Talk:Day/Archive 1

Boundaries of the day
I introduced and expanded this section, but it requires information from other societies on when the day begins and ends. It might also do with more wikilinks. -Acjelen 20:14, 18 May 2005 (UTC)

Change in day length over the time
Hi,

Could anybody offer a section in which length duration change on a geological time scale could be described. See for example http://www.religioustolerance.org/oldearth1.htm#skip

Cheers.

"24:00" doesn't feel like it fits
I edited the reference to "24:00" as midnight, since it is referenced just prior as "0:00", and added a qualifier in brackets. Hope noone minds this clarification. :) - Gingerkitteh 04:18, 24 August 2005 (UTC)

Daily shouldn't redirect here.
It's also a term for a british newspaper. Tlogmer 00:37, 7 September 2005 (UTC)

Interwiki problem
In Tamil language, there are two different words - one for the unit of time and the other for the sunlit state. The two articles are ta:பகல் (sunlit period) and ta:நாள் (unit of time). Which one do we link this to? -- Sundar \talk \contribs 05:17, 28 September 2005 (UTC)


 * I think ta:நாள் (unit of time) is marginally preferrable. I suppose the two articles refer to each other in disambig-like fashion?
 * Urhixidur 17:04, 28 September 2005 (UTC)


 * Not just Tamil! But very interesting to be able to add a non-european language to the list of languages with separate words! And extra bonus for being a language spoken so near to the equator.


 * For each language in that list, the interwikis are a total mess: Each language has chosen either the one or the other article for linking with Day. Depending on this choice, the other must either link to en:Daytime or Nychthemeron. And for each combination of these languages, links between them are either the correct or opposite one. These errors are _maintained_ by the bots, making it very hard in the long run for a few multilingual humans to keep the right links in place, not to mention that you would have to know every language on the planet to fix it all.
 * So the problem is, English has 3 articles (Daytime, Day, Nychthemeron) when there should be 2. My suggestion is to split this article into Day_(24_hours) and Day_(sunlit_state), and make this a disambiguation page. We don't need separate articles for Daytime (it is really not about time), or Nychthemeron (jargon).


 * Contrary to this article's standpoint of "day" primarily being 24 hours, it is etymologically the same word as Dutch and Nordic "dag", which is first and foremost the opposite of night. I think this can explain at least the Dutch, Danish, Bokmål, Nynorsk and Icelandic articles' long time disbelief in this standpoint.--129.241.30.97 (talk) 14:21, 10 May 2012 (UTC)

Beginning our broadcast day
I didn't see it mentioned, but Egypt is credited (or so I hear...) with inventing the 24h day. Trekphiler 09:23, 27 December 2005 (UTC)


 * You are correct, although I suspect these were seasonal hours (varying with the seasons), not equinoctial hours (constant throughout the year, equal to the nighttime or daytime hour at either equinox). — Joe Kress 23:14, 2 January 2006 (UTC)

Chronomera
I have invented a new form of measuring time and I would like to know if it would be considered Original Research so can't post it. It seems very intuitive to me so please read the whole specs before responding.
 * One "mera" is a day from Greek.
 * so a decimera is a tenth day or 2.4 hours.
 * Centimera = hundreth of day = 14.4 minutes.
 * Millimera = thousandth = 1.44 minutes.
 * Dilomera = ten thousandth = 8.64 seconds.
 * Silomera = hundreth thousand = .864 seconds.
 * Micromera = millionth = .0864 seconds

This can also be used to say time, for example 15:37(military time) = 6:50 (chronmera system). I am so far teh only person using this but I think it could catch on, if I cant post it here is there a seperate wiki I could put it in. (-Ozone-)


 * An original idea such as yours is not allowed on Wikipedia because it is included within No original research under "What is excluded?". You can post it on your own web site. — Joe Kress 07:24, 1 February 2006 (UTC)

This idea is not new at all. It was already proposed during the French revolution. See article decimal time. &minus;Woodstone 09:04, 1 February 2006 (UTC)

Really, wow. So I am not the first one. Thank you. I still think my wording is more elegant, though. Wow. (-Ozone-)

