# Talk:Axial tilt

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It is a popular misconception that the sun is at its greatest elevation above the horizon at noon every day. This misconception is reflected in the "Measurement" section: "..

## Term axial tilt and nutation, etc...

In physical terms any kind of axial tilt of the earth should be called nutation of earth. On the wikipedia-site "nutation" there is just the effect of the moon mentioned. But maybe I'm wrong. —Preceding unsigned comment added by 131.188.2.11 (talk) 16:09, 24 February 2009 (UTC)

==What is the tilt of the earth's axis? I really really need to know this for my sciennce homework.A.S.A.P.!!!! Please help me... Raven Rashal

## Teleise

the difference of the heights of the Sun above the horizon at noon on the longest and shortest day of the year is twice the obliquity."

Because of the eccentricity of the Earth's orbit around the Sun, the angular velocity of the Earth's orbital motion is not constant. The axial tilt of the Earth also causes a periodic variation in the time that the Sun reaches the meridian. As a result, the Sun may cross the meridian (and reach its maximum height above the horizon) as early as 16 min 33 s before noon (around November 3) or as late as 14 min 6 sec after noon (around February 12). This is explained in detail on the “Equation of time” page.

Of course there is always a tradeoff between maximum scientific accuracy on the one hand and the goal of keeping articles simple and straightforward for non-technical readers on the other hand.

One possibility: “...the difference of the maximum heights of the Sun above the horizon at midday on the longest and shortest day of the year is twice the obliquity.”

The last paragraph of the section would also need to be tweaked similarly.

Piperh 13:15, 29 January 2007 (UTC)

The text states that the Earth's axial tilt varies between 21.5° and 24.5°. But according to the image it ranges from 22.1 to 24.5. Marcel

I have a question about the Moon's 1.5 degree axial tilt. If astronomers were to give the Moon northern and southern tropical circles, and the equivalent of arctic and antarctic circles, would the tropics be 1.5 degrees north and south of the Moon's equator, and the arctic and antarctic circles 88.5 degrees north and south of the Moon's equator?

Thank you. Bruce McClure

Well I suppose you could, but no one does that. 72.231.18.127 06:52, 14 May 2006 (UTC)

It is not that simple. The axis of the moon inclines 1°32' with its orbit around the earth, which on its turn inclines by 5°09'. Therefore the total inclination of the axis with the ecliptic is 6°41' as the two angles always add up due to forced interaction. This was discovered by Cassini in 1721. The information in the moon article is wrong (was wrong, very soon now) --Tauʻolunga 09:25, 4 June 2006 (UTC)

## Wrong passage

I took out the sentence which said that for "the axial tilt is 0 or 180 depending on whether the planet is rotating in the same direction as its orbital motion". This does not make any sense for the following two reasons:

1. The axial tilt is the angle between the normal of the orbital plane and the axis of rotation of the body. So if the rotation axis is perpendicular to the orbital plane then the angle is 90 degrees, not more, not less! The axial tilt of Uranus is listed as 97 degress.
2. If the planet's axis of rotation actually lies in the orbital plane how do you define if it has proper or retrograde rotation? The rotation direction is determimed by the rotation of the "north pole" with respect to the ecliptic but in that case both poles actually lie on the orbital plane and neither one is to the north.

Smartech 03:41, 6 May 2005 (UTC)

I feel that the degrees, when referenced at greater than 90, are helpful. Anything greater than 90 degrees shows that the body is rotating in retrograde to its orbital direction. Example: Venus at 2.64 is true, but does not give a hint that it is rotating in a direction opposite from its orbital direction. Venus at 117.36 is likewise true, but shows that Venus is rotating in retrograde (to its orbital direction). As long as the point of relation is regarding the body's orbital plane and direction (i.e., obliquity), then using the 180º perspective works. In addition, in this context, concerns regarding the arbitrary "north-south" perspective is not an issue (in the context Obliquity to Orbital Direction, it need not be a perspective point). At least, this is true regarding the planets in our solar system (all revolve (orbit) in the same direction. Granted, when detailing aspects of some planetoids and satellites (those that orbit in the opposite direction, clarifying text should be added to such page to inform the reader that the orbital direction is opposite from the standard "northern-perspective, counter-clockwise" direction.
I think a problem stems from several of the pages attempting to use inclination, obliquity, and ecliptic (earth’s orbital plane) as relation points. In doing so, the pages need to be brought into some consistent term usage. See my entry, below, regarding Terms and Usage for Inclination, Obliquity, and Axial Tilt. Tesseract501 7 March 2007

## Axial Tilt

The tilt referenced in this article is different from the one in milankovitch cycles. The reference from that article came from NASA but I would like to know where the reference was obtained in this one.

