Talk:Longitude of periapsis

Applicability to exoplanets?
I've asked for some clarification of how the concepts described here apply to exoplanets on the argument of periapsis talk page. — Aldaron • T/C 03:28, 24 July 2009 (UTC)

Application in general
What is the practical purpose of this variable? It doesnt denote any physical angle that can be visualized or measured with a protractor. It is the linear sum of two angles existing in entirely different planes. It seems entirely abstract and purposeless. At what point in, say, the computations of planetary positions, for example, might this variable even be useful? The Argument of Periapsis and the Longitude of the Ascending Node are useable and practical and reflect real concepts. I guess that I (a novice) just dont see what this is for, how its used, why we need it, so on. It seems to me that at any point when Longitude of periapsis is being used, we are deducting from it one of the two variables it is comprised of in order to arrive at the other. I am left wondering why we ever derived this variable in the first place - was it just to confuse people? 64.134.140.40 (talk) 01:25, 10 June 2011 (UTC)


 * Most objects in the solar system have relatively small inclinations, so it just gives a rough idea of where the periapsis is in the orbit without having to do any complex calculations. --Lasunncty (talk) 03:18, 12 June 2011 (UTC)


 * I agree with the above comment - basically it tells you in what direction the perihelion (perigee, etc) point occurs when projected onto the reference plane, which is convenient for, say, publishing or other situations where a two-dimentional diagram is the only option. — Preceding unsigned comment added by 50.10.91.64 (talk) 21:34, 28 November 2013 (UTC)


 * I think it makes sense for objects which are roughly in the same plane. For example a bunch of objects that are anti-aligned to the hypothesized Planet Nine have a common Argument of Perihelion that is supposed to be 180° away from that of P9's.J mareeswaran (talk) 22:24, 25 October 2016 (UTC)

The figure on the page is very nicely executed. It is entirely legitimate to show an ascending node longitude as measured in the ecliptic plane, but if one wishes to use a perihelion longitude to find a planet's position, it should be the perihelion longitude as measured in the orbital plane, equal to the ascending node longitude plus the perihelion argument, both as measured in the orbital plane. The JPL website publishes a "low-accuracy" planet location document which directly uses the perihelion longitude. Use of their algorithm produces planetary positions that correspond closely to the JPL HORIZONS program output.

Name is wrong
Jean Meeus, in Astronomical Algorithms, 2nd edition, p. 411, convincingly argues that: “The word "periapse", used by some authors, is incorrect. The word perihelion means the point of the orbit that is closest to the Sun (from the Greek: peri = near + helios = Sun). Similarly, perigee is the point closest to the Earth (ge = Earth). Therefore, "periapse" would really mean the point closest to the apse; but this is ridiculous, because what is meant is the apse itself!” (Italics are his.)

He then proceeds to correct that “the best terms seem periastron and apastron, as for double stars.”

I suggest we rename this article “Longitude of the periastron” accordingly.

CielProfond (talk) 00:36, 18 February 2020 (UTC)


 * WP:Article titles states "Generally, article titles are based on what the subject is called in reliable sources." and "Article titles are based on how reliable English-language sources refer to the article's subject." This doesn't leave any leeway in continuing to use the present title "Longitude of the periapsis", despite Meeus's objection (he also objects to some other current astronomical standards). Any change would also affect similar articles like Argument of periapsis, not to mention the wholesale rewrite that would be required of these articles. — Joe Kress (talk) 02:05, 19 February 2020 (UTC)

Point of Vernal Equinox
The definition calls for the measurement of the longitude of the ascending node from the point of the vernal equinox, which requires knowledge of the inclination of the planet's axis of rotation to its orbital axis; for stars and exoplanets, this information is currently pretty reliably unknowable. It might be nice to make note of that being the reason that "the term longitude of periapsis is used to refer to ω, the angle between the ascending node and the periapsis" for "discussions of binary stars and exoplanets".

Also, it is a bit ambiguous, for solar system objects: the diagram shows the "Reference direction" with the symbol of Aries, implying the "first point of Aries", the direction of vernal equinox for earth. Does that mean that for longitude of perihelion, the measured angle should always refer to earth's direction of vernal equinox, or is it supposed to be calculated from each planet's own direction of vernal equinox - they all have rotational axial tilts, so they have equinoxes, but what would be "vernal"? Earth really has a different "vernal" equinox depending on which hemisphere you are in, but tradition assigns the northern one, naturally. I don't know how you would determine which is the appropriate "vernal" for other planets.2001:56A:F0E9:9B00:F035:F060:7504:921E (talk) 05:38, 18 July 2020 (UTC)justSomeWikiReader


 * For our own Solar System, we use Earth's vernal equinox as the zero-point of longitude for all orbital coordinates (as far as I'm aware). Changing this point of reference for every singe object would be confusing and tedious or even impossible since many objects' axial tilts are not known or not stable.  For exoplanets and binary stars, the zero-point is generally the direction of north (from Earth) measured tangent to the (Earth's) celestial sphere.  --Lasunncty (talk) 19:40, 18 July 2020 (UTC)

Update dead link
Please update the dead link to "Determination of the Earth's Orbital Parameters". It must be https://data.giss.nasa.gov/modelE/ar5plots/srorbpar.html. I don't feel qualified to do it. dcromley Dave C (talk) 00:31, 3 March 2021 (UTC)


 * ✅ --Lasunncty (talk) 05:15, 7 March 2021 (UTC)