Talk:Solar time

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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)

[edit] 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

[edit] 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)

[edit] 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)