People
I'm afraid I don't see the importance of having a list of people with "Day" in their name. WikiSlasher 12:36, 15 June 2006 (UTC)
 * Hey, good catch. Unfortunately, folks, although generally trying to be helpful, will add extraneous info to articles.  I created a disambiguation page (Day (disambiguation)) and moved that list over there.  Madman 15:20, 15 June 2006 (UTC)

"List of famous days"
Why is this list here at all? It would be endless! We already have got the WikiProject Days of the year which does a better job. Also the page is about the DAY as a unit of time, not about DATES. I propose to remove that section here permanently. Tom Peters 09:38, 28 October 2006 (UTC)

I agree. I have deleted that section before and have now deleted it again.


 * Fine, but please do not such things anonymously! An anonymous delete is vandalism by default. Tom Peters 15:45, 31 October 2006 (UTC)

Error in Introduction
Currently the introduction states that "definitions of the day are based on the apparent motion of the Sun across the sky". This is factually incorrect: the cause and basis of the day is the rotation of the Earth. The apparent motion of the Sun because of this is the basis for just one type of day; besides this, people have been looking to the apparent motion of the stars instead for ages (sidereal day; actually that refers to the moving aequinox, but well...). Originally I had put here a general descriptive phrase like "the word day is used for several different units of time based on the rotation of the Earth". Somehow it got lost in the successive rewrites. I propose to fix this. Tom Peters 13:03, 4 November 2006 (UTC)


 * The key word here is "apparent", that is; how it appears to us,what observation it is based on.  We observe the sun to move across the sky. The "fact" of the Earth's rotation was deduced after the initial observation (by many years).  All of the other definitions of the word  "day" derive from this initial observation.  The statement in the article is true as it stands. —Preceding unsigned comment added by Alexselkirk1704 (talk • contribs)
 * You're replying to a two year old comment. Verbal   chat  19:27, 10 November 2008 (UTC)

definition(s) of Day
As well as the dictionay definitions of the word "day", there is also a generally accepted or general use of "Day": e.g. "Remember the day when we thought we were invincible?" or "There was a day when women could not wear slacks in public." or "These days it seems as if more people use email than the telephone." Could this use be included in your encyclopedia? Joan Tytler 15:57, 17 July 2007 (UTC)


 * Go ahead and add it yourself. &minus;Woodstone 20:07, 17 July 2007 (UTC)

SI
I know there's a SI definition of second (http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html), as noted at second. However, I don't think there's a definition of day, so the section may need to be removed. Superm401 - Talk 07:58, 13 November 2007 (UTC)


 * Rewording would be appropriate, but it cannot be removed because the BIPM acknowledges that the day is 86,400 SI seconds in its Table 6: Non-SI units accepted for use with the International System of Units. A comparable non-BIPM source of this table is already referenced in the introductory paragraph of the article. — Joe Kress 21:45, 13 November 2007 (UTC)

What starts a new day?
I am having an argument with someone on what actually starts a new day? Is it 12:00:00 or 12:00:01, I believe that 12:00:00 is the start of a new day due to the fact that when we celebrate NYE we wait for the strike of 12:00:00, can anyone post a link to validate my argument? —Preceding unsigned comment added by 208.177.18.98 (talk) 14:05, 12 August 2008 (UTC)

See the discussion at 12-hour clock. The confusion arises from the ambiguity of 12:00 a.m.. Some legal documents in the U.S.A. state 12:01 a.m. to avoid the ambiguity, but actually the meaning is still midnight. In the 24-hour clock the problem does not arise. One day 24:00 = next day 00:00, so clearly the next day starts at 00:00 (=12:00 midnight). &minus;Woodstone (talk) 14:16, 12 August 2008 (UTC)

What's a solar hour?
I would remove the attribute "solar" where the approximate duration of the sidereal day is reported. I find it confusing, because no "solar hour" is previously defined. The best would be perhaps to put a link to the "hour" page where this is defined in terms of seconds. (160.45.24.185 (talk) 17:57, 30 October 2008 (UTC))

The Bible
Why is there so much christian theology in this article? —Preceding unsigned comment added by 88.130.74.157 (talk) 07:30, 29 May 2009 (UTC)

