be interesting to know under what conditions the analemma would be a line inclined to the ecliptic meridians, and when it would be a circle. i.e. - how is the phase of the equation of time relative to the solar declination determined ? It seems to me that the analemma is an example of a Lissajous figure, the figure 8 form being related to the large 2nd harmonic content of the equation of time. How does this 2nd harmonic arise ? Under what conditions would it be minimised, and is it possible that under some conditions higher harmonics could be present, giving 3 or more loops ? g4oep — Preceding unsigned comment added by 77.96.58.212 (talk) 2015-05-18T02:04:11‎

I wrote the sentences you have seen fit to delete, and I still contend that they were more accurate and complete than your substitutions for them. However, since you found them difficult to understand, I have left your substitutions, but have added some words about the analemma's apparent motions in the sky, which are not related to the description as a set of positions of the Sun at a fixed time of day. I've also done some minor fixes which you'll see if you compare the current version with one from a few hours ago. DOwenWilliams (talk) 20:49, 17 May 2015 (UTC)

Thank you. I feel that the lead is steadily improving as these contributions accumulate. I have made a slight edit to the sentence concerning the long axis of the analemma in order to pull it into greater accord with what I think you mean. This sentence has now moved away from the idea I had when I wrote it - namely to give some meaning to the line of the long axis. Can anyone identify this line ? Is it a line of celestial longitude, for example ? Also, any comments on my immediately preceding edit on this page concerning Lissajous figure, conditions for specific phase relationships (zero phase, phase quadrature), harmonic content ? Concerning the idea of the movement of the analemma, as I see it, the analemma is a fixed pattern in the sky representing the sun's apparent motion as seen under the conditions stated in the article. The figure will appear at different positions in the sky if the chosen time of day at which the observations are made changes. I think a difficulty will arise if you attempt to define the analemma in such a way that, as a whole, it can move continuously, following the daily path of the sun in the sky, or, as was originally indicated, being fixed relative to the celestial sphere and therefore showing diurnal movement. For example, the line drawn in the sky by the sun on a particular day will be a continuous curve, and on successive days through the year discreet curves will appear. But a collection of such lines will not be an analemma, nor will the supposed problem of the analemma as a collection of discreet points be solved in this way. I would suggest that the analemma is in fact a collection of points, and the supposed continuous line of the figure exists only in the imagination. Wikki has a policy of presenting what is considered to be the usual meaning of terms, so I would like to ask "how is the analemma usually understood: as a fixed pattern of discreet points or as a moving pattern formed from a continuous line ?"

On reflection, I can see that a continuous curve would result if the analemma were defined as a plot of solar declination against the equation of time (as mentioned in the article). But it now seems that these are alternative definitions, related, but non-equivalent. Perhaps both should be given as non-equivalent alternative definitions: one a pattern in the sky, the other a plot on a graph. Of course neither would move in the sense we are discussing. G4oep (talk)

Astronomers generally think of the analemma as a continuous curve. Insisting on discrete points is oounter-productive.

Please learn the distinction between "discrete" and "discreet".

The long axis is bisected (divided into two *equal* parts) by the equator. The figure is not.

This description was getting far too long and detailed to be in the lead, so I've moved a lot of it to the "description" section.

DOwenWilliams (talk) 15:32, 18 May 2015 (UTC)

Good - the article is steadily improving. One minor point - I don't like to argue over trivia, but I think that precise use of language is desirable. This is one of the definitions of "axis" given here: http://www.thefreedictionary.com/axis. "2. Mathematics: a. An unlimited line, half-line, or line segment serving to orient a space or a geometric object, especially a line about which the object is symmetric". This is the meaning I had in mind, including the "unlimited" part. It makes no sense to me to say that an unlimited line is bisected, but I can appreciate the meaning of the bisection of the figure along an axis which is itself unlimited. You say that the figure is not bisected, so I cannot follow your meaning. I am sure that we agree that the figure has equal angular extent above and below the celestial equator. Can we find a form of words that sorts out this difficulty as well ? I would still like to include a comment about the orientation of this axis, and, if possible, its relationship to some form of recognised co-ordinate system. G4oep (talk)

The axis was originally said to be part of a line of ecliptic longitude. That was just plain wrong. I tried using celestial longitude, but that led to some awkward links to equatorial coordinate systems. So I simply used the celestial equator, which is simple and clear.

Surely, saying that the axis is bisected by the equator and perpendicular to it fully defines its orientation.

The analemma is not symmetrical in the north-south direction. The small northern loop does not match the large southern one. So an east-west line such as the equator cannot bisect it into *equal* halves.

I have put in a definition of the axis, as a finite line segment.

DOwenWilliams (talk) 18:25, 18 May 2015 (UTC)

I still think the version of 13 April is better than the more recent ones. I've half a mind just to revert to it. DOwenWilliams (talk) 18:33, 18 May 2015 (UTC)

The only reason for the figure 8 pattern is the axial tilt. The horizontal asymmetry stems from the misalignment of the solstices and apsides. It is not possible to have three or more loops.
The analemma is continuous. The discrete-point version only arises from the usual method of photographing it.
The origin of the graph is the position of the mean sun. Thus the analemma moves around the celestial sphere once per year.
A line connecting the northernmost point and the southernmost point of the analemma is meaningless. It is not perfectly vertical, nor does it divide the analemma into equal halves. If however you are talking about the y-axis of the graph, that represents when the equation of time is zero, or when the real sun would be at the same right ascension as the mean sun.
--Lasunncty (talk) 12:16, 21 May 2015 (UTC)

Parenthetical comment

I removed "(mostly in items 5 and 6 of the table of contents)" from the second paragraph of the introduction, because future edits, could change where in the table of contents those topics fall. An editor who adds a new section could easily not notice that they need to change these two numbers. And, since this comment is parenthetical, it is not essential to the meaningNick Beeson (talk) 04:46, 7 June 2015 (UTC) of the sentence.

translation from greek

seems Analemma means: “pedestal of a sundial".

Saw it in the italian wikipedia and also here, as third explanation:

http://www.yourdictionary.com/analemma Hexagone59 (talk) 00:25, 21 November 2015 (UTC)

According to the Sawyer reference used for this article, that was how the word was used in Latin. The original Greek usage was the more general meaning listed here. BTW, the link you gave uses wiktionary as its source. I would support changing all the wikis to avoid contradiction. --Lasunncty (talk) 17:34, 2 December 2015 (UTC)

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Eccentricity effect on analemma

Note that the width (east-west component) of the eight-figure of analemma requires only an axial tilt. The effects of eccentricity (for the case of Earth) manifest primarily in a tilt of analemma, asymmetry of the lobes and a slight right-left assymetry. Please keep that in mind editing contributions of orbital parameters to the shape of analemma. L3erdnik (talk) 18:48, 4 July 2017 (UTC)

No, the asymmetry comes from the fact that the equinoxes are not lined up with the apsides. East-west motion could come from either eccentricity or axial tilt on their own. In the case of Earth, eccentricity is the larger contributor to the east-west motion, despite the fact that our eccentricity is less than 2%. See equation of time. --Lasunncty (talk) 20:45, 4 July 2017 (UTC)

Sorry, I apologize, I had it mixed up. The axial tilt's contribution is slightly more than the eccentricity's. I will change it back. --Lasunncty (talk) 20:55, 4 July 2017 (UTC)