Talk:Solar time/Archive 1

Archive 1

Is solar noon 0h or 12h?

I think this definition lacks the idea that the origin of these times is noon, ie it is 0h solar time when the mean/true sun crosses the meridian. Anyone to confirm this? If so needs editing this page and maybe the related as explanations are often done through comparisons.

Well, for one thing, mean solar time explicitly does not follow that rule. But it could be worked into that first section, as long as it's made clear that it's only the case for apparent solar time. -- John Owens 19:50 May 13, 2003 (UTC)

Do you have explicit references of that "exception"? Relying on some books here i have Meus say that the meridian crossing is the reference for mean time as well as true solar time. I agree that this is only a convention matter, but those details just screw half a day... It is precised though that civil time's origin is midnight.

I think it would be much better just to say that solar time is based on the apparent position of the sun in the sky. -Smack

Well, yes, centuries ago astronomers counted time from noon, so in registers of astronomical observations the date and time "January 1, 00:00" would mean January 1 at noon, and January 1, 12:00 astronomical would be midnight in the night from January 1 civil to January 2 civil. Currently astronomers use TU and some other timescales in all of which Greenwich noons fall near 12:00. The only exception is the "Julian day" day-count, in which a date like 24356789.0 means a noon and 24356789.5 means a midnight. "Greenwich mean time" used to be a valid timescale for astronomers in which 00:00 meant noon, but this name is no longer used by astronomers (some people use these words to mean Universal Time, but the IAU disallows this practice).

As far as I know, solar times are defined in terms of the hour angle, i.e. the diedral angle between the local meridian and the plane passing through the polar axis and the real, respectively mean, sun. Also, both true and mean solar day start at noon; civil time is mean solar time minus 12 hours, as stated above by someother poster. I need some references, however, please help. Rlupsa 20:58, 16 September 2007 (UTC)

Suggestions re: Mean Sun, etc.

I was linked to this page when I searched 'mean sun', but that term is not defined on this page. I realise that the sidereal measure is used in contructing the path of the mean sun, but an explicit explanation of how this is done would, though complicayed, be nice.

Done that, Rlupsa 20:58, 16 September 2007 (UTC)

Also, when the rate of one metrical process is compared to another in the form 'n units of this process = m units of that process +/- x time units' (e.g., '1 apparent solar day may differ from a mean solar day...by -22 seconds to 29 seconds'), it would be clearer if the standard process for the time units (e.g., "seconds") was given explicitly (e.g., 'atomic seconds' or 'sidereal seconds').

Thanks, Hemlock

Variations in Apparent solar time

The explanation that the earth's axial tilt is one cause of the variations in apparent solar time is confusing to me. Although I can't dispute it, the explanation is not convincing. In any case, if axial tilt is indeed a cause, wouldn't the dates given for shorter or longer apparent solar days be dependent on whether the observer is in the northern or southern hemisphere? If so, I think that should be stated. Also, I'd love to know why the dates don't correspond with the equinoxes and the solstices.

Thanks,

Bill Korbholz

I have (hopefully) improved the explanation to address your concerns. The location of the observer is irrelavent. The projection of the Sun's motion onto the equator is faster at both solstices and slower at both equinoxes than its mean motion. — Joe Kress 06:21, 17 March 2006 (UTC)

The first sentence strikes me as a bit dismissive. The "idea" that noon is when the sun is overhead? What else is noon? Yes, nowadays we say 12.00 is noon; but solar time is where the whole notion of time of day comes from. ("Notion" seems wrong too. This is pretty basic, pretty concrete stuff.)