Maybe because 'days' came into being when 'time' came into being, and according to Christian theology time was created by God. (Apparently the latest theories about the 'big bang' suggest that time started at that very moment... presumably the first 'moment' there ever was.) —Preceding unsigned comment added by JoJoUK2007 (talk • contribs) 15:32, 4 June 2009 (UTC)

some problems
When I read the introduction trying to understand "what is a day?" I get some problems, as So, it seems that this article does not cover day but is dealing with a unit of measurement called "day (d)". This unit has been defined to construct a reference for the different spans of days; certainly, "d" is not a day and it would function correctly the same way if called "lay" or somewhat else. In consequence of these inconsistencies, the first sentence makes a false proposition. It would be true to state: "Considering a day with view to the sun, it is not a unit of time and it is not equivalent to one entire rotation of a planet."--R*elation (talk) 09:06, 22 September 2010 (UTC)--R*elation (talk) 19:00, 22 September 2010 (UTC)
 * a unit is a unit, it does not differ from each other; therefore, if you call a day a unit, you cannnot speak of different days.
 * regarding the motion which turns the earth around itself - let's call this rotating - the period of a single entire rotation compared to an other may be of different length of time
 * if you fix a day to one single entire rotation, this might be nearly appropiate viewing the light of a far star (see sidereal day) - but it is not applicable in view to the sun.

Another Error in introduction
"The original length of one day, when the Earth was new about 4.5 billion years ago, was about six hours as determined by "

This value is correct for the earth after the impact which generated our moon. The length day before that event is unknown, and will remain unknown as long as we do not have details of that impact —Preceding unsigned comment added by 93.221.230.59 (talk) 15:20, 6 November 2010 (UTC)
 * Thanks for your amendment. Maybe you want to add anything else.--R*elation (talk) 16:35, 8 November 2010 (UTC)

Exact day
Many sources say an accurate day as 23.91 hrs. Any clarifications -- Extra   999  (Contact me  +  contribs) 02:15, 24 May 2011 (UTC)


 * I can't find 23.91 hours in any source. You may be thinking of the sidereal day which is 23 hours 56 minutes 4.091 seconds, or 23.93447 hours. But "day" usually means the solar day which is exactly 24 hours of mean solar time and about 1 millisecond more in atomic hours, minutes and seconds. — Joe Kress (talk) 03:30, 25 May 2011 (UTC)

It seems to me that the oldest tradition for a day would be the solar day, not the stellar (nor sidereal) day, which most people still do not even know about. The current first sentence favours the stellar day - unless one immediately takes the star to be referring to the local sun. The current lede paragraph is also excessively complex. It seems to me a better lede would be something like the lede of early October 2010, which also gets to 86,400 seconds much sooner and makes it clear that there are different definitions:


 * A day was traditionally defined as the amount of time it takes for the Earth to make one entire rotation with respect to its sun, measured most accurately from local noon to local noon. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a day as defined above (i.e. a solar day) is not exactly the same throughout the orbital year. The average length of a solar day for Earth is about 86,400 seconds (24 hours). In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. A day, redefined in 1967 as 86,400 SI seconds and symbolized d, while not itself an SI unit, is also a unit accepted for use with SI. The civil day is usually 86,400 seconds, but may occasionally have a leap second added or taken away once or twice in some years, and may regularly have an hour added or subtracted to adjust for a change from or to daylight saving time.


 * A stellar day is the time it takes for a celestial object to rotate once with respect to a distant star, which for the Earth is about 86,164.1 seconds -- almost 4 minutes shorter than the civil day. (There are about 366.25 stellar days in the mean solar year.) Other celestial objects (such as other planets and moons) also have their own solar and stellar days.


 * The word "day" can also refer to the (roughly) half of the day that is not night, also known as daytime. "Day" may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?".

