Talk:Sidereal time

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I just re-wrote this article, because the previous version was quite misleading and very unclear. I don't think what I've done is a model of clarity but is certainly more accurate than what was there before. I will try to create an illustrative diagram soon because this article really needs one. Worldtraveller 23:16, 30 Nov 2004 (UTC)ach

This has nothing to do with a rewrite but a sidereal day why do you need to say relative to the vernal equinox, it's relative to the stars each and every one of them. I propose a change so that it says relative to the stars instead. ^^^^ should have continued reading (talk) 14:09, 24 February 2010 (UTC)
No, sidereal time is not relative to the stars, it is relative to the vernal equinox. Earth Rotation Angle is relative to the distant stars, and more precisely, pulsars outside the Milky Way galaxy. Due to the great distance of these pulsars, it is expected that no angular motion of these sources would be detectable. See the Glossary of the Astronomical Almanac. Jc3s5h (talk) 14:56, 24 February 2010 (UTC)

the two contributors above are correct: sidereal day is measured as the interval between two successive transits of the vernal equinox. the stars define a stellar day or star day. due to precession, the vernal equinox moves westward among the stars at about 1/100th of a second of right ascension per day, making it slightly shorter than a stellar day. however my point in commenting here is that editors need to clean up the references to stars rather than the equinox in the discussion of sidereal day further down the page -- for example, in the figure and caption contrasting sidereal and solar days in relation to the earth's motion around the sun. (talk) —Preceding undated comment added 16:52, 31 October 2010 (UTC).


This keeps going in circles. I made comments on User:Garglebutt's talk page asking him to remove ET, which he had just inserted, because it is orbit related while sidereal time is rotation-related. He removed ET but mis-commented the change but put in the spurious word "not" in his explanation - "not related to orbits" when he must have meant "related to orbits" or "not related to earth rotation." Now someone put it back in perhaps due to Garglebutt's muffed commentary. Pdn 04:42, 30 August 2005 (UTC)

Yep, whoops! Garglebutt / (talk) 11:03, 1 September 2005 (UTC)

No problem. It all got me thinking about another huge difference: sidereal time is longitude-dependent (different in Greenwich, Athens, Brazzaville and Anchorage), while ET rolls on serenely, the same everywhere on Earth (relative, for example, to TAI).Pdn 17:50, 1 September 2005 (UTC)

No units[edit]

the article states "Greenwich sidereal time and UT1 differ from each other by a constant rate (1.00273790935)" but there is no units given for the number.

Both GST and UT1 are times, so the number is a ratio and thus does not have any units. — Joe Kress 07:12, 30 December 2005 (UTC)
What does the 1. represent in the ratio? Greenwich Sidereal, or UT1? Beetlecat 19:31, 18 July 2007 (UTC)
UT1. GST is always longer than UT1. I'll clarify the article. — Joe Kress 04:30, 19 July 2007 (UTC)
Ahh, that clarifies much. Thank you. Beetlecat 18:08, 23 July 2007 (UTC)

Merge proposal[edit]

I propose a merge with sidereal day as they are one the measure unit for the other - so there is only one concept behind - and this would be consistent with the solar day related pages. Rlupsa 06:18, 5 October 2007 (UTC)

I have no objection. — Joe Kress 17:15, 5 October 2007 (UTC)
Merge in progress. Old talk page Talk:Sidereal day —Preceding unsigned comment added by Rlupsa (talkcontribs) 18:23, 14 October 2007 (UTC)

Solar time or standard time?[edit]

There is a text Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours. However, AFAIK, solar time is local to the observer and depends exactly on its longitude, while standard time is related to time zones Rlupsa 06:18, 5 October 2007 (UTC)

You are correct if the meaning in clarified to exclude legal or civilian time. — Joe Kress 17:15, 5 October 2007 (UTC)

Galactic view[edit]

