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I am not too sure but this phenomenon always confused me
say you have a rotating disc and you put an inclined stick / on one end of the disc
when the disc rotates and the stick is on the other side, it becomes like \
however the same logic does not hold true for the earth. So does the axis actually tilt across when it goes from one side to the other? — Preceding unsigned comment added by Alokdube (talk • contribs) 12:31, 28 January 2009 (UTC)[reply]
The orientation in space of the Earth's axis stays the same all year round (except for a tiny amount of precession). The axis is not fixed in any way to the Sun or the orbit. The stick you have in mind is fixed to the rotating disk, and by this fixation, the disc's rotation exerts a torque on the stick and it rotates with it. −Woodstone (talk) 21:55, 28 January 2009 (UTC)[reply]
It is just observation. If the axis was like an unfixed thing on a disc too, it would still be bent in the same direction because, say it is heavier on one end and the centrifugal force causes it to bend, that bend would still have the same orientation. —Preceding unsigned comment added by Alokdube (talk • contribs) 08:23, 29 January 2009 (UTC)[reply]
I think the conservation of angular momentum would imply that the angular momentum has a magnitude and direction fixed w.r.t. a pivot, called the sun here. Assume there is no revolution, the angular momentum of the object is fixed at \ . now we take into account revolution causing another angular component around the pivot...sun going | . so | + \ has to be conserved in totality. —Preceding unsigned comment added by Alokdube (talk • contribs) 08:13, 11 February 2009 (UTC)[reply]
I'm not 100% clear on what your understanding of the situation is but perhaps you're considering the Sun's gravitational pull on the Earth to be like swinging a tennis ball on a string around yourself. It's not like that - the Sun's pull acts on the Earth's centre of gravity but it is not connected to it and has no effect on which way the Earth's axis of rotation points. This is why now (February) the north pole points away from the Sun while the south points towards it. This causes it to be winter in the northern hemisphere and summer in the south. Six months later the Earth's axis still points toward the same point in space (Polaris for north) but the Earth is on the other side of the Sun, so now the north pole points toward the Sun and experiences summer, and the south points away and has winter. — Preceding unsigned comment added by Secret Squïrrel (talk • contribs) 16:20, 12 February 2009 (UTC)[reply]
So you are saying a gyroscope on a spinning platform would point to the same thing in space once it reaches the other side so as to conserve angular momentum. In this case the assumption is that the gyroscope was already rotating before it was put on the spinning platform, correct? —Preceding unsigned comment added by 192.18.192.76 (talk) 12:39, 9 March 2009 (UTC)[reply]
Even if one was to assume this is a case of rotation about a fixed axis, the earth would point in the same direction relative to the axis.i.e / becomes \ http://en.wikipedia.org/wiki/Rotation_around_a_fixed_axis#Angular_Momentum .. The angular momentum can either be defined about a point or an axis, and I conclude it is not the Sun in case of the earth. 10:32, 7 April 2013 (UTC) — Preceding unsigned comment added by Alokdube (talk • contribs)
The revolution of the earth around the sun and the rotation of the earth on its axis are two separate motions, and thus have separate angular momentum vectors. The first points toward the ecliptic north pole in Draco, and the second currently points to the celestial north pole in Ursa Minor. The second wobbles around the first (see axial precession for more info.) --Lasunncty (talk) 03:47, 5 May 2016 (UTC)[reply]
Please note that while it is the Winter Solstice in the Northern Hemisphere, those of us in the Antipodes are presently enjoying the sunburnt splendor of the Summer Solstice, otherwise known as Lithia, and a few other things. It may simply be clearer to say Solstice, dropping the Winter/Summer designation as ambiguous. Mark Pesce (talk) 07:49, 12 July 2019 (UTC)[reply]
90 degrees below the horizon for sun angle at solar midnight[edit]
Defining solstices using right ascension of Sun[edit]
I like to define the solstices as the moments when the right ascension of the centre of the solar disk is precisely 6h (northern solstice) or 18h (southern solstice). -- Denelson8318:47, 21 December 2021 (UTC)[reply]
This is equivalent to using the geocentric longitude, as mentioned in the "Solstice determination" section near the end of the article. --Lasunncty (talk) 01:45, 22 December 2021 (UTC)[reply]
But the term "geocentric longitude" in this context seems a little vague. Should it not say "celestial longitude" or just straight up refer to right ascension instead? -- Denelson8322:16, 22 December 2021 (UTC)[reply]
I think "celestial longitude" is sufficient. I guess to be accurate, the actual coordinate system used is ecliptic rather than equatorial, although they yield nearly identical results. --Lasunncty (talk) 06:58, 24 December 2021 (UTC)[reply]
Two solstices and two equinoxes per […] year?[edit]
The section "Definitions and frames of reference" currently specifies "two solstices and two equinoxes in a tropical year" but is that really a criterion? Without much research (and it would have to be dismissed as {OR} anyway), I'd be fairly confident to bet there are two of either in any kind of year, be it tropical, sidereal, fiscal, or other, with the possible exception (or at least caveat) of a lunar "year", although even there I can't see how there'd be any number other than two.
The source given is cited as "For an introduction to these topics of astronomy refer to Bowditch, Nathaniel (2002)", which is rather broad in scope, and skimming pp. 228ff. I couldn't find any mention of tropical year specifically. Unless and until evidence to the contrary is presented, I'd suggest dropping the 'tropical' entirely, but just in case I'm missing the glaringly obvious, I won't be bold and edit it myself. — Preceding unsigned comment added by 2001:A62:1AAC:DC01:F20C:FFB6:11A0:7F59 (talk) 21:30, 26 December 2023 (UTC)[reply]
A tropical year is by defined by the cycle of the seasons, so that is the most technically correct version of the year in this context. The sidereal year, for example, is about 0.014 days longer, so would be off by a day about every 71 years. This precession is pretty slow, but eventually you'll have a sidereal year with an extra equinox or solstice. You make a valid point that this is rare enough that maybe it's not important for the wording to be so precise in this more-or-less conceptual paragraph. But in this case the more precise wording doesn't make the concept harder to understand, so I don't think it hurts anything to keep it in. --Lasunncty (talk) 09:27, 27 December 2023 (UTC)[reply]
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