## Query

Out of curiosity, how is the obliquity quantified? That is to say, what is the instrument by which it may be measured? If an instrument does not exist, what is the mechanism to do so?

That is a stupid question. But some teachers say that there are no stupid questions, only stupid answers. Well look and decide for yourself; see article. --Tauʻolunga 08:53, 4 June 2006 (UTC)

## A few odds and ends

I am by no means an expert on this subject but some things caught my attention. First, we sometimes use degrees minutes and seconds and at other times we use decimal degrees. Should both be used or only one or the other or does it depend?

Second, early in the article we say that the Earth has an axial tilt of about 23° 27’ and later we say that in 1976 ε equaled 23° 26’ 21.44". We also say that the degree of tilt is diminishing. Is ε the same as the axial tilt? If so, shouldn't the current degree of tilt be smaller than 23° 26’?

Last, the Tropic of Cancer article says the rate of change is about 0.5" per year but this article does not give a number. --YellowLeftHand 22:59, 26 January 2007 (UTC)

## Redirect request

Took me a while to find this page. Can a redirect be made from axial inclination to here? 62.136.137.117 14:34, 13 February 2007 (UTC)

Axial Inclination may be a mixing term usage. Equatorial Inclination or Axial Tilt might be the more accurate usages. A so-called inclination for the axis would be from polar. I believe that most technical references use the term "tilt" for such, and the term "inclination" regarding degrees "inclined" from an equatorial or orbital-plane perspective. When you did your search, that could be why the system sent you in a bit of trip around-about, side-to-side (inclination) or up-and-down (tilt)? Tesseract501 7 March 2007

## Terms and Usage for Inclination, Obliquity, and Axial Tilt

The pages detailing the Planets and the Planetoids (asteroids, TNO, dwarf planets) are great. The authors do great work and I appreciate all the effort and dedication they take. I do have a suggestion, however. I believe that we need some additional consistency regarding some items on the tables. A problem stems from several of the pages using inconsistent relationship reference points for the Inclination, Obliquity, and Ecliptic (earth’s orbital plane) data. In doing so, the pages need to be brought into some consistent term usage. For example:

1. INCLINATION: The use of the item for INCLINATION, under the Orbital Characteristics table section (under the various Planet/Planetoid pages), is inconsistent. Suggestion -- INCLINATION should relate to the body's orbital plane in relation to the Earth's ecliptic (orbital) plane. If the authors use something else, then they need to specify the "from-and-to" relationship. Otherwise, the reader is open to confusion, assumption, and the need to check alternative source documents. Examples of variations-of-use that need to be specified on various Planet/Planetoid pages: Axial Tilt in relation to the Ecliptic, Body's orbital plane in relation to the Sun's equator, Body's equatorial inclination in relation to the Ecliptic).
2. OBLIQUITY: The use of the items under the Physical Characteristics table section (on the various planet/planetoid pages) is inconsistent, as well. Some authors use the term: Axial Tilt. Others use the term: Equatorial Inclination. The ambiguous term, Inclination, is used as well. Suggestion -- OBLIQUITY should be used, and refer to the Body's equatorial inclination to its own orbital plane around the Host Star. In the case of satellites and sub-members of binary systems, the relationship can be to the Principal Subsystem Host (instead of the Host Star), but references to such should be added to the label or to the supporting text. Im addition, if the author wishes to indicate additional information on the table, that is super -- but doing so should include clarification text
Examples: The Pluto page referencing Pluto's axial tilt in relation to its orbital plane and to the Earth's Ecliptic (an unusual relationship reference for a planet’s axial tilt). Charon's relationship to Pluto (its principal. Etc.