Also, "measured most accurately from local noon to local noon" in the current lede (with respect to a star) is at least problematic (for stellar days), if not wrong. For Earth, the number of stellar/sidereal days in a solar year is 366.25 rather than 365.25. I think that is another indication that the "traditional" day is NEITHER the stellar nor sidereal day --JimWae (talk) 04:33, 25 May 2011 (UTC)

The solar day is also closer to the civil day (the unit of 86,400 SI seconds) than is the sidereal day. --JimWae (talk) 05:05, 25 May 2011 (UTC)


 * In my opinion, the problem the lede has to solve is not to get the 86.400 seconds as early as possible but to show a general conception, able to cover different definitions.


 * What do we need for "a day"? Apparently: light, ground, background, movement and someone seeing light and watching alteration. Terrestrial is not celestial, the Earth no celestial object. Obviously there are two different movements of the Earth, the rotating of itself and the revolving upon the sun, which may contribute to the span of time called "day". Thinking about whether a single rotation is an entire rotation - really, actually, effectively, in fact, essentially as a matter of fact, or de facto - you could walk around and around facing the problem like the top of a chapel, making a whole round. Then try to make round. - Did you rotate?


 * Thus, if these two movements would not have the same direction in case of the Earth - look at the case of its neighbour the Venus - we would have circumstances on Earth counting the same number of stellar/sidereal days in a solar year (366.25) but the number of solar days as 367.25.


 * I agree to you, JimWae, that the "traditional" day is neither the stellar nor sidereal day. Therefore, in the current first sentence a mistakable "traditionally" should be removed, "culmination" should be added to or substitute "local noon", and "most accurately" seems to be dispensable as beeing practice of methods of measuring and not a part of the definition.


 * The lede of early October 2010 did not deal with the movements of the Earth constituting solar day correctly nor does it give explanations acceptably. Telling "one entire rotation with respect to its sun" is at least not enough (see section above: 'some problems'). Sometimes subjects are not as simple as they look like at first sight. Meanwhile day and within time. --R*elation (talk) 08:53, 29 May 2011 (UTC)


 * The present lede focuses on the stellar day - which is not what people generally mean when they speak of a day. The proposal (above) covers the usual meaning and also the stellar day -- without giving it undue weight.--JimWae (talk) 09:03, 29 May 2011 (UTC)
 * Trying to come up with a single overarching definition obscures the fact that there are several definitions whose durations are NOT all the same--JimWae (talk) 09:15, 29 May 2011 (UTC)


 * The current lede presents the generally used general definition, mentions stellar day and then, focuses on the solar day. The proposed elder version swapped the related rotations: "to rotate once" and "to make one entire rotation" between solar and stellar day. Watching the rotation of Earth with respect to its background is the fundamental reference for the construction of Reference Systems today, e.g. used for timing by UTC (see below). We should not obscure the fact, that definitions are on one hand, and durations on the other. --R*elation (talk) 09:50, 29 May 2011 (UTC)--R*elation (talk) 10:17, 29 May 2011 (UTC)


 * The proposal is not to return to October, it is to use the version above. A day is NOT generally understood to be 86,164 seconds. The present lede is a complicated mess. The reference frame is of course an important concept, but it represents neither the traditional nor present primary meaning of "day", which is the subject of this article. Usage of "culmination" in the definition will not do in a general usage (non-technical) encyclopedia, when a simpler definition exists. Readers should not need to click on other words to figure out the first sentence of the article - and in this case, clicking on culmination does not help much either. Wikipedia is not a technical encyclopedia.--JimWae (talk) 19:19, 29 May 2011 (UTC)


 * The first sentence of the proposed lean version starts with the same proposition as in [October]
 * A day was traditionally defined as the amount of time it takes for the Earth to make one entire rotation ...
 * illustrating a picture of the world before Copernicus 1543. Since then, "The earth has, then, more than one motion." Nowadays, "one entire rotation" would define the stellar day or the rotation period of Earth - with respect to distant stars or celestial background, respectively. I think, to differentiate between rotating about itself and revolving around sun would be better than to mess around with those, when talking about the subject of the article. --R*elation (talk) 08:45, 30 May 2011 (UTC)

