# Talk:Sidereal year

## Question

Does anyone understand what " A true cycle will always compare two bodily objects that differ mathematically by exactly 1" means? I tried to tidy up the grammar without losing the sense. Strider52 | Talk 12:58, 1 March 2007 (UTC)

One is referring to the sun, the other to the stars. Since the earth goes around the sun it crosses the sky a different amount to the stars (ie different stars are above us at midnight each night of the year). Over a year this adds up to a difference of once across the sky, hence the year measured in solar days and the year measured in sidereal days differs by 1. If you look at the sidereal day page you will see that a sidereal day is shorter than a solar day for this reason.—Preceding unsigned comment added by 149.171.164.20 (talk) 06:10, 17 January 2008 (UTC)

## Measurement of the Length of a Sidereal Year

The value given in this article does not match the value in the article for Year. I've tried to resolve the discrepancy, but neither value is cited.

The value here seems to date back to Young's Manual of Astronomy Vol. 1 (1926), which is 31558149.45 pre-SI seconds. If that's the case, then it's not the appropriate source to use. The value in the Year article might reference back to Brownstein and Moffat (2006), but I have not been able to examine the source. The way that the value is expressed is correct in the Year article. It should be SI seconds of Terrestrial Time with the epoch specified, but a source must be cited, and uncertainty should be given.

For this article, I think the value should be rounded in the introduction to about seven digits (four after the decimal), and a careful presentation of the best accepted value should be available in another section. The new section might discuss reference frames for time measurement (very briefly), epochs, and the rate of change of the sidereal year. The rate of change is +0.00000011 days/century per E. Groten Subvolume A of Volume 2 ‘Geophysics of the Solid Earth, the Moon and the Planets’ of Landolt-Börnstein - Group V Geophysics. The value given in that source is 365.25636590 days in J1900.0, and the author seems to actually mean 86400 SI seconds of TT, which would be 31558150.0 seconds. This is a good careful source, but it seems to be based on calculations before the IAU 1976 System of Astronomical Constants (because it's not referenced to J2000.0).

The current best value that I've found is 31558149.8 seconds, given by the The Astronomical Almanac for the year 2007 prepared by the U.S. Nautical Almanac Office. (I suspect it's unchanged in the 2009 version.) This value is consistent with the value in the Year article, but has about three digits less precision. I haven't seen the Astronomical Almanac itself, so it's possible that the citation I found rounded down from a more precise value. The precision of the value in the year article is 31558149.7676 seconds, but I don't know the underlying source, so I don't know the uncertainty in that value. —Preceding unsigned comment added by Mathrec (talkcontribs) 17:58, 7 January 2009 (UTC)

## Much less than 20 minutes

I believe the incorrect equation is being used. The difference in the tropical and sidereal year is not 20 minutes. This is clearly not happening. Precession would be only 72 years(because 20 minutes covers 5 full degrees of the sky) if this were happening. The difference is only 50 arcseconds per year, which in real time is only 3.3 seconds.

To say this: "One sidereal year is roughly equal to 1 + 1/26000 or 1.0000385 tropical years." is not correct. It's 1 year plus 1/2600 of the angle of the sky, which is 50 arcseconds of angle. We've been lumping in the precession measure with the measure for the spin of the earth in a year. They are independent movements. The stars do not shift 20 minutes(5 degrees) compared to the sun at vernal equinox every year.--Markblohm (talk) 14:01, 9 December 2009 (UTC)

Please keep discussion at one location, that is at Talk:Axial precession (astronomy)#20 minutes per year - is that right?. — Joe Kress (talk) 21:40, 9 December 2009 (UTC)

## New age theories

This edit by 69.255.42.105 is riddled with errors, and neither sidereal astrology nor tropical astrology is appropriate here. Sidereal astrology did not replace tropical astrology in 2006. Both are currently used by astrologers. Calling a sidereal zodiac "sidereal years" is quite confusing. Nothing happened to the Earth 6000 years ago causing a "realignment". Sidereal stars are identical to tropical stars so no adjustment was made by either the Greeks or Hindus. Rather, the zodiac was either fixed relative to the stars or allowed to move (precess) relative to them. The zodiac is not "created" within one tropical year, nor does the tropical zodiac precess in 17,000 years. Earth's North Pole has been moving relative to Earth's crust ever since the last ice age, but that movement is extremely small, see polar motion for more information. Precession causes the north celestial pole to reach Vega in about 13,000 years, not 100,000 years. — Joe Kress (talk) 04:38, 6 September 2010 (UTC)