At present, several of the Planet and Planetoid (asteroid and TNO pages) use INCLINATION and AXIAL TILT as table items, without clarification as to the relationships (orbital plane, ecliptic, Sun's equator, etc.). Again, this ambiguity puts the reader in the position of having to go to secondary source documents to verify the intent and accuracy of the page data. I do not know if this is the page best suited to post these suggestions, but I will copy this under the Inclination and Obliquity sections too. Tesseract501 7 March 2007

Regarding the given formulae ε = 84,381.448 − 46.84024T − (59 × 10−5)T2 + (1,813 × 10−6)T3, b... Jean Meeus gives for the 2nd term -46,8150xT, which however gives a contradictory result (inversion of sign) with the example reckoned with the given Laskar's formulae. Signed: G. Campart

The paragraph starting Lower obliquity misuses the term; it perhaps should be Lower obliqueness of radiation, or something like that. --James G. Sack —Preceding unsigned comment added by Jgsack (talkcontribs) 21:03, 9 November 2008 (UTC)

## Glaciation

I modified the section on glaciation and climatic effects, adapting language from a NOAA site (which is based on papers by Pisias and Imbrie from 1986 and 1987) to render the meaning in a more straightforward way. Rsed 22:03, 25 August 2007 (UTC)

## measurement example

the measurement section contained an error in the example saying that an observer at 50 degrees latitude would see the sun at noon on the longest day at 50+23.5 degrees above the horizon which is incorrect, it should be (90-50)+23.5 as 90 minus latitude gives you the altitude of the sun at equinox not just latitude —Preceding unsigned comment added by 86.155.125.235 (talk) 12:24, 8 September 2007 (UTC)

## Baloney removal

"If we were off that axis by a degree we would burn up or freeze to death."

I'm calling 'baloney' on that statement and removing it. It's not even the least bit scientific. —Preceding unsigned comment added by Bizzybody (talkcontribs) 13:00, 20 October 2007 (UTC)

## Use of thin-space in table

This table uses a thin-space to slightly separate the 3rd and 4th fractional digits of the decimal number 23.439281 . However, the &thinsp; (" ") was displaying as a box in my browser. I changed it to use a unicode construction {{unicode|&thinsp;}} ("").

Axial tilt#Axial_tilt_of_major_celestial_bodies

Since the thin space makes hardly a difference in the display, if the unicode construct doesn't work for everyone, I suggest removing the separator entirely.

KerryVeenstra 17:21, 1 December 2007 (UTC)

## Citing, cleanup and more

Hi fellow wikipedians!

Due to the fact that this article was tagged with {{Citations missing|date=12 December 2007}}, I will scrutinize this article, find cites, do some basic cleanup etc. --Dna-Dennis (talk) 16:30, 25 December 2007 (UTC)

I've changed the tag to no footnotes and removed the previous Citations missing tag. The references sections is with references now, but the sources are unclear and its not cited in-text either. Its like some random reference. Venny85 12:39, 6 April 2008 (UTC)

## Angles more than 90 degrees

It was not clear from reading this article what angles over 90 mean. Looking at a diagram shown, it appears that the angle is measured opposite to the direction of orbit. In other words, Earth is tilted away from the direction of orbit, and Venus (over 170 degrees) is tilted towards the direction of orbit. Being a layperson, a readable clarification of this would be welcome. —Preceding unsigned comment added by 166.34.158.130 (talk) 19:22, 19 February 2008 (UTC)

I don't understand this either. Isn't 177 degrees toward direction of orbit the same as 13 degrees away from direction of orbit?69.250.30.33 (talk) 15:28, 25 August 2009 (UTC)

It's not 177 degrees toward direction of orbit. It's 177 degrees between the perpendicular and the axis of rotation. On its own just saying perpendicular and axis is ambiguous because 177 degrees between these lines at one end is equal to 3 degrees between these lines at the other. So you need to specify which pole of rotation and which pole of perpendicularity you are going to measure from. You do this by using the counter-clockwise pole. Setting zero degrees to be rotation in the same direction as the orbit selects the pole of perpendicularity. Zbayz (talk) 16:25, 25 August 2009 (UTC)

## Confused

I've read quite a number of articles now to try and understand how the seasons work. It's embarrassing that I don't already know!

This seems to be the key article, and having read it I'm still confused. The nub of my confusion is around this sentence:

The axis remains tilted in the same direction throughout a year; however, as the Earth orbits the Sun, the hemisphere (half part of earth) tilted away from the Sun will gradually become tilted towards the Sun, and vice versa.

It reads to me like there are two contradictory statements on either side of that semicolon. Obviously the second statement is correct (because we have seasons), so I must not be understanding the first statement. The axis remains tilted in the same direction relative to what frame of reference? Not the Sun's frame of reference, or surely the second statement would be false?