If we are going to discuss this productively, please refrain from omitting important parts of the sentences under consideration, in this case: "...with respect to its sun, measured most accurately from local noon to local noon." (Please see Straw man.)  There are about 5 or 6 different meanings of day to cover - all but one of which (daytime) is usually much closer to 86,400 seconds than a stellar day is. A stellar day is NOT the primary meaning of day - it is called a day only by analogy, for it has nothing to do with the cycle of day and night. A solar day can be explained without resorting to a pre-Copernican picture & w/o resorting to overly technical language. This article needs to begin with either the solar day or the day (unit) to be about the ordinary meaning of the concept. A stellar day may be a more advanced way of looking at the rotation of the Earth, but it is not a more correct way of looking at "day". (Btw1: A rotating Earth was part of the Copernican picture -- & not part of the predominant pre-Copernican one. The predominant pre-Copernican picture, you surely already know, involved a moving sun and an Earth at rest. So why did you bring pre-Copernican up? Btw2: The idea of the "Earth rotating about itself" is a picture that involves absolute space - a concept that is presumptive and inaccessible for measurement. Btw4: Please do not focus on any btws in any response.)--JimWae (talk) 23:05, 30 May 2011 (UTC)

Note that a source you yourself pointed to, whenever he says "day" (without qualification) means the solar day -- and gives an extensive caution AGAINST equating "day" with "sidereal rotation period". This article should begin by discussing the ordinary meaning of day - not the rotation period of objects in (absolute) space (Rotation period is a separate article). --JimWae (talk) 23:52, 30 May 2011 (UTC)

Your own words you added to the article ("motion of revolving around the Sun would perform one day within an orbital period by its own without any spinning" [OR?]) indicate you understand that even an object with NO rotation at all could properly & correctly be said to have a day.--JimWae (talk) 00:24, 31 May 2011 (UTC)


 * There have been various pre-Copernican pictures, the geocentric model with a resting Earth (as by Ptolemy) you mentioned and even those with Earth in motion - E. revolving with sun by Philolaos, E. rotating by Herakleides P., E. revolving in heliocentric model by Aristoarchos - but Copernicus was the first (as is) known, who stated definitely "more than one motion" for the Earth: rotating and revolving.


 * Let us have a look at these two motions (without referring to absolute space). We could use the 'walk around the chapel' from above and imagine its top to be Sun, the person being Earth. Just having completed a whole round, revolving within an orbital period, always facing the sun high noon: the Earth meanwhile performed one entire rotation about itself (360° around its axis), with respect to its sun as well as to its background. The rotation period and the orbital period have the same span of time. Do you agree?
 * Then, we continue the walk and complete the next round - now, make the Earth performing two entire rotations of 360° within the orbital period (rotation period half of orbital period) with a spin of equal direction, as before. And then, a third round with two entire rotations (rotation period half of orbital period) - but now, Earth rotating the other direction (retrograd).


 * And what about a day?
 * round: is there any? - by one entire rotation under these conditions: no single solar day
 * round: we got one solar day - by two entire rotations prograd
 * round: we got three solar days - by two entire rotations retrograd
 * round zero: yet done ("... you understand that even an object with NO rotation at all could properly & correctly be said to have a day.")


 * Thus, for an rotating object, you may define one entire rotation with respect to its sun or something else. For an rotating object revolving a round, "entire" seems already defined by the revolved circle of 360°. And you cannot define two different kinds of "entire" related to the same movement of the same object without getting troubles.
 * Therefore, if the Earth is revolving around the Sun, saying "one entire rotation with respect to its sun" would be a desirable simple definition for solar day, but it is more than a little bit incorrect, from my point of view, it cannot be true, because of logical collision. By the way: Copernicus, Kepler and followers had the same view.


