Talk:Apsis

message: I miss the definitions of the two kinds of Perihelion times, namely:
1. The perihelion of the earth itself, and
2. The perihelion of the centre of gravity of the earth-moon system.
The difference between these two times ("moments")can be as much as 26 hours, depending on the moonphase around January 4.
Alex Vermeulen, Zoetermeer, The Netherlands (Jordaan12 (talk) 15:24, 4 November 2009 (UTC)
thread_title: Missing definitions

message: That would explain with regard to most of the planets, then, the more obscure (to most of us) Greek vs. Latin form, and why "Perilune/Apilune" would be totally incorrect for "Periselene/Apiselene".  Excellent.
Rlquall 15:54, 20 Aug 2004 (UTC)
thread_title:  Excellent explanaiton 

message: It's not clear to me what the proper pronunciation is for "aphelion", from a linguistic point of view.  My copy of the American Heritage Dictionary (2nd college edition) gives the options of "a-feel-yon" and "a-feel-e-on". Indeed, almost every (perhaps every) Greek work that's come into English turns the greek letter Φ (phi) into a FFF sound.  
However, it is the case that, rather than being a mere hypercorrection, 'ap-helion' would indeed be the ancient pronounciation, if we were able to bring some ancient Greek speaker to this time and show him or her the written word.  (Source:  Vox Graeca, by Sidney Allen, the standard book on the prounciation of Ancient Greek)  I guess I'm uncomfortable hearing it called a Hypercorrection, since it does, in fact, reflect the classical pronunciation, which is being taught increasingly these days to students of Ancient Greek.  (When I was studying Ancient Greek twenty-five years ago, the pronounciation we were taught was the Erasman Pronounciation, which pronounces Φ (phi) as FEE.  The Modern Greek pronounciation of the letter is also FEE, and that's how I hear it pronounced when I attend a Greek Orthodox church service.)
The comment made in the article -- about "Greek από, from, which becomes απ before a vowel, and αφ before rough breathing)" -- is absolutely true.  However, the sound change only makes sense when you understand that the ancient Greeks really did pronounce Φ (phi) as an aspirated P sound, like the P that starts the English word "pin", where we release a puff of air, a sort of soft H, after the P.  Try it yourself, then compare with the way you say the P in "stop", where you don't release the puff of air.  Again, this change makes sense only if you understand that "apo helion", by the ancient sound transformation rules, naturally turns into ap' helion, which turns into "aphelion".   And the only way that change is natural is if it's pronounced in the classical way.  So go figure.
Saugart 06:04, 27 August 2006 (UTC)
thread_title:  Aphelion's Pronunciation

message: The article needs to define what mu and 'e' are.  I know e is called the eccentricity. I have no idea what mu is. --69.5.156.155 14:11, 14 Dec 2004 (UTC)
thread_title:  Missing Definitions 

message: I was redirected to this page from the term 'pericenter', as used in article S2_(star), but the page does not contain the word, let alone the definition.
--128.9.216.23 17:36, 17 August 2005 (UTC)
thread_title:  Redirection from Pericenter 

message: Are thre any useful mnemonics to help remember which one (peri- or ap-) refers to the closest point and which refers to the far point? I can never remember without checking this page. SigmaEpsilon → ΣΕ 05:14, 9 December 2005 (UTC)
thread_title:  mnemonics 

message: I wanted to find out what a perihelion is, but this article is completely incomprehensible. Could there be a friendlier explanation? I dont think it's really that complicated. —The preceding unsigned comment was added by 194.80.49.125 (talk • contribs) 15:43, 9 August 2006.
:I'm sorry that you find the article difficult. Please explain what could be done to make it more accessible/relevant. A perihelion is a point in space at which an object (usually in orbit around the sun) is closest to the sun. The Earth's perihelion is the moment when it is closest to the sun. THe furthest point from the sun is called the aphelion. — Gareth Hughes 14:58, 9 August 2006 (UTC)
thread_title: Question

message: 203.144.143.7 15:19, 4 September 2006 (UTC)
thread_title:  The diagram doesn't show any apsis 