The diagram doesn't help. According to the diagram the axis tilt is in the direction of orbit with the Southern Hemisphere leading the way. Perhaps the diagram represents a snapshot at a point in time where the axis of rotation and direction of orbit line up, perhaps at an equinox.

The only way my puny little mind can wrap itself around the geometry described in this article is if the axis of rotation rotates around the line perpendicular to the orbit, with one rotation taking the same time as an orbit. This would keep the axis tilted in the same direction relative to the fixed stars. (I believe the fixed stars are used to judge the period of an orbit.) This explanation fits what the article appears to be saying, but begs the question what remarkable coincidence caused the Earth's axis of rotation to line up with the fixed stars and not with something closer to hand.

So I'm guessing there's something that I just haven't been able to conceptualise about the geometries involved.

If you can help, or make the article a little easier to understand, that would be great!

Ben Arnold (talk) 11:35, 17 May 2009 (UTC)

The illustration in the article seems misleading. The direction of tilt is not fixed relative to what is labeled as the "orbit direction" in the illustration.
See the right way and wrong way examples here:
http://www.suntrek.org/earth-beyond/spinning-orbiting-earth/what-causes-seasons/watch-tilt.shtml
This animation is more interactive:
http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html
Other animations here:
-Ac44ck (talk) 00:03, 19 May 2009 (UTC)
Ben asks, "what remarkable coincidence caused the Earth's axis of rotation to line up with the fixed stars and not with something closer to hand." Yes, this is, in fact, too remarkable to be mere coincidence. And things can only be this way, if space is reasonably flat in the vicinity of Earth's orbit. The good news: it is! -- 99.233.186.4 (talk) 02:45, 19 November 2009 (UTC)

## Axial tilt: 177 degress = 3 degrees

Some user posted confusing text that if the angle between rotational axis and line perpendicular to orbital plane at one end is 177 degree then at other end it is 3 degrees.

Astronomers specify inner angle. Inner angle of axial tilt is 23.5 which corresponds to outer angle 156.5 but we do not say that axial tilt of earth is 156.5 degrees. Inner angle at both ends is same i.e 23.5 degrees. So I have removed confusing text. Rāmā (talk) 15:49, 17 October 2009 (UTC)

right. but do remember that the planet Venus has an axial tilt of 177.3 degrees, which means it spins in the opposite direction. -- 99.233.186.4 (talk) 02:59, 19 November 2009 (UTC)

Allowing obliquities greater than 90° is inconsistent with the IAU's definition since 1970. I'm not saying I agree with the definition, but that is what it is. See here. The right hand rule which is talked about in this article gives what the IAU calls the "positive pole". --Lasunncty (talk) 10:32, 4 December 2010 (UTC)

I am not a professional in this area and I may be missing the point entirely, but I think a word is being used inaccurately. "A smaller ε means a larger p (precession in longitude) and vice versa. Yet the two movements act independent from each other, going in mutually perpendicular directions." There may be a use of the word 'independent' that I do not know, but if it is always the case that a smaller ε means a larger p, and vice versa, and they move in mutually perpendicular directions, then they clearly are not independent in a statistical sense. If you are able to plot them against each other along x-y coordinates, will you get a scattergram? It seems not. If you get anything other than a scatter gram, they are not independent; if you can express one as a function of the other, they are not independent. Dependence does not necessarily mean that two things will change in exactly the same way; only that in some way the movement of one is a function of the movement of the other. — Preceding unsigned comment added by Emdrgreg (talkcontribs) 23:11, 8 February 2011 (UTC)

## context tags

i'm not understanding how this can be unclear. the named context is astronomy, where matters of spin and orbit are fundamental, afaik even to non-experts. perhaps an animation similar to above-referenced external links is best-suited to appear at the top of the page? -- 99.233.186.4 (talk) 02:59, 19 November 2009 (UTC)

## Missing redirects : 'figure axis' and 'north-south axis'

In today's NASA article Chilean Quake May Have Shortened Earth Days, the following statement is made: "Earth’s figure axis is not the same as its north-south axis; they are offset by about 10 meters (about 33 feet)." At this moment figure axis and north-south axis give red links. Could someone create redirects to the correct pages on Wikipedia? Wiki-uk (talk) 11:22, 2 March 2010 (UTC)

## Extreme variations in Earth's axial tilt

This statement:

Tentative evidence has recently emerged for extreme (> 50°) variations in terrestrial axial tilt.