This article is written from an Earth-centric perspective. Earth is not the only planet with sidereal time. For example, Mercury has a sidereal rotation period of 58.646 days. Would it be acceptable to make this article more about planets in general and less about Earth? Jecowa (talk) 13:19, 15 December 2007 (UTC)

I am reverting your "galactic view", because their is no such thing. Nor does the article have a systemic bias because sidereal time is only defined for the Earth—no other planet can have sidereal time according to its astronomical definition. The United States Naval Observatory's Explanatory Supplement to the Astronomical Almanac (page 736) defines sidereal time as "the measure of time defined by the apparent diurnal motion of the catalog equinox; hence a measure of the rotation of the Earth with respect to the stars rather than the Sun". Catalog equinox is Earth's vernal equinox fixed on the celestial sphere at a specific date/time. By this definition, sidereal time is the angle measured toward the west between an observer's meridian and the vernal equinox, but stated in units of time, hours, minutes, and seconds, like 15:35:25. A specific kind of sidereal time is Greenwich sidereal time. But the vernal equinox is defined by Earth's orbit around the Sun and by Earth's equator, not by any other planet's orbit and equator.
Your concern is not with sidereal time, but with sidereal day, apparently defined with respect to the fixed stars in general, not with respect to a specific point on the celestial sphere. Rlupsa merged sidereal day with this article, sidereal time, in October. You are welcome to add another section to this article listing other planets' sidereal days, appropriately defined. — Joe Kress (talk) 04:48, 16 December 2007 (UTC)
Thank you for the explanation. I think there might be an article describing sidereal days on other planets. A sidereal day is the same as a rotation period, right? It seems from what you've told me that a sidereal day isn't using sidereal time at all. Is this correct? Should the sidereal day section be merged into the rotation period article?Jecowa (talk) 11:13, 16 December 2007 (UTC)
Good catch. Rotation period already lists all 'planets' (including the Sun, Moon and the dwarf planet Pluto). Another article devoted to Earth itself is Earth's rotation. The Explanatory Supplement defines sidereal day as "the interval of time between two successive transits of the catalog equinox", hence it only applies to Earth. Not only does Earth's sidereal day use sidereal time, but one sidereal day contains exactly 24 hours of sidereal time, where each sidereal hour is slightly shorter than a solar hour. On the other hand, extending sidereal day to other planets is stretching its definition. I think rotation period is a better term. — Joe Kress (talk) 21:27, 16 December 2007 (UTC)
The Explanatory Supplement is a bit confused. Although it defines the sidereal day as relative to the catalog equinox on p.736, elsewhere on pp.48, 52, & 701, it makes abundantly clear that it is relative to the moving vernal equinox, either the mean equinox for the mean sidereal day, or the true equinox (including nutation in right ascension) for the apparent sidereal day. — Joe Kress (talk) 02:13, 19 December 2007 (UTC)
Good job clearing up the confusing information conflicts in the rotation period article and sidereal time article. Jecowa (talk) 06:38, 19 December 2007 (UTC)

Sidereal time and solar time[edit]

Article states: "During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the Earth moves a short distance (around 1°) along its orbit around the sun. Therefore, after a sidereal day, the Earth still needs to rotate a small extra angular distance before the sun reaches its highest point. A solar day is, therefore, around 4 minutes longer than a sidereal day."

However, the "therefore" is hasty -- to require an "extra" angular distance, resulting in a "longer" solar day, you also need to know that Earth rotates the same direction around its axis as it revolves around the Sun. If Earth rotated the opposite direction from the direction of its solar orbit, then the solar day would be a few minutes *shorter* than the sidereal day. Without this information the conclusion following "therefore" is not fully justified. —Preceding unsigned comment added by (talk) 04:21, 3 March 2008 (UTC)