message: Is there a reason that there is an article for perigee, but not apogee? Perhaps these should all be merged? Or should the apogee article be created? Lunokhod 21:35, 16 January 2007 (UTC)
thread_title:  [[Perigee]] but no [[apogee]] article 

message: Could anybody please explain the difference between, say, the Perihelion of a planet and its semi-minor axis?
--Overpet 02:14, 10 June 2007 (UTC)
replies: text: A semi-minor axis is half of the shortest possible diametric section of an ellipse. It is measured from the centre of an ellipse to the ellipse itself. In an elliptical orbit, the gravitational centre is not at the centre of the ellipse (unless it happens to be a circle, it has zero orbital eccentricity), but at one of the two foci of the ellipse (a circle is an ellipse with both foci at the centre). As you can see from the diagram, the place where the ellipse is closest to F1 is A, and closest to F2 is B, whereas the semi-minor axis can only be measured to C or D. The place of farthest approach is at the opposite end of the major-axis from the closest. Perhaps it's good to think of an extremely eccentric orbit, like that of a comet orbiting the sun: it disappears off out of the solar system fo long periods, and so the sun is clearly not at the centre of the ellipse, but far away at one of its foci. — Gareth Hughes 11:01, 10 June 2007 (UTC)
replies: text: Wow, I didn't expect such a good response, thanks very much. If you wouldn't mind clarifying one thing for me. If, for example, the sun was at the centre of an ellipse, which I assume from your above comment is not possible, as it would be at one of the two foci, but for arguements sake, if it were at the centre, then would the Perihelion of an orbiting planet and its semi-minor axis be the same? Thanks very much. --Overpet 12:25, 10 June 2007 (UTC)
replies: text: It is possible for the gravitational centre of attraction of an orbit to be at the geometrical centre of the orbit, but then it creates an ellipse with zero eccentricity: a circle. A circle is an ellipse in which the axes are equal and the two foci merge on the geometric centre. Of course, there is no notion of closest approach with a circular orbit: the orbit remains equidistant at all points. If an object were at the centre of a true ellipse (one that is not a circle or a hyperbola), it would not be at a focus and thus would not be the gravitational centre of attraction. But, in such a case, the orbit would pass closest to it at the two points where the minor axis intersects the orbit. — Gareth Hughes 16:40, 10 June 2007 (UTC)
thread_title:  Confused 

message: I don't know if the other two dwarf planets besides Pluto have special names for their apses.  Wouldn't they be "-demetrion" for Ceres and "-eridion" for Eris? Or am I wrong?  —Preceding unsigned comment added by 151.203.56.155 (talk) 01:21, 23 December 2007 (UTC)
thread_title: Ceres and Eris

message: This article needs to start off simply and answer the most common questions first, such as when and why the earth is closest to and furthest from the sun.  Going to "perihelion" results in a redirect to this article, so someone looking for information on perihelion wants something at the top.  The article is too technical too quickly and will leave the average non-expert baffled from the start.  It could use some editing to make it more user-friendly.Tmangray (talk) 00:54, 6 January 2008 (UTC)
thread_title: Simplify Intro, at least

message: I noticed the remaining text still refers to two images, though there is only one image. I was going to correct the text, but upon viewing Image:Inner Planet Orbits.jpg I couldn't figure out why it is deleted. I found it very informative, and its copyright annotations seemed all in order.  —Preceding unsigned comment added by 99.226.205.27 (talk) 15:42, 28 November 2008 (UTC)
replies: text: I see no reason why it was removed and so have re-inserted it. AstroMark (talk) 22:35, 14 December 2008 (UTC)
thread_title:  What caused Image:Inner Planet Orbits.jpg to go up for deletion per CSD 

message: Does anyone know why the word aphelia (Greek for brotherly love) redirects here? Jeffrey Pierce Henderson (talk) 07:22, 11 December 2008 (UTC)
replies: text: Aphelia is the point of an orbit furthest from the Sun (Sun being helios in Greek. Are you perhaps getting confused with philia? Also see other Greek words for love. AstroMark (talk) 13:05, 11 December 2008 (UTC)
replies: text: Thank you. Jeffrey Pierce Henderson (talk) 16:41, 11 December 2008 (UTC)
thread_title:  aphelia? 

