Talk:Equinox (celestial coordinates)

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Citations

Now this article has 2 citations, and 1 external link. Coronellian (talk) 18:53, 15 July 2009 (UTC)

Doubtful distinction between equinox and epoch

I don't think the distinction made in this article between equinox and epoch (see epoch (astronomy)) accurately reflects modern usage. In astronomy literature, I have never seen the word equinox refer to a time basis; the word epoch is always used as a basis for the coordinate system itself, i.e. precession and nutation changing the celestial equator used for declination and right ascension. The epoch (astronomy) article seems to indicate that using equinox in this sense is archaic. As such, that seems to undermine the existence of this article entirely. CosineKitty (talk) 18:14, 22 March 2010 (UTC)

I have done a little more reading and found this: Oliver Montenbruck and Thomas Pfleger. Astronomy on the Personal Computer. Springer-Verlag. p. 17. ISBN 0-387-57700-9., which indicates that the phrase equinox of date is used to refer to the orientation of the ecliptic at a particular date. However, epoch still is used interchangeably with equinox of date as far as I can tell. I don't see any useful distinction, and epoch is much more commonly used in literature over the last 100 years. CosineKitty (talk) 15:31, 23 March 2010 (UTC)

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I call tell you that a lot of astronomers who don't deal with objects that move at an appreciable proper motion use the terms interchangeably. Astronomers who do deal with solar system objects or objects with high proper motion cannot afford to have the distinction blurred. Epoch refers to a time for which the position of an object is given. Equinox refers to the coordinate system in which the position is given. If someone gave me the coordinates of an object and told me the epoch was J2000, I would assume those were the coordinate on J2000.0 in equinox J2000.0 coordinates.

One example where people make mistakes is the Hipparcos catalog. The header lists the coordinates as "ICRS (J1991.25)" which everyone assumes means equinox J1991.25 coordinates. Unless they actually look up ICRS and find out that it's essentially identical to equinox J2000.0. J1991.25 is actually the epoch for which the coordinates are valid, but the coordinates are J2000.0 equinox. I've seen a number of veterans mess that one up. SETIGuy (talk) 00:20, 23 August 2011 (UTC)

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I came to this Wikipedia article to educate myself further on this particular usage of the term equinox, so I lay no claim to speaking as an expert on the subject, but I can tell you that on the USNO site, which manages and provides information on a whole array of star catalogs, these two terms are differentiated. In the case of NOMAD (The Naval Observatory Merged Astrometric Dataset) for example, to retrieve information from it, two of the input parameters that you specify are equinox and epoch. You may specify the same for both, but you don't have to. The help page for it says:

Equinox: J2000.0 or B1950.0.

Epoch: A year between 1900 and 2050.

So for equinox, a standard equinox is expected (one of the two most recent ones set by the IAU), whereas for epoch, you can specify any value (within the range) that is suitable for your purpose (e.g. for comparison). It also says under equinox that "If the equinox and epoch you select are NOT those of the catalogue, the returned data will be precessed [my emphasis] to the equinox and epoch you gave as input." Mottelg (talk) 02:36, 19 March 2012 (UTC)

Infobox

I've added an infobox with a pair of screenshots from Celestia, depicting the location of vernal equinox in the sky. The whole article discusses a lot of theory behind establishing the point/direction of equinox, various caveats etc, but in essence the equinox is a very concrete point in the universe and I found the article ultimately lacking in relating all the theory to the reality surrounding us, just *showing* the Equinox next to telling what it is.

I'm not a professional, so if I made any blunders in the description I'd be grateful if someone could check and correct them. Sharpfang (talk) 22:05, 31 January 2016 (UTC)

I removed the infobox because I couldn't read the image, didn't understand what the two yellow splotches in the image were for, and couldn't understand the caption. Jc3s5h (talk) 11:03, 7 February 2016 (UTC)

The two "yellow splotches" were marking the point of the direction of equinox. This whole article is about what it is and the whole theory about how to calculate it, it only lacks the actual result of said calculation. It also lacks the only actual purpose of why would anyone want to perform that calculation, which is establishing the origin of the celestial coordinates. What exactly didn't you understand about the caption? And what was your problem with reading the image? I might fix it, but your explanation doesn't say what needs to be fixed. Was the file corrupt? Or did you have problems reading the star chart or understanding the connection between the celestial coordinates, celestial sphere and the direction of equinox? Sharpfang (talk) 01:34, 19 February 2016 (UTC)

The text in the image was too small to read. Also isn't clear why there would be two different splotches so far apart. Also, there is insufficient information in the image for any reader who wants to to verify it is correct. Jc3s5h (talk) 11:32, 19 February 2016 (UTC)

Did you actually open the image to see it at full resolution? If you look at any article with semi-complex images in the side boxes, they very rarely are of resolution sufficient to read without zoom-in.

