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Missing definitions[edit]

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))

Is that really correct? The original Latin term is apsis, plur. apsides. From apsis derives Eng. apse which can hardly have apsis as plur.

Sebastjan 11 March 2004 10:45 (CET)

Isn't there something slightly odd in the term "furthest approach"? Evercat 01:40, 15 Mar 2004 (UTC)


There is a link on this page to apogee which simply links back to this page. This page says nothing about the term apogee though.

Perijove and apojove[edit]

Why should "perijove" and "apijove" be avoided? Sez who? They are probably plainer.

Rlquall 19:00, 19 Aug 2004 (UTC)

It is considered very bad form to mix Latin roots with Greek, which is what "perijove" does. Jupiter is Zeus in Greek, and the corresponding Greek root is Zeno, hence perizene/apozene.
Urhixidur 15:41, 2004 Aug 20 (UTC)
"This is the kind of impertinence up with which I shall not put!"--Churchill Seriously, to say it's "very bad form" is extreme pedantry, along the lines of "never split an infinitive". Wikipedia doesn't seem to have a List of words combining Latin and Greek roots, but 'automobile' is the canonical example. Perijove and apojove are far more commonly used than perizene and apozene. NASA uses them; why shouldn't we?
—wwoods 19:37, 4 Oct 2004 (UTC)
If NASA uses them then that should at least deserve a mention in the footnote. Wikipedia should be descriptive rather than prescriptive, IMO; we should use the most widely-used term even if it's "wrong", though its "wrongness" should still be pointed out. Bryan 23:27, 4 Oct 2004 (UTC)
And the "wrongness" is of the "Thou shalt not wear a cloth woven of wool and linen" (Deut. 22:11, I ♥ Wikipedia) variety. The usual terms are better, since someone who hasn't seen them before stands a good chance of understanding -jove = -Jupiter, while nobody is going to make the leap -zene = -Zeno = -Jupiter. Without context, I'd have guessed they were geologic periods.
—wwoods 16:12, 7 Oct 2004 (UTC)

Further to this, I have just toned down the 'it is considered more correct to use the greek form for the body'. It's not specified who considers it correct, and as these are all scientific terms, I think it's best to go with what the scientists actually use, regardless of any linguistic concerns. Otherwise, you might as well put a note in the Television article saying that 'proculvision' or 'teleopsis' are considered more correct. Also, I couldn't find any use in the literature of the '-cytherion' or '-areion' terms, so I moved mention of those from occasionally used to never used. Worldtraveller 08:43, 22 Apr 2005 (UTC)

Absence of evidence is not evidence of absence, as Einstein used to say. The problem with trying to find quotes in the litterature through the Web is that there are still huge gaps. You wouldn't be able to find the first use of -saturnium mentioned in the article because it only exists as scanned-in GIFs on the web (I stumbled across it serendipitously). A lot of the litterature of the mid-20th century has yet to be made available either. The term -cytherion, apparently, was introduced by Carl Sagan in the 1960s...But just try and prove it! Anyway, I think the article is reaching a pretty decent equilibrium point by now.
Urhixidur 17:32, 2005 Apr 22 (UTC)

Indeed, very true, but if we can't cite our sources, then the information is not verifiable, and I think that's very important for an encyclopaedia. I'd say anyway that the gaps in online coverage of astronomy literature are not that huge. The Astrophysics Data System contains scanned versions of a huge number of journals right back to issue 1 in most cases ([1], [2]), and even allows you to search OCR'd text of scanned journal articles [3]. Checking now uncovers a total of 87 refs to -saturnium including the one you chanced across [4], of which 8 are in the last 50 years. Still no sign of perizenes, -hermions or others though. I still question, I'm afraid, the statement that some scholars object to these combined forms. Who are these scholars? If you can show me an example of a scholar actually saying that perijove should not be used, then that is worthy of including, but seeing as perizene is never used, it seems odd to try and give it credence by including it here in the table with perijove. We don't do the same with other hybrid words such as television, although they do have an article about them at hybrid words. I've now commented out the forms which do not appear anywhere in the available searchable astronomical literature. If they are actually used in journal articles anywhere, then they should be included, and sources should be cited, but otherwise they're just neologisms. Worldtraveller 19:18, 22 Apr 2005 (UTC)

Excellent explanaiton[edit]

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)

Aphelion's Pronunciation[edit]

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)

Missing Definitions[edit]

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

"MU" is gravitational parametr for central body, i.e. product of its mass and gravitational constant. 3 July 2005 19:06 (UTC) (A.Vitek from

earth's apogee/perigee[edit]

When are Earth's apogee and perigee? (Presumably, it's at around the same time each year.) I know it's not the same as the solstice, so when are they? --Carl 01:59, 1 Jan 2005 (UTC)