 * My intention was to point out "entire", as said before; not to omitt for refuting a straw man. (Btw.4 indicates you know and make the difference ([Btw.3]) between object language (Btw.1 and 2) and metalanguage and (perhaps] know the problems, which may arise if there would be none)--R*elation (talk) 14:19, 31 May 2011 (UTC)


 * We should not deprive the reader of the enligthenment, that the counted solar days within a year do not equal the number of rotations of the Earth - and do not do for any other planet or moon, either. These numbers are connected by logical implication, therefore, the number of solar days has to be one more (+1) or one less (-1) (and nothing else) compared to the number of rotations (called stellar "day") through the course of the year (for the Earth and any celestial body; doubters may observe this difference each night). We should mention this in the lede and give a chance to understand it.
 * "In former times ..." people have been told, a solar day would appear by an entire rotation of Earth. We know, since 1543, that this proposition cannot by true, if the Earth revolves around the Sun. --R*elation (talk) 08:01, 3 June 2011 (UTC)

Not everything needs to be covered in the lede. The lede already gives the length of both "day"s, one being one more than the other. I am not convinced stellar "day" even needs to be in the lede at all. Some things need to be left for the body of the article. The lede is the introduction, mentioning what will be covered - it is not the entire "course".--JimWae (talk) 09:39, 3 June 2011 (UTC)


 * I agree that the stellar day does not belong in the lede. The lede should summarize the article which only meantions a stellar day once in passing, whereas large sections are not even mentioned. The only way it could be mentioned in the lede is if a much larger section of the article is devoted to it. — Joe Kress (talk) 22:41, 3 June 2011 (UTC)


 * One thing needs to be covered in the lede in any case and in all cases: motion of the Earth. Any whatever conception of "day" refers to the movements of rotation and revolution of the Earth (or/and the Sun) and accordingly their relationship. In the introduction, we should discern accurately and treat correctly.
 * I don't mind, if neither stellar or sidereal would be mentioned. I am arguing for not to echo fairy tales. A lot of people (like me and maybe you alike) have been told in former days belike (1) "the Earth is revolving around the Sun" and (2) "one entire rotation makes a whole day". One of these propositions must be false. I believe in the course of the Earth to be an orbital revolution. So I disagree to the second.--R*elation (talk) 08:46, 4 June 2011 (UTC)

Introduction
The Introduction section gives conflicting numbers for the maximum deviation from 86,400 seconds for the solar day. It says 7.9 seconds and it says 30 seconds. I have not yet found a source that will clarify this. Does anyone know of one?--JimWae (talk) 09:50, 3 June 2011 (UTC)


 * The deviation in day length is the derivative of the equation of time. So:
 * $$\Delta d(n) = \frac {2 \pi \cdot 60} {365.242} [-7.657\cos M + 9.862\cdot 2 \cos(2M+3.599)] \;\mbox{sec}$$
 * It shows a double dip graph, with local extrema +18.94 s on 25 Mar, -12.68 s on 17 June, +22.51 s on 12 Sep and -28.07 s on 19 Dec.
 * The total variation in day length is thus almost 51 seconds.


 * &minus;Woodstone (talk) 15:56, 3 June 2011 (UTC)

Lede again
This edit was made with the comment "At first define". I agree! However, nowhere does "A day, commonly understood as the time it takes for the Earth to make a rotation with respect to the Sun, measured from local noon to the following local noon, is called a solar day" say what a day IS. It claims only what "a day... is CALLED", with the reference back in "called..." being unclear & strangely syntaxed. Furthermore, the time of rotation is an explanation, and it is not clear that a day is "commonly understood AS" a certain time of rotation. However, the previous wording, "A day, commonly understood as a solar day, is the time it takes for the Earth to make one rotation with respect to the Sun, measured from local noon to the following local noon" DOES say what a day (understood one way) IS. Time to revert this as well as this empty wordiness and this unclear, convoluted, barely-English syntax, both re-inserted after being reverted. Next step is to de-emphasize stellar day in lede. Would other editors please express their opinions on this, or do some reverting themselves, so we have more than 2 people going back & forth. --JimWae (talk) 20:23, 4 June 2011 (UTC)


 * "When timing": "Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the environment by external cues called zeitgebers, the primary one of which is daylight." "Although not the only parameter, the changing length of the photoperiod ('daylength') is the most predictive environmental cue for the seasonal timing of physiology and behavior, most notably for timing of migration, hibernation and reproduction." And without: "A circadian rhythm is an endogenously driven roughly 24-hour cycle in biochemical, physiological, or behavioural processes." So much for 2-wordy emptiness (Please see Circadian rhythm). For what it's worth, this edit had been reverted and obviously has not been "re-inserted". Nor that was a re-insertion.
 * It would be unclear or an unnecessary complication, if "Other planets and moons also have their own stellar and solar days." To give a definition like "span of time from local noon to the following one" and to measure the duration of this interval is not the same. --R*elation (talk) 09:16, 6 June 2011 (UTC)--R*elation (talk) 10:00, 7 June 2011 (UTC)

