Talk:Sidereal time

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[edit] Rewrite

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)

[edit] ET

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)

[edit] No units

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)

[edit] Merge proposal

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 (talk • contribs) 18:23, 14 October 2007 (UTC)

[edit] Solar time or standard time?

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)

[edit] Galactic view

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)

[edit] Sidereal time and solar time

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 76.120.107.55 (talk) 04:21, 3 March 2008 (UTC)

[edit] References to Explanatory Supplement

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 (talk • contribs) 14:26, 28 March 2008 (UTC)