As for the two causes of variation in length of the solar day: could we have some idea of their relative importance? I didn't think that the elliptical orbit was much of a factor here; the orbit isn't very eccentric. And one wouldn't expect that to be reliably seasonal, to fit in with the effect due tilt of the axis. Anyone? Omicron18 11:00, 9 August 2006 (UTC)

There is a more detalied explanation on the equation of time page, Rlupsa 20:58, 16 September 2007 (UTC)

Civil time

As the civil time is exactly mean solar time minus 12 hours, shouldn't this page be merged with that for civil time? Rlupsa 20:58, 16 September 2007 (UTC)

That is only true in astronomy. In civilian or legal use it has other meanings. Astronomical civil time is only true for the central meridian of a time zone. Elsewhere in the time zone and whenever daylight saving time is in effect, it is not true. Hence it cannot be merged. — Joe Kress 06:30, 22 September 2007 (UTC)

Link needed

The pages which deal with the annual variation of the time between sunrise and sunset, and the time between noon and noon, should each have a listed link to the pages which deal with the long-term variation of the number of noon-noon days per year. I've not yet found where the links should best be to - maybe Earth rotation, which I'll add. Is any other sort of link needed? 82.163.24.100 (talk) 11:02, 11 November 2008 (UTC)

Suggestion to add the Formula for Calculating Solar Day

It might be helpful if the page showed the formula for calculating the mean solar day (i.e., when one knows the sidereal rotation period, and the sidereal orbital period). [[User:Tesseract501|Tesseract501] 20:45 7 March 2008 (UTC)

How to improve inexact language and category-mistakes

The main article starts off with this statement:

Solar times are measures of the apparent position of the Sun.

I never heard of time being in itself a measure of position, and doubt very much that it is such a thing. The statement appears to involve a category-mistake. How about this for a possible improvement?--

Solar times are times measured by the sun, sometimes by its apparent position.

(Such a change would also deal with the factual point that not all solar times are measured by apparent position anyway.) I'm hesitating to edit this, because a number of other parts of the article also appear to involve related category-mistakes. It would take a lot of work to fix. Do other editors/potential editors hold strong or different views? Terry0051 (talk) 23:29, 21 March 2009 (UTC)

Solar time is the apparent angular position of the Sun, relative to either noon (apparent/sundial or mean/clock) (upper culmination), midnight (apparent or mean) (lower culmination), sunrise or sunset (both seasonal) as implied by the second sentence. — Joe Kress (talk) 07:28, 22 March 2009 (UTC)

The difference I'm talking about is the difference between saying that time _is_ position, or that time _can be measured by_ position. (And I believe we're not talking about relativity here.) The first seems to me to be not true, while the second is the familiar fact. So I would be inclined to re-phrase your statement as -- Solar time can be measured by the apparent angular position of the Sun etc -- and I would deny that solar time _is_ the apparent position etc, while admitting that the apparent position gives a _measure of_ solar time. Do wiki-reliable citations support the _is_ type of statement, as distinct from the _can be measured by_ type of statement? If they do, then I'll just plead guilty to pedantry for the present purpose if accused, but it does seem to me that the first (arguable mis)statement can lead into a number of substantial errors. Terry0051 (talk) 22:11, 22 March 2009 (UTC)

I do not accept your metaphysical interpretation that time is somehow different from the physical processes which define it, whether they be atomic transitions or dynamical motion within the solar system, and neither do most astronomers. "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.", not is "measured by". The IERS states that the recently introduced "Earth Rotation Angle" IS Greenwich Mean Sidereal Time excluding precession and nutation. See How does the stellar angle theta (or Earth Rotation Angle) differ from GST?. Greenwich Mean Sideral Time has historically been defined as the "hour angle" of the vernal equinox. Here we have both time and angular position within the same term. Simon Newcomb states, "Mean time at any place and at any moment is the West hour angle of the mean sun at that place and moment, each 15° of arc counting one hour." and "Sidereal time at any moment is the West hour-angle of the vernal equinox". (A compendium of spherical astronomy (1906) page 119). — Joe Kress (talk) 21:17, 23 March 2009 (UTC)