I am confused, the calculation for a sidereal day 365.25/ ( 365.25 + 1) should be a vector equation; unfortunately one cannot simply add the Earth’s polar and orbital axis since they are not collinear. A more proper solution would be 365.25*[365.25 + cos(23.5)]/ [(365.25 + cos(23.5))^2 + (sin(23.5))^2] which yields 23 hr, 56 min, 23.5 sec. In this case the length of a year and the polar tilt angle are approximate but the calculated error of some 19.5 seconds per day would change little with more accurate numbers. This formula obviously can be applied for any angle from 0 thru 180 deg which in the later would yield a retrograde planetary rotation rate of negative 1. I would like to hear the rational for using the default angle of zero. Johnerner (talk) 21:50, 3 March 2010 (UTC)

References to Explanatory Supplement[edit]

The Explanatory Supplement to the Astronomical Almanac (1992) is a great book -- but outdated in some details. Since it was written, there have been several changes to the definition of time scales. See IAU resolutions 2000, IAU resolutions 2006, USNO Circular 179.

In practice, the procedure is now like this: (1) Measure Earth Rotation Angle (ERA), relative to certain quasars. This is done via an international VLBI network. (2) Derive UT1 from a linear relationship with ERA. (3) Derive Greenwich Mean Sidereal Time as linear in UT1, plus a fifth-order polynomial in TDB time since J2000. (4) Derive Greenwich Apparent Sidereal Time as GMST + equation of the equinoxes (which accounts for the motion of the equinox due to nutation).

None of this depends on the physical equinox. It is no longer necessary to try to observe, predict, and get international agreement on, where and when the Earth's spin axis is orthogonal to the Earth-Sun line (or Earth - Solar System Barycenter). Or should we use the Earth-Moon Barycenter? And which definition of "spin axis" should be use? At the sub-millisecond level, this was problematical in any case. --ExtonGuy (talk) 14:30, 28 March 2008 (UTC)comment added by ExtonGuy (talkcontribs) 14:26, 28 March 2008 (UTC)

Useless definitions[edit]

Is this article written to any particular audience? I think we need another wiki rulebook for audience classification.

I am an electrical engineer and a software engineer, but I am completely ignorant as to matters of astrophysics. I came to this article to learn what a sidereal day, and sidereal motion precisely means, but found its definitions to be circular and self-referencing (i.e. useless).

The article uses the phrase "rotation about its axis" as though the axis somehow defines a reference frame. It doesn't. Any solid body rotation inherently defines an axis, but wihtout an EXTERNAL reference frame, measurement of angular motion is impossible.

Most of us understand the 24 hour day to be relative to the sun as the external reference frame. We also know that the solid body axial rotation is concurrent with orbital motion, which, I presume, describes the distinction between some of the terms diurnal, sidereal, etc.

From this article, I have no clue, since the initial statement "Sidereal time is a measure of the position of the Earth in its rotation around its axis,..." is false. As I stated, in order to have meaning, axial rotation must be defined in terms of an external reference.

The rest of the sentence, "or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars" is likewise fairly useless to a lay person, since the distinctions between sidereal and diurnal presumably define each other - and this explanation is thus circular (or possibly elliptical? a little orbital humor) and self-referencing.

The phrase "motion of the stars" really sticks in my craw, since my understanding is that on a relative scale to days and years, the stars are virtually motionless. The term "apparent" absent context, is likewise hanging in the breeze, defining, by the law of implied duality, a distinction between "actual" and "apparent," but without giving a clue as to what that distinction might be.

So, I wouldn't say that this article is unclear, I would say that it's dead wrong. Please somebody who is A) well versed in celestial motion, and B) has children under 10 years old and is capable of explaining this to them, fix this. —Preceding unsigned comment added by (talk) 10:59, 20 November 2008 (UTC)