Apogee around July, perigee around January, I think. However, unlike Mars, the Earth's orbit has low eccentricity and thus the distance difference between perigee and apogee is more or less negligible and doesn't affect the weather noticeably. -- Curps 07:46, 1 Jan 2005 (UTC)
Shame on you Curps for not spotting the mistake. Earth has no apogee or perigee because it does not revolve around itself! For Epoch J2000, Earth's perihelion occurred on January 3, 1999, aphelion on July 5,1999. The following years, perihelion occurred on January 4, 2000, January 3, 2001-2003, January 4, 2004. Aphelion was on July 4, 2000-2001, July 5, 2002-2003, July 4, 2004. Next occurrences are January 3, 2005 and July 5, 2005.
Urhixidur 15:25, 2005 Jan 1 (UTC)

Pronunciation of aphelion[edit]

I'm sure that many books stating that the correct pronunciation of aphelion is ap-helion can be found. However, the Greek word suggest the exact opposite. The classical Greek word for sun is ἥλιος (hēlios). The preceding preposition is ἀπό (apo). When this is prefixed to another word that begins with a vowel it looses its final ο, and if that word begins with the spiritus asper, the π is aspirated into φ. Thus the compound word is ἀφήλιον (aphēlion), not ἀπήλιον (apēlion). If you feel that the ph in photograph, pheromone and physics should be pronounced with an /f/, then the same holds true for aphelion. Gareth Hughes 01:31, 6 Mar 2005 (UTC)

Presumably your typical classical Greek would have a lot more trouble with the meaning of the word than with its pronunciation. :-) How would a modern Greek pronounce it? Anyway, whatever its roots, it's an English word, and not bound by its ancestry.
—wwoods 19:00, 22 Apr 2005 (UTC)
All right, let's start looking at sources. For now I'll put just this one because that's all I've got here. More to follow:
  • Webster's Ninth New Collegiate Dictionary (1989) : afelyon (also attests -cynthion, -gee, -lune; note that -astron and -apsis are absent)
Urhixidur 19:16, 2005 Apr 22 (UTC)
  • The OED concurs with the /af-/ pronunciation; they cite that Kepler coined the term around 1596, and also that there was an alternative form, aphelium, for a time.

Fcw July 4, 2005 00:34 (UTC)

Redirection from Pericenter[edit]

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. -- 17:36, 17 August 2005 (UTC)


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. SigmaEpsilonΣΕ 05:14, 9 December 2005 (UTC)

Use the word "away" (as in away from the earth) to remember aphelion. Russ


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 (talkcontribs) 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)

The diagram doesn't show any apsis[edit] 15:19, 4 September 2006 (UTC)Chris

Perigee but no apogee article[edit]

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)

Advance of perihelion[edit]

Obviously the perihelion of the Earth varies a few days correponding to a few degrees or some thousend seconds of arc every year. That's much more than the relativistic advance of 43“ per century – not year – as predicted by general relativity for the planet Mercury. 09:38, 21 March 2007 (UTC)

As clearly seen form the table of data, the advance of perihelion is fluctuating by some days (one day is about 3,600“) from year to year. There is not a continuous advance every year or century. Therefore, it just nonsense to say Earth or Mercury have a certain advance of perihelion per century caused by some effect due to general relativity, since the fluctuations every year are larger than this effect in one century. 18:21, 2 April 2007 (UTC)
The variations are caused by the moon and to a lesser extent the other planets in the solar system. This is not explained in the article (it should be). -- B.D.Mills  (T, C) 06:42, 28 May 2010 (UTC)
If what you say were true Milankovitch theory would be impossible as the orbits would be too noisy to extrapolate them with the necessary accuracy to a million years.
Just as annual climate is made meaningful by averaging out the daily and seasonal temperature fluctuations, so can an orbit of a million years be made meaningful by averaging out the fluctuations. Even more so than climate because the orbital fluctuations are more predictable than the temperature fluctuations since the orbits of the perturbing bodies are predictable.
Apropos of the influence of the Moon, NASA JPL's orbital elements for Earth are preferably given for the Earth-Moon barycenter so as to eliminate that influence. The remaining two big influences are then Jupiter and to a less extent Venus, which appear as pulses along an otherwise almost perfectly elliptical orbit coinciding with when those planets are respectively in opposition and inferior conjunction, essentially their points of closest approach to Earth.
I would however think that giving distances to the Sun to the nearest km only makes sense for a best-fit Keplerian (elliptical) orbit defined by a choice of orbital elements, and even then 1,000 km precision would be more appropriate. Giving the actual distance to the nearest km at any specified instant conveys nothing for the reason you give, namely those perturbations. Vaughan Pratt (talk) 23:44, 19 December 2013 (UTC)


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)

F1 & F2 are foci, a is the semi-major axis, b is the semi-minor axis.
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)
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)
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)

There is no real definition for the terms listed on this page, or if I am mistaken, the article needs some clean-up. I was looking up perihelion in this article and fail to understand it.. WinterSpw 05:46, 12 June 2007 (UTC)