[from Terry0051] The problem previously raised is a physical matter, not merely metaphysical: expressed in other terms, it is a problem of commensurability and dimensional analysis. That is, it is an error to say that time _is_ position, because "The most basic consequence of dimensional analysis is: Only commensurable quantities (quantities with the same dimensions) may be compared, equated, added, or subtracted." (No criticism was made about statements that time (given a suitable setup and conversion factor) _can be measured by_ position.) It can be seen, by reference to the linked articles and also to related reliable sources (for example see Quang Ho-Kim et al., "Invitation to contemporary physics", World Scientific, 1991, ISBN 9810207247, 9789810207243, section starting at p.489), that time on the one hand, and either length or else dimensionless quantities on the other hand, are mutually incommensurable (even if there are various statements in loose language giving dimensionally incompatible comparisons, or equations and statements that effectively ignore the dimensional issue). "Hour angle" is a special measure of angular distance or position given in terms of the time _by_ which it is measured: and where Simon Newcomb stated that each 15° of arc counts one hour, he was giving the conversion factor between angle and time in this special situation, he was not saying that angle is time. Equations involving both of two incommensurable units inevitably also involve some additional coefficient or conversion factor. E.g., in the example at hand, the conversion factors would be in units of length/time, or [dimensionless]/time, or their inverses.

(No criticism was made, in my earlier post, against the definition of a unit of time in terms of some other 'duration', and no generic criticism was made that includes that. It is beside the point to quote the definition of the second as if that had been the subject of some criticism. No lack of commensurability is introduced by defining a second, or other unit of time, in terms of a duration defined in some suitable -- and dimensionally commensurable -- way.) Terry0051 (talk) 13:31, 13 April 2009 (UTC)

Bias in 'History'

'Many methods have been used to simulate mean solar time throughout history. The earliest were clepsydras or water clocks, used for almost four millennia from as early as the middle of the second millennium BC until the early second millennium. Before the middle of the first millennium BC, the water clocks were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night.' There is a clear, controversial, overweaning, unscientific and unexplained assumption here that solar time is not a good and beneficial system. —Preceding unsigned comment added by 79.72.255.48 (talk) 11:59, 1 November 2009 (UTC)

Seasonal variations

Article says solar time is realized with the UT1 time scale and after that: "the length of a mean solar day does not change on a seasonal basis". But in Universal Time I read that UT1 is not smoothed by seasonal variations - it is done only for UT2! So seems "does not change on a seasonal basis" is incorrect. --Dryzhov (talk) 11:03, 20 April 2010 (UTC)

Mean Solar Time

The paragraph on Mean Solar time should be restricted to the relation between Mean Time and Apparent Time, that mean time is the year-round average of apparent time, the Equation of time, the analemma, how to find Local Mean Time, the fact that GMT is Local Mean Time at Greenwich, etc. This is what someone wants to know when they look up Mean Solar Time. The additional information on the increasing Length of Day should be in a separate paragraph or perhaps simply referenced to the appropriate Wikipedia article. Alexselkirk1704 (talk) 12:54, 30 September 2010 (UTC)

I have made an edit along the lines suggested by Alexselkirk1704. Jc3s5h (talk) 14:31, 30 September 2010 (UTC)

Mistake in length of day

"In 2010, the greatest UT1 time interval between apparent midnights (at Greenwich) is 86471 seconds and the shortest interval is 86325 seconds."

Pretty sure both of those are wrong-- the variation isn't that great. Greenwich apparent midnight was at
23-58-46.57 on 22 Dec 2010
23-59-16.335 on 23 Dec
23-59-46.08 on 24 Dec
00-00-15.77 on 26 Dec

Forgot to check 21 Dec, but as you see an apparent day is never much over 86430 seconds, if any. Tim Zukas (talk) 19:08, 28 July 2011 (UTC)

Why would you think checking a few days in December would cover the whole year?