message: Hi. This article still doesn't say anything about perigees of the moon, which is what I was really looking for. Today, the moon is the closest to Earth in all of 2008. Apparently in 1999 the moon was the closest for a century. Either there should be an article on lunar apses, or this article should contain the specific information. Or, maybe this article could link to such articles. Either way, please add the information. Thanks. ~AH1(TCU) 03:07, 13 December 2008 (UTC)
thread_title:  Lunar perigee 

message: In case anyone wonders why I'd interfere in the article just to remove 'AD' from "2000 AD", the reasoning is basically like this: "Hmmm, someone's put 'AD' after the year number when, a popular British comic title aside, it should really be before it ("the Year of Our Lord 2000").  But then, that's going to seem a bit religiously fussy.  Let's switch it to 'CE'.  On the other hand, 'CE' isn't really any better, despite common practice, since it's still just the Gregorian calendar based on the traditional birth year of Jesus and therefore still carries specifically Christian overtones.  And let's face it, most of the time we don't bother with either: you rarely hear someone say that the Second World War started in AD1939, or that it ended in 1945CE, so let's just leave it out entirely and avoid the whole issue."
Sorry.  I have far too much time on my hands.  - Laterensis (talk) 08:29, 4 August 2009 (UTC)
thread_title:  Tiny tweak to date 

message: The opening paragraph of this article is     "An apsis, plural apsides (pronounced /ˈæpsɨdiːz/) is the point of greatest or least distance of a body from the center of its elliptical or eccentric orbit. In modern celestial mechanics the center is also the center of attraction, which is usually the center of mass of the system. Historically, in geocentric systems, apsides were measured from the center of the Earth."   The use of "center" in sentence 1 is misleading because, without a qualifying adjective before it, the reader's default interpretation is bound to be the "geometric center", which is not correct. Also, the word "focus" never appears in the opening paragraph, although the diagram makes it clear that the "center" referred to is really one of the foci.
I propose to replace the opening paragraph with the following:
"An apsis, plural apsides (pronounced /ˈæpsɨdiːz/) is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system. Historically, in geocentric systems, apsides were measured from the center of the Earth."
Would anyone object to this?  Duoduoduo (talk) 23:14, 22 June 2010 (UTC)
thread_title: Opening paragraph: Center vs. focus

message: :The dates of perihelion and aphelion progres through the seasons, making one complete cycle in 22,000 to 26,000 years. This is a mechanism behind one of the many Milankovitch cycles, but represents an insufficient period to cause the ice ages which occur on a period exceeding 100 million years. (emphasis mine)
The highlighted time period is dubious because so far as I remember Milankovitch cycles operate in periods of 100,000 years and not 100 million years. Ice ages are about 100,000 to 200,000 years apart so 100 million years is probably incorrect here. --<SPAN STYLE="font-family:monospace; color:#008800; background-color:#ffffcc; border:1px solid #ff4444;"> B.D.Mills </SPAN> (T, C) 07:08, 6 July 2010 (UTC)
replies: text: You're right. The article on Milankovitch cycles says ice ages happen on a 100,000 year cycle. I don't know where the 100 million year number came from but it's wrong. I'm going to correct it. Dr. Morbius (talk) 19:11, 6 July 2010 (UTC)
thread_title:  Dubious time period for Milankovitch cycles 

message: :The point of farthest excursion is called the apoapsis (ἀπό, apó, "from", which becomes ἀπ-, ap- or ἀφ-, aph- before an unaspirated or aspirated vowel, respectively), apocentre or apapsis (the latter term, although etymologically more correct, is much less used).
I'm confused. Some articles use apoasis (61_Cygni, Season, Earth's_orbit and Seasons1). Should we add this form to the article, or should we correct the others? Sharkb (talk) 10:43, 25 July 2010 (UTC)
replies: text: The others are wrong. They are misspelled. Dr. Morbius (talk) 20:26, 25 July 2010 (UTC)
thread_title:  Apoasis 