... appears to be a misinterpretation of the researcher's findings and the information in their news release (http://www.princeton.edu/main/news/archive/S15/64/72A37/index.xml?section=newsreleases). Although they hypothesize that the position of the earth's poles may change substantially with relation to the continents, they state explicitly that "the poles themselves would still point in the same direction with respect to the solar system." Thus it is not the orientation of the rotational axis relative to external referents that changes so dramatically, but rather the orientation of the planet and its continents with respect to the rotational axis. The wording of the headline of the news release is rather unfortunate, as it may easily give the wrong impression to someone who does not read the entire text carefully.

Since this hypothesized phenomenon does not involve or affect axial tilt, it would not seem to belong in this article, so it has been removed. Piperh (talk) 06:31, 12 July 2010 (UTC)

Should the angle measures in radians be relative to pi? (that is, instead of 3.10, 0.95pi)? Radians is kinda meant to be used this way. D O N D E groovily Talk to me 03:55, 3 January 2011 (UTC)

## Eight day week?

Two things remain unclear to me. Is day length affected? If so, how (if the axial tilt is more like Uranus, frex)? And what impact does this have on seasons & temperatures at the equator (again, taking the Uranus example)? It would appear the day would be months long, & the equator would always be cold, & there'd be two seasons, hot & cold. Is this accurate? TREKphiler any time you're ready, Uhura 16:06, 12 January 2011 (UTC)

I know this is an old question, but it's worth an answer. Day length really only relates to the planet's rotation rate, not its tilt. If, however, you mean the hours of sunlight per day, then yes the tilt does matter. For any planet, if you are on the equator, there is always half a day of sunlight and half a day of night. At the poles, there is always half a year of sunlight and half a year of night. The polar circles separate the areas with sunrise and sunset every day from areas lacking sunrise or sunset at least one day of the year. The latitude of the polar circles is directly related to the tilt. For the earth, they are about 23° from the poles, at 67° N and S, but for Uranus, they would be about 82° (or 98°) away from the poles, at only 8° N and S.
Uranus's day is about 17 hours long, and its year is about 43,000 of these days. If we take latitude 45° as an example, there would be about 11,000 days or 190,000 hours around the summer solstice where the sun wouldn't set, and the same amount of time around the winter solstice where the sun wouldn't rise. The other 21,000 days of the year the sun would rise and set every day like most earthlings are used to.
As for temperature, the poles of any planet would be warmest around the summer solstice and coldest around the winter solstice. In Uranus's case, however, the poles would actually get warmer than the equator at the summer solstice since the sub-solar point would be only 8° away. The equator would be warmest around the equinoxes and coolest around the solstices. --Lasunncty (talk) 11:57, 4 July 2015 (UTC)
Thx so much for the explanation. (Even taking pity on my imprecision... :D ;) ) Yes, I was taking "hours of sunlight" rather than "period of rotation". I'm much clearer now than before. TREKphiler any time you're ready, Uhura 19:21, 4 July 2015 (UTC)

## HELP!

What is the tilt of the Earth's axis? please please please tell me! i do not have time to read ALL of the info so if someone could post it in the first few paragraphs after answering my question, that would be AMAZIING! thanks, Roza (talk) 00:37, 18 March 2011 (UTC)Raven Rashal

## Removed offtopic stuff about Japan

Removed bullshit article with overdue weight and offtopic on Japan. Precision of measurement could be discussed and that the Japan earthquake(and perhaps other things) was measurable could be mentioned as an aside. (Maybe also mention that they can see weather effects in Earths rotation.)82.169.255.79 (talk) 14:18, 12 April 2011 (UTC)

## Minutes and seconds of arc

In the section about "Obliquity of the ecliptic" the text states that the obliquity "is decreasing at a rate of about 47" per century". The term 47" links to the article about Minute of arc, which is reasonable since that is where second of arc is also described. However, when you let your mouse hover over the 47", a tooltip appears saying Minute of arc, which is confusing to the reader since the unit here really is seconds.