If you actually open the image, it isn't too small to read, and if it is for you, then it's a problem with your computer - lower your screen resolution or upscale the page until it's comfortable. Also, if you would have read the text and looked at the image paying some attention to the content, you would have noticed this is in fact two images composed together, one above the other; the same piece of the sky - the same set of constellations, but two different sets of coordinates. The two points are in exactly the same point of the sky, and denote the origin of both sets of coordinates, although the axis of these sets of coordinates are pointing in different directions ( equator in case of equatorial, the plane of ecliptic in case of ecliptic ). Sharpfang (talk) 10:40, 3 March 2016 (UTC)

This is my final post in discussion mode. The image is of low quality and too confusing to be in the article. If you reinsert the image into the article I will seek to have it removed through the various dispute resolution channels. Jc3s5h (talk) 12:24, 3 March 2016 (UTC)

Okay, so the image was unclear. The article is STILL lacking the crucial information. It's the kind of omission of answering "What is the Earth radius?" with "It's the distance between center of Earth and its surface, as measured..." (and delving into discussion of how to perform that measurement) without ever giving the actual value of Earth radius. Sharpfang (talk) 17:16, 12 December 2016 (UTC)

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This article is totally wrong

Equinox in this context is not a moment, it is a place. I am so sorry if this ends up starting an argument, but the article really needs to be completely rewritten. (I have already started.) LaurentianShield (talk) 16:06, 23 December 2018 (UTC)

Equinox in the context of celestial coordinates is not a place, it is a direction. More specifically it is a ray from the center of the Earth along the line formed by the intersection of the equatorial plane with the ecliptic plane, in the general direction of the constellation Pisces. Jc3s5h (talk) 13:31, 24 December 2018 (UTC)

Agreed, but I was distinguishing it from a moment in time, which is what the article originally said. LaurentianShield (talk) 16:07, 24 December 2018 (UTC)

Epoch and equinox, again

Epoch in this context, does not strictly and simply mean the date for which a particular equinox is chosen. In contrast to the date of an equinox, epoch refers to the date of any astronomical observation. In this context, it typically refers to the location of an object which varies over time, and hence must be specified at a particular time. Hence, to specify the coordinates of an astronomical objects, two dates are required: the date of the equinox and the date of the observation (or calculated coordinates). These two dates are generally referred to simply as the equinox and epoch. These are typically the same but need not be. For example, the Hipparcos catalog uses an equinox of J2000 (from the Guide Star Catalog and Hipparcos Input Catalog, actually ICRS in the Hipparcos output catalog)) but an epoch of J1991.25; ie. with reference to the coordinate system based on the position of the equinox at J2000, it gives the positions of objects as measured at J1991.25. Hence, despite the statement that the equinox and epoch are often confused being tagged, the equinox and epoch have been confused in this article. Also, to some extent, it isn't made obvious that while the equinox is the direction of the origin of a coordinate system, the same term in the same context is given a value which is a date. Calling that value an epoch, which technically it is because any reference date is an epoch, only confuses the fact that in this context epoch refers to something different. Many professional and academic sources still confuse the two (completely at random, this glossary gets it wrong while this blog gets it right), but a quick look at Simbad shows that there are two separate dates called equinox and epoch. Lithopsian (talk) 14:35, 24 December 2018 (UTC)

I've updated the lead to address the two distinct dates, referred to as equinox and epoch, required to define a celestial position. It may need further work, in particular to address the likely confusion between an equinox being a location/direction but being given a value which is a date. The body almost certainly needs work to be clear about the distinction between the two dates (and potentially alternatives to an equinox such as ICRS). Lithopsian (talk) 15:43, 24 December 2018 (UTC)

What you wrote is an improvement, but I don't think I agree with what you said that "two dates are required" to specify coordinates. Coordinates come from a star catalog, such as FK5, which is specified according to an epoch, J2000.0. That's fixed, and has even become more fixed because ICRF3 nails it down even more using the VLBI. Stars do move slightly, but most of them are pretty fixed. The observer moves, and the observer needs to set the telescope or other instrument taking that into account, but that's a separate question. Capitaine explains this nicely in the paper I cited BTW. LaurentianShield (talk) 16:15, 24 December 2018 (UTC)

Perhaps we can start by agreeing that there are two dates? A coordinate system is defined wrt the direction of the equinox on a particular date, and object positions can be observed or calculated using that coordinate system. An observation is made, or a position calculated, for an object at a particular epoch, but can be expressed using a coordinate system with any equinox from any date. For example, the location of α Centauri in AD 100 can be reported using a coordinate system using an equinox from AD 100, and then its position in AD 2000 can be reported using the same reference system. Forget for a moment whether you personally would want to do that, or see the need for it, it is the case that those two dates can be different. One is definitely called epoch, the actual date that an object is in that location. The unambiguous name of the other (the date for which the equinox is chosen) is unclear, partly because they are only infrequently different, partly because people are confused and call them both epoch, and partly because the name epoch can apply to the defining date of anything, including of the position of the equinox. Agreeing that would help a lot because we can then try to express it in the article.

Now, why does it matter that they can theoretically have different values? The positions of "static" objects like stars are almost always given using an epoch and equinox with the same date. It matters for two reasons that I can see: first, to actually calculate object coordinates for a catalog at a given equinox and epoch, you have to modify an observed position by applying both a proper motion between the observation date and the desired epoch, and apply the equation of equinox between the observation date and the desired equinox; second, calculations of "local" objects like comets are most easily done against a static reference frame, so that different epoch coordinates are all calculated for a single equinox, which may later be precessed to equinoxes matching the epoch. So you may not notice all that sleight of hand when you look at a published position, but without the knowledge that both the position of an object and the reference frame it is measured against change independently over time, you'd never have your catalogue.

Of course, this has all more or less been expressed in a previous topic on this talk page. Previously they didn't reach any useful consensus and we were left with a poor article, but maybe we can do better now. Lithopsian (talk) 20:30, 24 December 2018 (UTC)

For now I will say that I agree, I think we can come to consensus/agreement. However it being the holiday, for the time being I am just going to mull over (ha ha) what you said -- probably respond in a day or two. I may also make some less controversial changes to the article -- there is just some other logic and wording clean up needed, especially since I rearranged some of the sections. LaurentianShield (talk) 21:17, 24 December 2018 (UTC)