Talk:Ecliptic coordinate system

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Celestial latitude and longitude[edit]

I thought this was the place to explain a series of related changes I've made. I restored the Ecliptic latitude and Ecliptic longitude articles, because the orphan Celestial latitude and Celestial longitude articles had become redirects to Declination and Right ascension, which are quite wrong. They needed something to point to and they now redirect to the restored Ecliptic latitude and Ecliptic longitude articles. SteveMcCluskey 05:22, 20 February 2007 (UTC)

A question: the stars on the ecliptic are steady, so is more or less the ecliptic pole, the pole around which the north or south pole turn in about 25000 years (I mean here the ecliptic pole and the ecliptic are steady in millions of years, that have to do with the movement of the stars and the galaxy and not the precession). I have the impression that this is not so clear in the article (of course it can be, that i havn't understand it well)Yormomo (talk) 00:23, 23 February 2012 (UTC)

conversion algorithm[edit]

I've been trying to implement functions based on the text provided in this section and I must confess to being totally confused. If we consider the first cas eof converting from ecliptic (RA,Dec) to equatorial (lat,lon) then to state the obvious we know (RA,Dec) but we do not know (lat,lon), but the article reads as if we know all 4 variables. For instance, the first line of the algorithm states "Calculate the terms right of the = sign of the 3 equations given above", but how is it possible to do this if we don't yet know (lat,lon)?

If you know (RA,Dec), you compute the right sides of the second set of equations. If you know (lat,lon), you compute the right sides of the first set of equations. —Tamfang (talk) 07:39, 4 April 2008 (UTC)

The second line of the algorithm states "taking the cos α cos δ as the X value". Are we to interpret this x=cos α cos δ or x=cos λ cos β because if it's the 2nd version, again we don't know lambda and beta.

I would suggest interpreting "α" as α and "δ" as δ, but that's just me, you may find some other approach more useful. —Tamfang (talk) 07:39, 4 April 2008 (UTC)

The closing line of the algorithm section states "Similarly for the equatorial to ecliptic transformation". After coding what I thought was the algorithm was, doing the inverse calculation I don't get back the original (RA,Dec) values from the (lat,lon) computer values. What's really needed for this page is an example. gmseed 28/03/2008

If I understand it well, the conversion algorithms don't take the precession in considerationYormomo (talk) 00:17, 23 February 2012 (UTC) But I see now that precession is included in the equatorial coordinates, so they are ok 93.82.149.44 (talk) 17:41, 23 February 2012 (UTC)

Rectangular coordinate system (ecliptic)[edit]

The article seems to be exclusively about spherical ecliptic coordinates and says nothing about the 3D (x,y,z) rectangular coordinate system that is used extensively in orbital and positional computations. Perhaps the article could be better titled "Spherical ecliptic coordinate system"? Pomona17 (talk) 13:11, 8 November 2008 (UTC)

Diagram is needed[edit]

Per the guidance of the header [The article may be too technical for most readers to understand], this topic needs a diagram to explain the topic. Bcwilmot (talk) 20:53, 25 July 2010 (UTC)

Spherical Trigonometry[edit]

This article should make some reference to the fact that the formulas shown were derived using Spherical Trigonometry. This would address most of the concerns expressed above, including the need for a diagram. — Preceding unsigned comment added by Alexselkirk1704 (talkcontribs) 16:13, 8 January 2011 (UTC)

Units of measurement[edit]

Should astrological signs be mentioned? Sometimes astrological signs are used as the units of measurement for ecliptic longitude, just as hours can be used as the units of measurement for right ascension. (Astrological signs are also a unit of angular measurement (used only for ecliptic longitude), although some people do not know this.) --Zzo38 (talk) 05:44, 18 June 2012 (UTC)