Any suggestions on how to solve this?--Oz1cz (talk) 19:45, 30 July 2013 (UTC)

We could instead link to "Second of arc" or "arcsecond", which redirect to the "Minute of arc" article. --Lasunncty (talk) 21:26, 3 August 2013 (UTC)
Done --Oz1cz (talk) 17:16, 5 August 2013 (UTC)

## October 2013

this is about the tilted axis of the earth it seems to me the axis remain tilted outside of its orbit[ north pole remain tilted away from the sun when the earth is just at the right hand side of the sun, during 22nd December when it is winter in the northern hemisphere]but the images especially the video is showing the difference. if the axis is as it is shown in the video[the moving blue object around the yellow ball] then the it is summer in the northern hemisphere when the earth is at the right or at the eastern side of the sun when it is the month of December as the earth is revolving the sun from west to east. so it seems to me that the diagram needs to be changed. — Preceding unsigned comment added by 117.241.232.91 (talk) 15:31, 14 October 2013‎ (UTC)

The right side of the diagram is June, the left is December. The diagram is correct... does the caption need clarification? -Lasunncty (talk) 00:04, 18 October 2013 (UTC)

## Tilt amplitude and tilt direction

Fig. 1: reference direction to Aries for Orbital Inclination

I could not find a clarification in this here article : To describe an object spin's axis in a 3D coordinate system one need either a 3-vector of positions on the 3 axis or a 2-vector of angles, with amplitude and direction. It seems obvious that the word tilt refers to the amplitude, but i can't find anything to give a clear understanding of the direction of the axial tilt. I cannot imagine that each and every planet has the same tilt direction.
I have been trying to understand this in reading many articles here and in other websites. In the article about Orbital inclination there is a reference direction (Aries, which is neither of the solstices), but only in the picture, with no reference in the text).
I don't have a problem with understanding why some say, for Venus tilt : 177° and other say 3° with a mention about reversed rotation, as these views refer to the two political bodies (IUA and NASA) having different perspective on things. What i don't understand is that almost every website i have read that lists planets tilt neglect this parameter in their description of the concept, as if it was obvious. I have seen one reference to summer solstice for Earth, but this is the same thing as direction of tilt, which is forthright unknown for the other planets and one needs to know the direction of their tilt to define the solstice ! I personnally think that the orientation should be the first thing to specify/clarify after the plane of reference !
Further clarification : If you think of Uranus having a tilt close to 90°(97.8°), you still need to know if its poles axis is tangent or perpendicular to the orbit, or anywhere in between, or rather when it is in a 'standard' direction, which is part of the definition of the solstice.
Zigomar7 (talk) 11:48, 25 January 2014 (UTC)

The section Axial tilt#Other objects of the Solar System lists the directions of the north poles, by NASA and IAU definitions. Is that what you were looking for? --Lasunncty (talk) 08:50, 27 January 2014 (UTC)
Thanks Lasunncty, but the picture about Earth's axial tilt in the middle of the main page does not refer to R.A. or Dec to explain the tilt, and when i search for Right ascension and Declination, I find them to be refering to the planet's equator, not the ecliptic, if so it (tilt) should always be 90°. And why is the Earth's RA 'undef' in that table you point to me. So there is still something obscure for me... --Zigomar7 (talk) 20:02, 29 January 2014 (UTC)
If you compare the coordinates of the rotational pole and the coordinates of the orbital pole, that difference is the axial tilt. You can use any celestial coordinate system you like to describe those coordinates, as long as you are consistent. For most planets, the ecliptic pole is a good approximation for the orbital pole, since they are all inclined by small angles relative to the ecliptic.
A Dec of 90° means that our north pole is 90° from the equator, not that the tilt is 90°. The reason the RA of the Earth's pole is undefined is because all meridians intersect at the poles, so picking one (like NASA did) would just be arbitrary. --Lasunncty (talk) 11:07, 30 January 2014 (UTC)
I withdraw my 30 January 2014 (UTC) comments as useless to the topic Zigomar7 (talk) 10:30, 1 February 2014 (UTC)

## Origins of tilt

Can somebody add a section, paragraph, or link on theories of how the earth's tilt came about? — Preceding unsigned comment added by 98.235.167.63 (talk) 01:50, 20 June 2014 (UTC)

## found this clarify tag with comments

clarify|date=January 2013
I suspect this should be "before", i.e. T is positive for times in the past, and negative for times in the future? the linear term is positive, but above we claim that "it is currently decreasing".