message: This is another of those annoying pages that is only useful if you already know what is being explained.  It's of no value to anyone who is trying to understand either the definition or mechanics of perihelions.  It's just one of those pages where the preacher is speaking to the converted.  Of what use is that to Wikipedia?  Anyone who wanted this sort of technical information would not rely upon Wiki, which is notoriously inaccurate anyway, but a more technical source.  Very disappointed.  NaySay (talk) 19:37, 8 April 2011 (UTC)
replies: text: I agree, if I want to get an idea of what an apogee is, I either have to infer it from the jumble of words in the opening section, or just look somewhere else where the writers aren't so impressed with themselves. The Wikipedia isn't a technical journal.  — Preceding unsigned comment added by 24.11.145.12 (talk) 01:15, 15 October 2012 (UTC)
thread_title:  Not useful 

message: The tables or article should make it clear. As an example, Solex 11.0gives, from 1900-2100, Earth perihelions ranging from 147.083345 to 147.112547 Gm, with a mean of 147.098078 Gm. Saros136 (talk) 10:26, 10 April 2011 (UTC)
thread_title:  Apsis distances vary 

message: "On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years"
Could it please be included in the article which way the dates are currently tending. For example is the July apsis currently tending towards june or august!? I think this would be a very good informative addition to this article. Thanks in advance!  —Preceding unsigned comment added by 119.18.19.65 (talk) 02:03, 24 April 2011 (UTC)
thread_title:  Dates of Perihelion and Aphelion shifting (long term) 

message: The standard system for Astronomical measures is the decimal system.
Distances should always be expresed as Kilometers and if available the milage in parenthesyst and not the other way arround.  —Preceding unsigned comment added by 108.38.23.35 (talk) 06:22, 12 May 2011 (UTC)
replies: text: Agreed, except that Unsigned does not mean the decimal system.  The standard system of measurements used in 99% of countries in civil life and in 100% for almost all scientific work is the SI. for which the unit of length is the metre; and, for Solar System distances, the kilometre is a popular multiple. 94.30.84.71 (talk) 10:51, 23 September 2011 (UTC)
thread_title:  International Units Astronomical Measures 

message: In the first diagram, the periapsis is not the nearest point on the ellipse to the focus.  It should be.  The diagram should be re-drawn in true proportion.  The diagram in "Confused" above looks correct in this respect, and could be copied and modified. 94.30.84.71 (talk)  —Preceding undated comment added 10:55, 23 September 2011 (UTC)
thread_title:  First Diagram 

message: So the article about the International Space station says that it has a perigee of 380 km and an apogee of 410 km ( or numbers like this ).   So according to the stated formula,  the major axis of its elliptical orbit is 790 km  and the length of its semi-major axis is 395 km ?  Which would put its orbit where !   Umm no.  And repetitive and pedantic waffle about the distinction between the centre  of the earth  and the barycenter of the Earth-ISS system does not solve this problem.  It is, what, about 3 centimeters in the centre of the earth ?
The "altitude" of satellites and other man-made objects in earth orbit is often described in this way,  which people can clearly understand.   Yet the actual major axis length of the ISS orbit is more like 14,000 km (whatever).
Is it actually correct to state the apogee and perigee of the space station in this manner ?  I think this issue needs to be clearly and plainly discussed at some part of this article.Eregli bob (talk) 00:56, 1 July 2012 (UTC)
replies: text: According to policy and style guidelines, it is correct for Wikipedia to reflect real-world usage. NASA has traditionally used the altitude definition since the beginning of the space age. I would assume this is because the public is more interested in how high a spacecraft is above the ground, than in its distance from the Earth's center of mass. The problem is, this article seems to have been written for and by physicists and astronomers, rather than engineers and people interested in real-world spaceflight. I absolutely agree, this article should mention the common usage in the altitude sense. JustinTime55 (talk) 17:33, 25 July 2012 (UTC)
thread_title:  Terminology problem. 