Outlines of Astronomy by John Herschel, 7th ed. in 1864, makes no mention of zodiac signs as a measure of ecliptic longitude. I've seen this work described as highly influential. I have not come across any work on astronomy that actually uses zodiac signs for longitude, although I think I've seen this mentioned as a historical practice. I think the thing to do is leave it out until some editor with access to sources on the history of mathematical, scientific, or astronomical terminology is able to look up when this terminology faded out. Jc3s5h (talk) 11:44, 18 June 2012 (UTC)
Yes, astrological signs were used (360°/12 = 30° each), maybe back around Kepler's time and before, but this was dropped pretty quickly once measuring devices were developed. I don't think there will be much mention of them in any English references... it was far back enough that they will mostly be in Latin. Might be worth a sentence or two if we can find a reference. Tfr000 (talk) 13:23, 18 June 2012 (UTC)
I found a reference. Tfr000 (talk) 15:00, 21 June 2012 (UTC)
It is not true that they are not used any more today; perhaps they are not used as often today, though, and usually astrologers use it, astronomers rarely use it today (in the past, before science was invented, it was the same thing). Astronomers more commonly using equatorial coordinates today, anyways (since equatorial coordinates is more useful for observation of stars by telescopes, than ecliptic coordinates are). (I still use both ecliptic and equatorial coordinates, and will use different units too depend what is being done. Notice also that, you need to have some zero longitude reference, and whatever zero of ecliptic longitude is will be called 0 Aries, even if it is not near that constellation (signs and constellation are two different things; tropics of Cancer/Capricorn correspond to astrological signs not to constellations).) --Zzo38 (talk) 23:41, 2 November 2012 (UTC)

This was "dumped" in the article. I relocated it here.[edit]

Ecliptic latitude and longitude[edit]

Ecliptic latitude and longitude are defined for the planets, stars, and other celestial bodies in a broadly similar way to that in which terrestrial latitude and longitude are defined, but there is a special difference.

The plane of zero latitude for celestial objects is the plane of the ecliptic. This plane is not parallel to the plane of the celestial equator, but rather is inclined to it by the obliquity of the ecliptic, which currently has a value of about 23° 26′. The closest celestial counterpart to terrestrial latitude is declination, and the closest celestial counterpart to terrestrial longitude is right ascension. These celestial coordinates bear the same relationship to the celestial equator as terrestrial latitude and longitude do to the terrestrial equator, and they are also more frequently used in astronomy than celestial longitude and latitude.

The polar axis (relative to the celestial equator) is perpendicular to the plane of the Equator, and parallel to the terrestrial polar axis. But the (north) pole of the ecliptic, relevant to the definition of ecliptic latitude, is the normal to the ecliptic plane nearest to the direction of the celestial north pole of the Equator, i.e. 23° 26′ away from it.

Ecliptic latitude is measured from 0° to 90° north (+) or south (−) of the ecliptic. Ecliptic longitude is measured from 0° to 360° eastward (the direction that the Sun appears to move relative to the stars), along the ecliptic from the vernal equinox. The equinox at a specific date and time is a fixed equinox, such as that in the J2000 reference frame.

However, the equinox moves because it is the intersection of two planes, both of which move. The ecliptic is relatively stationary, wobbling within a 4° diameter circle relative to the fixed stars over millions of years under the gravitational influence of the other planets. The greatest movement is a relatively rapid gyration of Earth's equatorial plane whose pole traces a 47° diameter circle caused by the Moon. This causes the equinox to precess westward along the ecliptic about 50″ per year. This moving equinox is called the equinox of date. Ecliptic longitude relative to a moving equinox is used whenever the positions of the Sun, Moon, planets, or stars at dates other than that of a fixed equinox is important, as in calendars, astrology, or celestial mechanics. The 'error' of the Julian or Gregorian calendar is always relative to a moving equinox. The years, months, and days of the Chinese calendar all depend on the ecliptic longitudes of date of the Sun and Moon. The 30° zodiacal segments used in astrology are also relative to a moving equinox. Celestial mechanics (here restricted to the motion of solar system bodies) uses both a fixed and moving equinox. Sometimes in the study of Milankovitch cycles, the invariable plane of the solar system is substituted for the moving ecliptic. Longitude may be denominated from 0 to \begin{matrix}2\pi\end{matrix} radians in either case.

Tfr000 (talk) 15:24, 28 February 2015 (UTC)