Thank you for your criticism. I'll try to reword the lead and the rest of the article, taking your concerns into consideration. Unfortunately, I've already hit a road block, because the Explanatory Supplement to the Astronomical Almanac, written by the director of the Nautical Almanac Office at the United States Naval Observtory, which publishes the ephemerides used by all professional astronomers, explicitly defines a sidereal day as "the interval of time between two consecutive transits of the catalog equinox" (of a star catalog), that is, it assumes the stars are revolving around a non-rotating Earth. His definition of sidereal time is even worse from your point of view because it is "the measure of tme defined by the apparent diurnal motion of the catalog equinox; hence a measure of the rotation of the Earth wirh respect to the stars rather than the Sun." The first half uses the phrase you find confusing, apparent diurnal motion, which also means that the stars are revolving around a non-rotating Earth, even though the second half would be more to your liking. Apparent notifies the reader that the writer knows that the stars don't actually move, even though he finds it simpler to use that terminology. Furthermore, he explicitly uses diurnal, a vague term which you also don't like. Another problem is that the sidereal day has two distinct definitions, one relative to a catalog equinox which is fixed relative to the stars (at least for a few years before it jumps), and another relative to a precessing equinox which moves relative to the stars. The IERS has introduced the term stellar day for rotation relative to the stars, which either clarifies or muddies the situation, depending on your point of view. — Joe Kress (talk) 01:17, 21 November 2008 (UTC)

Formula for determining Solar Day from Sidereal Day[edit]

I think you did a good job with this page. I like that you included the illustration regarding angular differences from Sidereal position and Solar position. Along the same line, it might be helpful to include the formula for calculating the length of the MEAN Solar Day from the known Sidereal Roation period (and Sidereal Orbital period). This would give readers to determine the solar days (view from imagined surface of said body) for various planets and planetoids (assuming they know the sidereal rotation and orbital periods for such). Just a thought. User:Tesseract501\Tesseract501 20:05, 7 March 2009 (UTC)

Recent Edits[edit]

The length of a sidereal day is uniform, because it reflects only the rotation of the Earth, while the solar day only averages to 24 hours, due to the Equation of Time. I have made the appropriate corrections to the image used on the page, but have left its original author credited as the author of the image. If that is the incorrect procedure for a derived work, I apologize.

I've tried to make this page a bit more nontechnical, as requested, as well.

Some of what I have done has been spurred as a reaction to a poster on sci.astro.amateur, who has been using the original image on this page as evidence that the concept of sidereal time is a terrible mistake visited on the world by Newton and Flamsteed, and so my changes could involve some overreaction to that. Quadibloc (talk) 16:14, 29 May 2009 (UTC)

Also, it should be noted that I avoided removing any content on this page. Except for the one error in the image, I found the content, although indeed technical in nature, to be excellent in quality. Thus, rather than rewriting the entire page, or replacing it with a less technical page, I simply added a less technical part at the beginning, as well as re-ordering some of the previous text so that the less technical part of what was already there appeared first. Quadibloc (talk) 16:20, 29 May 2009 (UTC)

Yes, adding is better than subtracting! Good stuff. HarryAlffa (talk) 19:48, 3 June 2009 (UTC)


(The actual time difference is likely to be one hour greater, because Britain has changed the time zone it uses to facilitate communications with continental Europe.)

This appears to be have been written sometime between 1968 and 1971! This was the only period when the United Kingdom (and Ireland) tried moving to Central European Time. It proved unpopular as it made the mornings darker.

Sergeirichard (talk) 13:06, 12 June 2009 (UTC)

Sergeirichard is nearly half right. The UK never changed its time zone; in 1968-71 we had GMT+1 all the year round, and called it British Standard Time. It has been thought that, while the Irish clocks undoubtedly always, since about 1916, agreed with the British clocks, the RoI may have altered their time zone in 1968 (probably right), and in 1971 may have retained by law their advanced zone and just set the clocks back during Winter time (questionable) [under the EU, they must advance clocks for Summer; therefore, they are now in the same Time Zone as the UK, since the clocks agree]. (talk) 17:47, 12 June 2009 (UTC)

recent earth rotation change[edit]

due to the fact that the recent earthquake in Chile spinned the earth faster for 1.26 microseconds, is the info correct on this page? maybe a link for this should be added too? --Pixy909 (talk) 12:38, 5 March 2010 (UTC)

According to an interview on National Public Radio (in the US) there are theories that indicate a change on the order of a microsecond, but efforts on the part of the US Naval Observatory to measure the change were unsuccessful because there are many other sources of change of the same magnitude, and it was not possible to isolate the effect of the earthquake from the other sources of change. So I wouldn't change the article. Jc3s5h (talk) 16:13, 5 March 2010 (UTC)

Definitions from other sources[edit]

This article has stuck to the IERS definitions.