1 true day equals 47 hours
From after midnight of the day on one side of the International Date Line until in reaches the next midnight on the other side of the international dateline is 47 hours. — Preceding unsigned comment added by 75.181.99.157 (talk) 08:35, 15 September 2011 (UTC)

Civil Day: Historical Record
I need to know when the date changed in historical times, so far as civil records go.

We are all in agreement that the date presently changes at midnight, but this is clearly a recent development, based on electrical clocks.

Mechanical clocks, being mechanical, and when they could be had at all, could not be considered accurate, much less reliable, which means that before electricity and before railroads and before standard time zones, when Mean Time or Apparent Time was used, sundials were the civil standard of time keeping and therefore the date changed at NOON, as it was the one moment of the day that could be agreed upon by all.

Hence, anti-meridian, or AM, and post-meridian, or PM.

In support of this, all early European ephemerides, dating back to at least the 16th century, were invariably set for local NOON, in the place of publication. Midnight ephemerides did not appear before the middle of the 20th century.

Why is this important? Astrologers are time-keeping mavens. In William Lilly's Christian Astrology, of 1647, he generally sets his many charts in AM and PM, as we would expect, but on occasion he does not. On pg. 385 of Christian Astrology, Book 2, he gives a time of 19:20 pm, June 16, 1646 (Julian) and then supplies the chart. Transposing both the time and the date, this is June 27, 1646, 7:33:50 AM on the next day.

On pg. 473 of the same book, another chart, set for 12:29 pm, May 13, 1644. Which in modern notation is May 24, 1644, 12:24:35 AM. In both cases Lilly has supplied charts, which are nothing more than maps of the heavens drawn for a specific moment in time. All we are doing is making an exact copy and noting the modern time. Thus proving that, in London of 1647, the civil day began and ended at noon, not midnight. And also discovering that Lilly could draw very accurate charts with the means he had at his disposal.

Where this gets puzzling is with the birth of noted Austrian conductor, Herbert von Karajan. His birth was recorded, on his birth certificate, as 10:30 pm on April 5, 1908, in Salzburg. Regrettably, the resulting chart is nonsense. The time must be 10:30 am.

So far as astrology, this is a tempest in a teapot, but so far as accurate historical records, this is critical. At some point in the 20th century, the time of day when the date changed was shifted from NOON, to MIDNIGHT. It was presumably by international treaty and is a matter of historical record, and should be included on this page. Dave of Maryland (talk) 17:10, 25 June 2012 (UTC)


 * Cultures determine their own points of the day where a date changes, regardless of the points used by other cultures. I presume you are not interested in any culture that does not used the Julian/Gregorian calendar, because Jews, ancient Babylonians and Muslims changed their dates at sunset, and ancient Egyptians changed their dates at dawn, both using their own calendars.


 * In the West, dates changed at both midnight and noon for the last 2,000 years. Midnight has been used to change all civil dates since the ancient Romans did so until the present. Mechanical clocks had nothing to do with that choice. Roman sundials were marked from sunrise to noon to sunset by VI to XII to VI indicating seasonal hours, that is, hours whose lengths changed with the seasons, being long during summer and short during winter. The Romans used the Latin terms ante meridiem for hours before noon and post meridiem for hours after noon, which is where we get our abbreviations AM and PM, even though they changed their dates at midnight. The Latin meridiem literally means midday, that is, the middle of the day, indicating that the limits of the day were equally spaced on either side, implying midnight. "Meridian" refers to the celestial great circle passing through the celestial poles and the observer's zenith. It is a corruption of meridiem and obscures the meaning of meridiem.