T is the time since the "epoch" J2000.0. It would be exactly opposite this comment - negative before J2000.0, positive after. T is not tilt, it's just a parameter for the equations given here. Tfr000 (talk) 03:33, 7 May 2015 (UTC)

## split off Earth's Axial Tilt

Ok, someone has proposed that we split off an article about Earth's axial tilt only. Discuss here. Tfr000 (talk) 12:03, 24 May 2015 (UTC)

Oppose, because this article is mainly about Earth already. Earth isn't special, other than that we happen to live upon it, which is why much of the discussion here is about it. Otherwise, it's another planet with a tipped axis. Tfr000 (talk) 12:40, 24 May 2015 (UTC)
Oppose, I cannot see any reason to discuss earth's axial tilt differently than other planets. Allecher (talk) 01:44, 25 May 2015 (UTC)
Oppose - Too much overlap between the two concepts. --Lasunncty (talk) 01:24, 27 May 2015 (UTC)
Oppose - I removed the {{split section}} template and revised the entire article.-- Cheers, Rfassbind -talk 05:53, 14 August 2015 (UTC)
Not bad. It needed something like that. Tfr000 (talk) 23:21, 14 August 2015 (UTC)

## absurd and unusable (and also incorrect) units

I came to this page searching for an expression for the obliquity which would help me to calculate its value during the bronze age (~2000BC). The equation (attributed to Lascar) is there, but in my view is unusable because of the use of degrees, minutes & seconds. How on earth is anyone expected to be able to evaluate this expression in this form !? I have never seen anything so absurd and useless. What can possibly be in the mind of the person who wrote that ? Why not quote the expression in the form given here, where the coefficients are in units of seconds of arc: http://www.neoprogrammics.com/obliquity_of_the_ecliptic/

Or even better, divide by 3600 and express the coefficients in units of degrees with decimal fractions, which is, I suppose, what real people want and use. g4oep — Preceding unsigned comment added by 77.96.58.212 (talk) 16:58, 20 June 2015 (UTC)

Well you have several options... 1) click the link and get the data from Laskar's original paper and put it in whatever form you want, 2)start -> all programs -> accessories -> calculator... 23 degrees = 82800" and 26' = 1560" and the rest is already in seconds, 3)have the computer do those two tedious calculations for you in whatever program you're writing. Tfr000 (talk) 20:35, 21 June 2015 (UTC)

No - my most useful option is to go here, and I recommend it to others who want this equation in a usable form. http://www.neoprogrammics.com/obliquity_of_the_ecliptic/

By deleting my edit to the main page you retain the wikki option in a form which is not only unhelpful to potential users, but is also inaccurate. The units of the coefficients in the equations quoted in the article are all wrong except for those of the zeroth order terms. The reference I give above is readily usable and also dimensionally correct (insofar as it quotes dimensions at all). g4oep — Preceding unsigned comment added by 77.96.58.212 (talk) 13:49, 23 June 2015 (UTC)

The two sources appear to say the same thing. Unfortunately the article failed to provide a correct definition of the time variable for Laskar's equation. I fixed it. Jc3s5h (talk) 14:50, 23 June 2015 (UTC)

Hey guys, do you realize that the article now says:

```In the following equation t is measured in units of 10000 julian years from J2000.
[blah blah blah equation]
where here T is multiples of 10,000 Julian years from J2000.0.[12]
```

Tfr000 (talk) 23:33, 23 June 2015 (UTC)

Fixed. Jc3s5h (talk) 02:03, 24 June 2015 (UTC)

## Contradictory statements in the Introduction

In the Introduction it states that the 177 degree axial tilt of Venus means that the planet's rotation is "retrograde". However, in section 1.1 "Two standards" it states that the right-hand-rule gives a 177 degree axial tilt and rotation is "direct" (prograde).

I'm surprised such an obvious contradiction has survived. Surely the right-hand-rule can't result in a rotation that's defined as retrograde, because it's arranged to produce a rotation towards the East, like Earth's. Anyway, it would be nice if the contradiction were resolved one way or the other. — Preceding unsigned comment added by 91.235.65.1 (talk) 09:20, 15 July 2015 (UTC)

The article pretty clearly explains the contradiction. Since there is no "official" way of defining tilt/rotation, we live with it. Tfr000 (talk) 22:34, 15 July 2015 (UTC)

Update: At least someone's read my comments, but I still say the article does contradict itself. It says the rotation is retrograde then says it's direct (prograde). It's nothing to do with the different definitions of tilt. The contradictory statements use exactly the same definition of axial tilt (the right-hand-rule). — Preceding unsigned comment added by 212.121.220.12 (talk) 13:05, 16 July 2015 (UTC)