Checking a little further, I see that I inserted that information back in September 2009. There does appear some kind of error. Back in May 2011 I found the equation of time for every day in 2008. I chose that year because it is the most recent year for which MICA uses actual differences between UT1 and TT, rather than estimates. I need to work with my spreadsheet a while to make sure I'm calculating the duration of the apparent day correctly. Jc3s5h (talk) 21:24, 28 July 2011 (UTC)

The "longest day of the year" (in this sense) will happen when the slope of the equation-of-time curve is steepest, which is near 23 Dec-- no question about that. Don't recall offhand when it's steepest in the other direction, but that's when the "day" will be shortest. Tim Zukas (talk) 21:29, 28 July 2011 (UTC)

Jean Meeus, the internationally acknowledged expert in mathematical astronomy, states that the true solar day varies from 24 hours + 29.9 seconds on 22 December to 24 hours − 21.3 seconds on 16 September (1998) (Mathematical astronomy morsels, p. 346). [1] and [2] agree with Meeus, but [3] uses the opposite sign (blue curve and right-hand scale). — Joe Kress (talk) 05:11, 29 July 2011 (UTC)

The last source mentioned by Joe clearly has the sign wrong for duration of apparent day. Consider the apparent day beginning at apparent noon on 14 September 2001 and ending at apparent noon on 15 September 2001. http://www.jgiesen.de/Divers/Sonnenuhr/eot.html claims this has duration 24 h + 21.4. But if one calculates the apparent topocentric local hour angle of the sun at the Royal Observatory, Greenwich, using MICA, one finds apparent solar noon on 15 Sept. is 11:55:10.05 UT1, and for 14 Sept. it is 11:55:31.34 UT1, giving a duration of 24 h - 21.29 s.

I'd appreciate it if Joe could update the article and supply a citation to Meeus, since neither I nor my nearby libraries have that book. Jc3s5h (talk) 15:26, 29 July 2011 (UTC)

In 2010, Greenwich apparent midnight at
15 Sep 23:55:04.29
16 Sep 23:54:42.87
17 Sep 23:54:21.42
18 Sep 23:53:59.98
19 Sep 23:53:38.56

So shortest "day" in 2010 was about 86378.55 seconds. All the above figures from http://ssd.jpl.nasa.gov/horizons.cgi Tim Zukas (talk) 16:35, 29 July 2011 (UTC)

The ephemeris transits on page C15 of the 1998 Astronomical Almanac and on page C17 of the 2010 Astronomical Almanac confirm that the September peak in the length of the true solar day increased from −21.34 to −21.45 seconds, or from 86378.66 to 86378.55 seconds. Although Meeus does not state how he calculated the true solar day, I presume he derived it from the equation of time which he calculated using the same method described by Müller and to the same number of terms. He shows that the November peak in the equation of time remains essentially unchanged from +2000 to +3000 while the July peak increases from −6m 30s to −8m 41s, increasing the intermediate September peak in the length of the true solar day correspondingly, as implied by its increase from 1998 to 2010. Because the article mentions all four peaks in the length of the true solar day, I think I'll add the complete table provided by Meeus, which includes four peaks and four zeros. — Joe Kress (talk) 05:07, 30 July 2011 (UTC)

Too many solar time articles

Please see Wikipedia talk:WikiProject Time#Too many solar time articles Jc3s5h (talk) 17:30, 2 August 2011 (UTC)

Tone of article needs revising.

The tone of this article is highly informal. It should be revised to meet encyclopedic standards. — Preceding unsigned comment added by Gabsvillalobos (talk • contribs) 21:59, 2 January 2012 (UTC)

Show an example.Tim Zukas (talk) 21:53, 21 January 2012 (UTC)

"The tone is wrong." That is absurd, and borders on something like racism, something to do with how different people use the English language. Your implication is that your "tone" is the "correct" tone. Again, it is absurd. Br77rino (talk) 00:13, 25 September 2012 (UTC)

Merger/Redirect Proposal

I am proposing that Apparent sun be merged/redirected to this article. The page as it stands is a dictionary definition which is adequately covered in the Solar time section on Apparent solar time.Primefac (talk) 21:19, 1 September 2014 (UTC)

• Comment it should be moved to wiktionary. After that, it can be redirected here. -- 65.94.169.222 (talk) 04:50, 2 September 2014 (UTC)

External links

Best page on the topic regarding visualization, interactive:

• Worldwide Suntime Calculator in a OSM map

--Leiamada (talk) 20:20, 17 February 2019 (UTC)

Error in page

The page says that the hour angle is measured from the "zenith". I think it should say it is measured from the "local meridian".