I never mentioned Vega in 100,000 years. It has reached vega and will do so again. It is currently doing so rapidly which I found unusual. In a span of two years, all of the stars in the night sky went a month backwards as if they were in sidereal. I found this strange because I relied on tropical data all of my life and this precession could mean something bigger. As for earth shedding its skin, we did have records of destruction at about 3000 bc. Hence the triangular pole of the North stars. Although 75,000 bc is a current serpentine zodiac of about 13 star signs, I think 10 came around 17,000 years ago and Vega will 'somehow' align once again. To me, the 12 star zodiac is 5 million years old and its 7 star even older (@least 260 million years in the hebrew bible i think). Since 1980 earthquakes have increased drastically in an upscale as well. Not to feed the serpent's nest, except 2012 to the year 2060 should be a very great threat. Basically something is changing the alignment of the earth without us knowing. Srry, bye! comment added by 69.255.42.105 (murriemir)

## Beginning of sidereal year

This article does not mention when the sidereal year begins (I.e. When sidereal time = solar time) i beleive this occurs on the autumnal equinox? I have not been able to find a source for this though. TimL (talk) 12:14, 27 November 2010 (UTC)

The sidereal year does not have a specific beginning. Solar time would not be appropriate because it is not related to either the sidereal year or sidereal time. A solar day is longer (24 hours) than a sidereal day (23 hours 56 minutes), so even if a sidereal day were to begin when a solar day began, it could never again begin at the same point, certainly not one year later. This means that the two times are incommensurable, that is, no multiple of one can equal another multiple of the other. Sidereal time, despite its name, is not related to the stars, but is defined as the hour angle of the vernal equinox, which precesses at a regular rate (50.3" or 20 minutes in time per Julian year) relative to the fixed stars. The Astronomical Almanac shows in its "Universal and sidereal times" table (B17 or B18) that the Greenwich transit of the mean equinox occurs twice on September 21 in 2008 and 2009 and twice on September 20 in 2010 and 2011. The autumnal equinox during those years occurs on September 22 in 2008 and 2009 at 15:44 UT and 21:18 UT, respectively, and on September 23 in 2010 and 2011 at 3:09 UT and 9:04 UT, respectively, so the autumnal equinox occurs one or three days after the double date. Moreover, these transits occur near Greenwich midnight, not noon, even though "transit" means the upper culmination, when the location of the vernal equinox amongst the stars is highest in the sky. Thus the old per-1925 noon-to-noon astronomical calendar day would be better, so both would occur near March 21. But even then, the two times can never occur at noon exactly. None of this has anything to do with the sidereal year. — Joe Kress (talk) 01:49, 8 December 2010 (UTC)

## Accuracy

Why can the length of the sidereal year be given to 12 digits (and not, say, to 10, or to 14)? To what accuracy do 86400 SI seconds equal a mean solar day? The second is defined relative to a property of caesium 133, so any correlation to celestial mechanics is subject to measurement. The suggestion that the mean solar day is somehow per definition equal to 86400 SI seconds implied in the current revision of the article may have been correct in the early 20th century (when there was no hope of measuring anything to 12 digits' accuracy), but clearly it has been obsolete since at least 1967. --dab (𒁳) 06:16, 16 January 2015 (UTC)

after quick googling, this 1992 paper may be relevant to my question, but I did not have time yet to understand what it is saying. --dab (𒁳) 06:18, 16 January 2015 (UTC)
upon further research, it seems that we are simply looking at a mistake in the hpiers.obspm.fr page. They claim "365.256363004 days" without any specified uncertainty, but their source is the publication of precession data which gives the precession speed as 50288.200 arcseconds per millennium, i.e. to 8 digits, so it is difficult to see how they propose to reach 12 digit accuracy from that. From the value 50288.200 in the 1994 paper, I naively calculate a sidereal year of 365.256362 days, which is consistent with the value given here to the expected accuracy of 8 digits, so I guess I'll just reduce the figure in the article to this accuracy for now. 365.25636 days are 365 days, 6 hours, 9 minutes and 9.68 seconds, compared to "365d 6h 9m 9.76s" given by obspm.fr, so we are looking at a discrepancy of 0.08s per year. --dab (𒁳) 07:54, 16 January 2015 (UTC)

## Absurd phrase

"... effectively using the sidereal year." Given that there is no difference between a sidereal and tropical year over the time period over which Hesiod's advice applied, it is absurd to suggest that Hesiod was using sidereal time: the same stars would appear each spring century after century. Writing such an odd thing in the intro is ridiculous, and the whole story about Hesiod should be left out. — Preceding unsigned comment added by 174.88.11.173 (talk) 02:40, 16 March 2015 (UTC)