message: The Apsis/Mathematical formulae section states:
*The geometric mean of the two limiting speeds is \sqrt{-2\epsilon}
First of all, the geometric mean should not be an imaginary number; second of all, using the speeds given the g.m. appears to be simply \sqrt{\mu/{a}} hgilbert (talk) 11:32, 3 December 2012 (UTC)
replies: text: Specific mechanical energy of an orbit is \epsilon=-\frac{\mu}{2a}, so the radicand is actually positive. Your second point is correct though. Jaxcp3 (talk) 15:26, 7 August 2013 (UTC)
thread_title:  Geometric mean of two positive values cannot be an imaginary number 

message: I spent a lot of time watching Walter Cronkite and the Apollo program on TV.  I never heard pericynthion and apocynthion.  I heard perilune and apolune several times.   Randall Bart    Talk   22:14, 3 December 2012 (UTC)
replies: text: Pericynthion and apocynthion were the terms NASA used. I remember tuning in during the early hours of the Apollo 13 accident and hearing the astronauts discussing a post-pericynthion abort with mission control. The networks may have preferred the shorter terms.--agr (talk) 00:42, 3 January 2013 (UTC)
thread_title:  Pericynthion and apocynthion  

message: No useful information about apogee and perigee.  This is especially bad because those definitions are complex -- there are a number of different standards used by different countries depending on what Earth radius they used (equatorial radius, mean radius, height above geoid).  This leads to a lot of confusion when people get historical orbit info from US, British or Soviet sources, because they all used different definitions. DonPMitchell (talk) 20:24, 11 March 2013 (UTC)
thread_title:  Apogee and Perigee 

message: Being as the distances being given are precise to a distance smaller than the Earth's diameter, are the measurements mean-surface to mean-surface, or center to center?  That is, does the distance between the objects include portions of the sun and Earth, or does it only refer from the surface of the Earth to the surface of the sun? GBC (talk) 02:24, 5 July 2013 (UTC)
thread_title:  What does the distance refer to? 

message: (This topic has already been mentioned on this page, but that was all the way back in 2005)
I added the alternate pronunciation of aphelion to the lede. As far as I know, both pronunciations are used(I've only ever heard it as "ap-helion"), regardless of what the "correct" version is Jaxcp3 (talk) 16:12, 6 August 2013 (UTC)
thread_title:  Alternate pronunciation for ''aphelion'' 

message: I just tweaked the following so as to be more accurate.... "The center of mass of the Earth-Moon system or Earth-Moon barycenter, as the common focus of both the Moon's and Earth's orbits about each other, is about 74% of the way from Earth's center to its surface." - Benjamin Franklin 65.34.181.75 (talk) 14:24, 21 June 2014 (UTC)
thread_title:  Earth-Moon barycenter is ~74% of the way from Earth's center to its surface 

message: Can anyone explain these terms to me? If an apsis is the point at which a body is nearest or furthest from the body it is orbiting, then what is the periapsis close to? Itself?
I see these terms have spread around the web, but they aren't in my Collins English Dictionary which is usually pretty good on technical terms. Even in the OED, the first occurrence of periapsis is 1959, whereas the first occurrence of perigee is 1595, quite a difference. Neither apoapsis or apapsis are in the OED at all (but pericentre and apocentre are there). It seems a rather misguided generalisation. Do we need them in the article? Chris55 (talk) 11:45, 10 August 2014 (UTC)
replies: text: I think you have a point. To quote Meeus (Astronomical algorithms, 2nd ed., p. 411, note): “… ‘periapse’ would really mean the point closest to the apse; but this is rediculous, because what is meant is the apse itself!”. — Leen 94.212.249.212 (talk) 22:45, 10 December 2020 (UTC)
thread_title:  Periapsis and apoapsis 

message: The dates and times of the perihelions and aphelions vary much more than those of the equinoxes and solstices due to the presence of the Moon. Because perihelion and aphelion are defined by the distance between the center of the Sun and the center of the Earth, the Earth's position in its monthly motion around the Earth–Moon barycenter greatly affects the time when the Earth is at its shortest or longest distance from the Sun.
I removed the sentence. The anomalistic year is greater that the calendar year, so perihelia and aphelia advance through the dates in the calendar, eventually happening on each. The design of the calendar year keeps the equinoxes and solstices from varying much. In any case the the perihelion dates in this century for Earth are never more than two days different from those of the EMB. Saros136 (talk) 11:37, 4 October 2015 (UTC)
thread_title:  explanation of sentence removal, regarding effect of Moon on dates. 