Fair enough, but perhaps a discussion of prescriptivism/descriptivism is in order. The fact is that a lot of authors use sidereal day to mean the day relative to the distant stars, including some well respected reference works.

Encyclopedia Britannica

Eric Weisstein's World of Astronomy

Ask A Scientist

I don't think it is good enough to say other reference works are simply _wrong_ because they use a non-IERS definition. Conventions are built by usage, not fiat.

Something to consider, anyway. Ordinary Person (talk) 00:05, 11 November 2010 (UTC)

sundial time[edit]

This is similar to how the time kept by a sundial can be used to find the location of the Sun.

If I can find the Sun using a sundial, I don't need the sundial. ;) I'm far from sure what this sentence intends to say. —Tamfang (talk) 21:24, 30 November 2010 (UTC)

I think the editor got carried away with the similarity of using sidereal time to find stars (via their right ascension given in units of time). A little later in the paragraph a better statement appears, "The basic difference between the two is that solar time maintains orientation to the Sun while sidereal time maintains orientation to the stars in the night sky." — Joe Kress (talk) 16:21, 1 December 2010 (UTC)

Faster or slower?[edit]

"Therefore sidereal time correlates closely to the inertial rotation of the Earth, whereas common time on a typical clock correlates to a slightly faster rotation, accounting not only for the Earth's axial rotation but also for the Earth's annual revolution around the Sun ..." This seems backwards to me. Of course there is only one rotation, so the author surely meant "...a slightly faster apparent rotation...". However, an (apparent) rotation that takes 4 minutes longer than another is a slower rotation, not faster. I shall try to revise the sentence along these lines. —Preceding unsigned comment added by (talk) 20:12, 11 March 2011 (UTC)

Tamfang' s change is an improvement, but "inertial rotation" is a poor phrase; a rotating frame of reference is not inertial. Jc3s5h (talk) 21:13, 11 March 2011 (UTC)
Tamfang's wording "sidereal time is defined by the inertial rotation of the Earth" is incomplete. He is assuming that sidereal time is rotation relative to the stars or intertial space, but, strictly speaking, sidereal time is rotation relative to the precessing vernal equinox, that is, relative to the equinox OF DATE, not relative to a fixed, inertial or catalog equinox. The Explanatory supplement to the Astronomical Almanac (page 48) sidesteps this problem by stating "Apart from the inherent motion of the equinox, due to precession and nutation, sidereal time is a direct measure of the diurnal rotation of the Eath." Both its apparent sidereal time and mean sidereal time are relative to the equinox of date. Later, it states "Classically, ... sidereal time was actually based on the hour angle of the zero point of right ascension of the catalog, known as the catalog equinox. This point is distinct from (but close to) the dynamical eqinox defined by the orbital motion of the Earth." The two times differ by 8.4 ms per day, so 11 years after J2000 this difference amounts to about 34 seconds, which is significant when using high magnification. Thus there are two forms of sidereal time, a modern definition relative to the precessing vernal equinox, and a classical definition that uses a frozen catalog equinox at a fixed point in time. One definition is not sufficient. — Joe Kress (talk) 05:25, 13 March 2011 (UTC)

