Talk:Axial tilt

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Please see also last 3 lines of this same page. Signed: G. Campart

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: "...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)

Contents

[edit] 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

[edit] 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.

[edit] 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)

[edit] 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)

[edit] 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

[edit] 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

[edit] 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)

[edit] 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)

[edit] 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 (talk • contribs) 13:00, 20 October 2007 (UTC)

[edit] 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   (" ") was displaying as a box in my browser. I changed it to use a unicode construction {{unicode| }} (" ").

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)

[edit] 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)

[edit] 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)