International Astronomical Union... rotates retrograde... the right hand rule does not apply
NASA... with the right hand rule... rotates direct
Seems pretty clear, maybe I'm missing something. Tfr000 (talk) 01:17, 17 July 2015 (UTC)

Second Update: That part is OK, but you've still not seen the problem. Anyway, I am now going to remove what I think is the error. Thanks. — Preceding unsigned comment added by 212.121.220.12 (talk) 09:13, 17 July 2015 (UTC)

One of the sources used to support the section, Alexandre C.M. Correia, Jacques Laskar and Olivier Néron de Surgy, said "The crossing of this zone in the past,can lead Venus' spin to its present retrograde configuration..." [[Jacques Laskar] is a very widely cited astronomer. If those authors say Venus's spin is presently retrograde, then that is an acceptable description if the spin. No Wikipedia editor is competent to deny this. Furthermore, you must not misrepresent the contents of cited sources. I have reverted the edits by 212.121.220.12. Jc3s5h (talk) 13:34, 17 July 2015 (UTC)

```— Preceding unsigned comment added by 91.235.65.1 (talk) 09:00, 18 July 2015 (UTC)
```
I think the problem here is the definition of the words retrograde and prograde... as if they defined a specific amount of tilt. Retrograde means simply in the opposite direction or backwards compared to its orbit or to everything else in the system, or whatever. To say that a planet's tilt is 177 degrees and its rotation can be considered retrograde is not necessarily a contradiction. We can write a sentence or two to explain that, if we think it will avoid confusion. Tfr000 (talk) 14:00, 18 July 2015 (UTC)

Fourth update: Thanks, but that still won't do it. Perhaps this is a bit more subtle than I thought. The only way you can get a tilt of 177 degrees is by using the "right hand rule". That puts the Noth pole of Venus to the South (Earth's South) of the orbit of Venus. That means the rotation of Venus is counterclockwise from above its North pole, the same as Earth. On that criterion, the rotation is not retrograde. But that rotation direction is opposite to the direction of orbital motion of Venus. Perhaps that qualifies the rotation as retrograde. I've contacted some astronomers who may be able help on this, but it will take a week or two to get replies. Maybe there is a genuine ambiguity here, but I hope to get a clear answer. I will put any results here. Until then I recommend leaving things as they are. — Preceding unsigned comment added by 91.235.65.1 (talk) 14:16, 20 July 2015 (UTC)

You are assuming the term "retrograde" depends on how the coordinate system for measuring axial tilt is defined. But one interpretation that is consistent with the cited sources is that "retrograde" is independent of how coordinate systems are defined, and that for purposes of applying the words "direct" or "retrograde" the direction of rotation about the pole that is north of the invariable plane of the solar system is always used. (North of the invariable plane means on the same side of the plane as the north ecliptic pole.) Jc3s5h (talk) 14:31, 20 July 2015 (UTC)

Fourth update (part 2): Maybe, but I'm just waiting now for some replies from people who work in astronomy. — Preceding unsigned comment added by 91.235.65.1 (talk) 14:55, 20 July 2015 (UTC)

Fifth update: I've just noticed that the contradiction has now been removed from the article. That still leaves the question of what exactly is the correct way to define "retrograde". I've had one a reply from an astronomer. I expect to be able to give the results of all my enquiries in about ten days from now.

Some of this information is in the Retrograde and prograde motion article, although the section on that page about the axial tilt doesn't take into account the two definitions. --Lasunncty (talk) 17:15, 25 July 2015 (UTC)

Sixth update: Yes indeed, I have an eye on that article too! It may or may not need editing. I'm not sure yet, so I'm leaving it as it is for now. Thanks.

Seventh (and probably final) update: From the replies that I've now received, I don't think there is any need to edit the article(s) as they are now. I'm still not totally happy with the wording, but I can't justify any significant change. I realise Wikipedia is an encyclopedia not my personal blog! — Preceding unsigned comment added by 91.235.65.1 (talk) 09:42, 1 August 2015 (UTC)

## Caption on the first graphic

```It is the angle between the perpendicular to the plane of the ecliptic (vertical black line) and the planet's rotational north pole (red line).
```Here, a vertical line (black) is drawn perpendicular to the plane of each planet's orbit.