 * Noon was used to change all astronomical dates since Ptolemy did so about 140 (for calculations) until December 31, 1924. However, Ptolemy also changed his dates at midnight for observations. The International Astronomical Union agreed that astronomical dates would begin at midnight beginning January 1, 1925 to agree with civil dates. So you must determine whether the date was written by an astronomer or someone else. Until the 17th century, all Western astronomers were also astrologers. Indeed most of them earned their living casting horoscopes for rich patrons, which enabled them to perform astronomical observations, which no one paid for. Ptolemy wrote the greatest ancient astronomical book (Almagest) and the greatest ancient astrological book (Tetrabiblos). Kepler was one of the first modern astronomers, composing the Rudolphine Tables in 1627, which allowed emphemerides to be calculated. But he also cast numerous horoscopes. — Joe Kress (talk) 06:13, 26 June 2012 (UTC)

Definition of the second
The article states that "In 1967, the second was redefined in terms of the wavelength of light". This is not correct as to convert from wavelength to time (frequency) requires a conversion using the speed of light. In fact, in 1967 the second was redefined as "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 caesium 133 atom".(Wikipaedia article on the second ). That is, the second was redefined in terms of the frequency of an atomic transition and not its wavelength. The distinction is important. --Gordone (talk) 17:14, 10 September 2012 (UTC)

'Astronomy' section is hopeless
I started to Wikify and copy edit the 'Astronomy' section, but in the process of doing so I realized that the section is hopeless. The section attempts to give general astronomical definitions, applicable to all planets, of stellar day, sidereal day, and mean solar day. However, these terms are only well defined in relation to Earth. The 'Astronomy' section is an object lesson in why citing reliable sources is essential to Wikipedia. The section could not have come out as badly as it did with citations to reliable sources. The erroneous internal links were also a warning that the section was not well grounded.
 * The given internal link to Stellar day redirects to a section of the article on Earth's rotation: Earth's rotation. That does not support a general definition applicable to planets generally.
 * The given internal link to Sidereal day redirects to Sidereal time, which is defined primarily with respect to Earth. Section 5 of the article does apply the concept to other planets in our solar system, but not elsewhere: Sidereal time The discussion shows that the concept is not useful when, as in the case of Mercury and Venus, the sidereal day is either a large fraction of or greater that the planet's period of rotation about the Sun or, by extension to other planetary systems, the system's central star. Furthermore, other planets, in our solar system or elsewhere, will not have a March equinox, which is essential to the 'Astronomy' section's definition of sidereal day.
 * The given internal link to Mean solar day redirects to Solar time. That entire article is about timekeeping on Earth. Also, piping Star to "its central star" does not help, and indeed misleads, the reader. It promises to explain central stars in planetary systems, but leads only to a general discussion of stars. All our readers know what a star is.

The section, with my little bit of editing (nowhere near enough), follows as a level 3 subsection. I suggest that we conduct any discussion up here. If anyone thinks they can turn this section into something informative, with appropriate citations to reliable sources and apt internal links (not links to redirects), feel free to try.—Finell 04:02, 12 May 2016 (UTC)

Astronomy
A day of exactly 86,400 SI seconds is the base unit of time in the science of astronomy.

Astronomy defines three types of day for every planet:


 * Stellar day – An entire rotation of a planet with respect to the fixed stars
 * Sidereal day – An entire rotation of a planet with respect to the March equinox
 * Mean solar day – The average time of an entire rotation of a planet with respect to its central star

For Earth, the stellar day and the sidereal day are nearly of the same length and about 3 minutes 56 seconds shorter than the solar day. Relative to the fixed stars, the Earth spins just over 366 times upon its axis during one complete orbit. The Earth's orbit around the Sun reduces (by one) the number of transits the Sun makes across the Earth's sky in a sidereal year.

Number format
I had no problem recognizing SI/ISO-style digit groupings. However, this article is aimed at the general reader, many of whom will be utterly mystified. Manual of Style provides, "In general, use a comma to delimit numbers with five or more digits to the left of the decimal point." Further, while WP:MOSNUM authorizes thin spaces to delimit number groupings in technical articles, no style guide authorizes use full spaces to delimit long numbers, as this article does. Therefore, I propose converting numbers to conventional English language use of commas as delimiters in numbers.—Finell 04:55, 12 May 2016 (UTC)