The etymology for 'sidereal' or 'Sidus' refers to the return of Sirius as an orbital event where it becomes distinct from behind the glare of the Sun or what effectively is a line of sight observation as the Earth moving along it orbital circuit brings Sirius into view from its roughly two month disappearance from behind the Sun.Loading this astronomical event which affirms the orbital motion with a daily stellar circumpolar connotation is not helpful in that it obscures,for want of a better word, the original significance of the return of a star and particularly Sirius in 1461 days as the basis for the 365/366 day calendar cycle.It is noteworthy that the root meaning of the word 'consider' comes from 'sideratus' or the contemplation of the celestial arena. Gkell1 (talk) 10:43, 25 February 2012 (UTC)

Mathematical symbols and equations error[edit]

Mathematical symbols and equations are not being properly displayed on the page. Anyone who can fix this please do so urgently. Ahmer Jamil Khan (talk) 14:11, 27 August 2012 (UTC)

It seems OK to me. I'm using Windows 7 64 bit and Firefox 14.0.1 Jc3s5h (talk) 14:20, 27 August 2012 (UTC)

Introduction still conflicts with itself[edit]

The introduction is still a crapfest. It shows all the marks of being the scarred battlefield of an edit war. It starts out: Sidereal time /saɪˈdɪəriəl/ is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky. Briefly, a sidereal day is a "time scale that is based on the Earth's rate of rotation measured relative to the fixed stars."[1]

This is completely in conflict with the later paragraph in the intro: A mean sidereal day is about 23 hours, 56 minutes, 4.0916 seconds (23.9344699 hours or 0.99726958 mean solar days), the time it takes the Earth to make one rotation relative to the vernal equinox.[clarification needed] (Due to nutation, an actual sidereal day is not quite so constant.) The vernal equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 26,000 years, so the misnamed sidereal day ("sidereal" is derived from the Latin sidus meaning "star") is some 0.0084 seconds shorter than the Earth's period of rotation relative to the fixed stars.

It can't stay like this. Either: a) we write the article to make it clear that sidereal time relates to the RA of the vernal equinox, OR b) we write the article saying sidereal time relates to the celestial positions of the distant stars, OR c) we write the article reflecting the fact the IERS definition relates to the RA of the vernal equinox, but that the term is also commonly used to refer to time based on the distant stars.

The current conflict appears "non-encyclopedic". Ordinary Person (talk) 09:45, 8 April 2013 (UTC)

wrong values in introduction and meaning of "stellar day"[edit]

In the introduction (3rd paragraph) it says: "A mean sidereal day is about 23 hours, 56 minutes, 4.0916 seconds (23.9344699 hours or 0.99726958 ..." these valuse are slightly off (see more precise values later in the article). It should be "A mean sidereal day is about 23 hours, 56 minutes, 4.0905 seconds (23.9344696 hours or 0.99726957 ..." I'll correct this occasionally (if that's ok).

By the way: what is given here as a "stellar day" (some 8.4 msec longer) seems to correspond to the *momentary* rotation rate of the earth. (Note that a rigid body doing some general movement always at any given time has a momentary rotation axis and momentary angular velocity.) But this does not exactly correspond to the number of rotations the earth does (relative to distant stars) over a long time, say over a whole precession period. Then the difference (to the sidereal day) would be 86400/(25900*366.2422)=9.1 msec longer. [I think the difference corresponds to adding the angular velocity vector of earth to that of precession either vectorially or simply by length.] Don't know whether a brief clarification of "stellar day" could be made without adding confusion to the article...? Maybe just use the words "momentary rotation rate" (about "momentary rotation axis")..?

  Greetings, (talk) 22:50, 3 January 2015 (UTC)

May need a rewrite?[edit]

A reader wrote to Wikimedia (ticket:2016070710007063) and shared the following comment: I find this very well written but too complicated for me. I am a university educated professional, career researcher (aka journalist) with a particular interest in science who is over 50 and I don't understand this. Can someone please add to this excellent page an even more simple definition of sidereal time for a non-scientist. --S Philbrick(Talk) 13:55, 7 July 2016 (UTC)