Ceres and Eris[edit]

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 (talk) 01:21, 23 December 2007 (UTC)

Simplify Intro, at least[edit]

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)

time of periastron[edit]

what is it? a field named like this apears on the exoplanet infoboxes, like in 70 Virginis b, XO-1b, WASP-1b and others. can anyone explain it? thanks. (talk) 02:40, 25 April 2008 (UTC)

It is a moment of time when (exo)planet passes periastron. — Chesnok (talk) 17:28, 27 April 2008 (UTC)
I don't quite catch it. On 70 Virginis b says "Time of periastron (T0)= 2,447,239.82 ± 0.21 JD" . That means that the planet passes it's apsis each 2,447,239.82 days? or that passes it 2,447,239.82 days after j2000? thanks again, by advance (talk) 04:07, 28 April 2008 (UTC)
JD is Julian day (number of days since noon Universal Time (UT) Monday, January 1, 4713 BC).
JD 2,447,239.82 ± 0.21 is a moment of time when planet passes periastron. — Chesnok (talk) 08:23, 28 April 2008 (UTC)
Now I understand. (talk) 20:44, 28 April 2008 (UTC)

What caused Image:Inner Planet Orbits.jpg to go up for deletion per CSD[edit]

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 (talk) 15:42, 28 November 2008 (UTC)

I see no reason why it was removed and so have re-inserted it. AstroMark (talk) 22:35, 14 December 2008 (UTC)


Does anyone know why the word aphelia (Greek for brotherly love) redirects here? Jeffrey Pierce Henderson (talk) 07:22, 11 December 2008 (UTC)

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)
Thank you. Jeffrey Pierce Henderson (talk) 16:41, 11 December 2008 (UTC)

Lunar perigee[edit]

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)

Tiny tweak to date[edit]

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)

Opening paragraph: Center vs. focus[edit]

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)

Dubious time period for Milankovitch cycles[edit]

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. -- B.D.Mills  (T, C) 07:08, 6 July 2010 (UTC)

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)


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)

The others are wrong. They are misspelled. Dr. Morbius (talk) 20:26, 25 July 2010 (UTC)

Stop redirect, create two less technical pages. (Apogee/Aphelion & Perigee/Perihelion)[edit]

I think this article could gain a lot by avoiding a redirect from the commonly-used words of Apogee and Perigee. Apsis is an overarching concept, and to understand the basics of "Apogee" one need not first grasp the overarching term "Apsis". It's confusing to the general encyclopedia reader, who may be seeking to answer the question of "What is an Apogee?" or "What is a Perihelion?"

I suggest that Perigee/Perihelion and Apogee/Aphelion are given their own, less technical pages (each having a link to the general concept of Apsis). -PhysGrad —Preceding unsigned comment added by (talk) 20:49, 19 October 2010 (UTC)

Sounds like duplicating information to me. I think the 3rd para in the intro partially remedies for the complications you're adressing, although improvements could well be considered. Weird items such as perimelasma for black holes could redirect to Apsis#Terminology. Rursus dixit. (mbork3!) 20:00, 22 January 2011 (UTC)
The trouble is, the 3rd paragraph doesn't explain what these words mean. Currently, one has to wade through a lot of irrelevant notes about greek grammar to try and extract the basic meanings of the two apsis forms, and then figure out how to apply this to the derived terms. I think a simple paragraph near the beginning, perhaps under a separate heading, defining in simple terms what perigee/apogee and perihelion/aphelion mean would be useful.
Also, although we are told what the genitive and plural forms are and given some (irrelevant?) info about aspirated/unaspirated vowels, it doesn't actually say what the greek word ἁψίς means. If anyone thinks the apo/apho stuff is relevant, or even interesting, then maybe it should be moved to a footnote - or maybe add a separate etymology section. Thirteenangrymen (talk) 11:51, 4 April 2011 (UTC)

Not useful[edit]

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)

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 (talk) 01:15, 15 October 2012 (UTC)

Apsis distances vary[edit]

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)

Dates of Perihelion and Aphelion shifting (long term)[edit]

"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 (talk) 02:03, 24 April 2011 (UTC)

International Units Astronomical Measures[edit]

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 (talk) 06:22, 12 May 2011 (UTC)

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. (talk) 10:51, 23 September 2011 (UTC)

First Diagram[edit]

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. (talk) —Preceding undated comment added 10:55, 23 September 2011 (UTC).

Terminology problem.[edit]

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)

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)

Geometric mean of two positive values cannot be an imaginary number[edit]

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)

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)

Pericynthion and apocynthion[edit]

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)

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)

Apogee and Perigee[edit]

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)

What does the distance refer to?[edit]

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)

Alternate pronunciation for aphelion[edit]

(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)

Earth-Moon barycenter is ~74% of the way from Earth's center to its surface[edit]

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 (talk) 14:24, 21 June 2014 (UTC)

Periapsis and apoapsis[edit]

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)