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thread_title:  External links modified 

message: I found the first (upper right) diagram deceptive with these labels, since the focus (3) doesn't coincide with the larger body, but it also makes a smaller eclipse around the center of mass. So I added this diagram. The original diagram might be considered accurate if the primary body is considered stationary, but that would be a noninertial reference frame. I'm not sure how to best explain the difference, but my new diagram could show a third image with that, and then the center of mass could be drawn as a red ellipse perhaps? Tom Ruen (talk) 10:29, 21 October 2016 (UTC)
replies: text: Would it be possible to redraw this diagram to label the apoapsis and periapsis as being the maximum and minimum distances from the orbiting body to the center of mass, rather than to the other body, to make it consistent with the text? Geoffrey.landis (talk) 18:56, 29 September 2017 (UTC)
thread_title:  Diagram with (1) farthest (3) focus (2) nearest 

message: On 10 January 2018,  has proposed to merge Perihelion and aphelion into Apsis (this article); and he posted: The reason is that the "Apsis" article has much more information about the subject, including detailed info about perihelion/aphelion of Earth and other planets.
* neutral — I'm not sure if a merger makes the subject simpler, as there are already too many terms to cover under the  title "Apsis", but I have no strong opinion. I do, however, think that any proposal for a merger should be accompanied with a post on the corresponding talk page in order to allow others to easily join a conversation. Rfassbind – talk 23:06, 19 January 2018 (UTC)
replies: text: About the most likely words to access this page, redirects do the job fine, and "Perigee" would appear in the search box in bold. In addition, each redirect could point to the appropriate section where this particular case is discussed, i.e.  would redirect to the "Geocentric orbit" section, and  would redirect to the "Solar orbit" section. That would indeed be an improvement for readers. — JFG talk 13:57, 24 March 2018 (UTC)
replies: text: On the origins of the words, surely you mean etymology, not entomology, unless we are discussing orbital insects.  — JFG talk 13:57, 24 March 2018 (UTC)

text: Jackhammer, the point is that there's very little to say about the apsides of orbits around the Sun that's different from those of orbits around Earth, the Moon, or another body. Rather than duplicate all the same material about what apsides are, the etymology of the terms, and so forth in a half dozen different articles, it would be more helpful to write one good article about apsides with sections about the apsides or particularly important kinds of orbits. As JFG says, redirects would bring all of those terms to this article. -Bryanrutherford0 (talk) 20:03, 6 November 2018 (UTC)

text:  Yet the minimal amount of original content in Perihelion and aphelion is not enough to justify an entire separate article from the modestly-sized Apsis. – PhilipTerryGraham (talk · articles · reviews) 23:36, 8 December 2018 (UTC)
thread_title:  Proposed merger January 2018 

message: As per this page here, some of the terms in this article were made up by one of the contributors to this article. If we strip each of those terms out (as we should, until and unless they can be cited to sources older than the point at which they were first added to the article), the table will be left with some blank spaces. What should we do about those? DS (talk) 20:28, 5 February 2019 (UTC)
replies: text: Bah. Not only was the issue raised two years ago, citing the exact same source I just raised, the issue was argued about in 2005. "Should we do this", it went. No. No we shouldn't. I have removed the unsourced neologisms, from here and from Wiktionary both. Please do not restore them unless you have a source older than 2005. DS (talk) 20:47, 5 February 2019 (UTC)
thread_title:  Terminology and loops 

message: This section kind of repeats itself and duplicates it's information several times just structured differently. It clearly needs restructuring. Question is in which way the information could be most clearly presented?
I would prefer to remove the point list, expand the normal text (under the etymology heading) slightly and keep the table of different terms. If no one objects, I will try to get back and do it. In the meantime suggestions are welcome and by all means feel free to edit it (who knows if I ever get around to do it). Amphioxi (talk) 12:53, 30 April 2021 (UTC)
thread_title:  Terminology section 

message: due to the presence of leap years, it is difficult to guess the length of the years. Is it possible to add this datum? I suppose that should be easy for someone that manages well the software. thanks. pietro 151.29.59.56 (talk) 23:40, 27 May 2022 (UTC)
thread_title:  table of aphelion and perihelion dates