question: Why is noon 12 pm, midnight 12 am, and not the contrary?
comment: AM: “ante meridiem” Latin for “before midday” PM: “post meridiem” Latin for “after midday”
passages: Section::::Confusion at noon and midnight.
  
  It is not always clear what times "12:00 a.m." and "12:00 p.m." denote. From the Latin words "meridies" (midday), "ante" (before) and "post" (after), the term "ante meridiem" (a.m.) means before midday and "post meridiem" (p.m.) means after midday. Since "noon" (midday, "meridies" (m.)) is neither before nor after itself, the terms a.m. and p.m. do not apply. Although "12 m." was suggested as a way to indicate noon, this is seldom done and also does not resolve the question of how to indicate midnight.
   Strictly speaking, it is incorrect to use "a.m." and "p.m." when referring to noon or midnight. The abbreviation a.m. stands for "ante meridiem" or "before noon", and p.m. stands for "post meridiem" or "after noon". Since noon is neither before nor after noon, and midnight is exactly twelve hours before and after noon, neither abbreviation is correct. However, many digital representations of time are configured to require an "a.m." or "p.m." designation, preventing the correct absence of such designators at midnight. In such cases, there is no international standard defining which arbitrary selection is best. 
   "The Canadian Press Stylebook" says, "write "noon" or "midnight", not "12 noon" or "12 midnight"." Phrases such as "12 a.m." and "12 p.m." are not mentioned at all. Britain's National Physical Laboratory "FAQ-Time" web page states "In cases where the context cannot be relied upon to place a particular event, the pair of days straddling midnight can be quoted"; also "the terms 12 a.m. and 12 p.m. should be avoided."
   Originally there were two cycles; one cycle which could be tracked by the position of the Sun (day) followed by one cycle which could be tracked by the Moon and stars (night). This eventually evolved into the two 12-hour periods which are used today, starting at midnight (a.m.) and noon (p.m.). Noon itself is rarely abbreviated today, but if it is, it is denoted M.
  
  The 12-hour clock can be traced back as far as Mesopotamia and ancient Egypt.
  
  Both an Egyptian sundial for daytime use
   Midnight
  
  Midnight is the transition time from one day to the next – the moment when the date changes. In ancient Roman timekeeping, midnight was halfway between sunset and sunrise (i.e., solar midnight), varying according to the seasons. By clock time, midnight is the opposite of noon, differing from it by 12 hours.
   "The American Heritage Dictionary of the English Language" states "By convention, "12 AM" denotes midnight and "12 PM" denotes noon. Because of the potential for confusion, it is advisable to use "12 noon" and "12 midnight"."
  
  E. G. Richards in his book "Mapping Time" provided a diagram in which 12 a.m. means noon and 12 p.m. means midnight. The style manual of the United States Government Printing Office used 12 a.m. for noon and 12 p.m. for midnight until its 2008 edition, when it reversed these designations, later maintained in its 2016 revision.
   Many U.S. style guides, and NIST's "Frequently asked questions (FAQ)" web page, recommend that it is clearest if one refers to "noon" or "12:00 noon" and "midnight" or "12:00 midnight" (rather than to "12:00 p.m." and "12:00 a.m."). The NIST website states that "12 a.m. and 12 p.m. are ambiguous and should not be used."
  
  "The Associated Press Stylebook" specifies that midnight "is part of the day that is ending, not the one that is beginning."
   Solar midnight is the time opposite to solar noon, when the Sun is closest to the nadir, and the night is equidistant from dusk and dawn. Due to the advent of time zones, which regularize time across a range of meridians, and daylight saving time, it rarely coincides with 12 midnight on the clock. Solar midnight depends on longitude and time of the year rather than on time zone.
   An earlier standard of indicating noon as "12M" or "12m" (for "meridies"), which was specified in the "U.S. GPO Government Style Manual", has fallen into relative obscurity; the current edition of the GPO makes no mention of it.
  
  However, due to the lack of an international standard, the use of 12 a.m. and 12 p.m. can be confusing. Common alternative methods of representing these times are:
  
  BULLET::::- to use a 24-hour clock (00:00 and 12:00, 24:00; but never 24:01)
  
  BULLET::::- to use "12 noon" or "12 midnight" (though "12 midnight" may still present ambiguity regarding the specific date)
   In the United States and Canada, digital clocks and computers commonly display 12 a.m at midnight. The thirtieth edition of the "U.S. Government Style Manual (2008)", in sections 9.54 and 12.9b, recommended the use of "12 a.m." for midnight and "12 p.m." for noon. However, the previous 29th edition of the "U.S. Government Printing Office Style Manual (2000)", in section 12.9, recommended the opposite. There is no further record documenting this change. The US National Institute of Standards and Technology (NIST) recommends avoiding confusion altogether by using "00.01 am" and the date instead of "midnight."
   The terms "a.m." and "p.m." are abbreviations of the Latin "ante meridiem" (before midday) and "post meridiem" (after midday). Depending on the style guide referenced, the abbreviations "a.m." and "p.m." are variously written in small capitals ("" and ""), uppercase letters without a period ("AM" and "PM"), uppercase letters with periods, or lowercase letters ("am" and "pm" or, more commonly, "a.m." and "p.m.").
   A possible explanation for the shift from having the first hour being the one after dawn, to having the hour after noon being designated as 1 pm (post meridiem), is that these clocks would likely regularly be reset at local high noon each day. This, of course, results in midnight becoming 12 o'clock.
   The word "noon" is derived from Latin "nona hora", the ninth hour of the day, and is related to the liturgical term none. The Roman and Western European medieval monastic day began at 6:00 a.m. (06:00) at the equinox by modern timekeeping, so the ninth hour started at what is now 3:00 p.m. (15:00) at the equinox. In English, the meaning of the word shifted to "midday" and the time gradually moved back to 12:00 local time (that is, not taking into account the modern invention of time zones). The change began in the 12th century and was fixed by the 14th century.
   Noon
  
  Noon (also midday or noon time) is 12 o'clock in the daytime, as opposed to midnight. The term 12 p.m. (for post meridiem, and also written as 12 pm or similar variations thereof) is sometimes used for noon.
  
  Solar noon is the time when the Sun appears to contact the local celestial meridian. This is when the Sun apparently reaches its highest point in the sky, at 12 noon apparent solar time. The local or clock time of solar noon depends on the longitude and date.
  
  Section::::Etymology.
   The shortest time spans occur twice each year, around 25 March (24 hours minus 18 seconds) and 13 September (24 hours minus 22 seconds).
  
  Section::::Nomenclature.
  
  In the US, noon is commonly indicated by 12 p.m., and midnight by 12 a.m. While some argue that such usage is "improper" based on the Latin meaning (a.m. stands for "ante meridiem" and p.m. for "post meridiem", meaning "before midday" and "after midday" respectively), digital clocks are unable to display anything else, and an arbitrary decision must be made. 
   Likewise, some U.S. style guides recommend either clarifying "midnight" with other context clues, such as specifying the two dates between which it falls, or not referring to the term at all. For an example of the latter method, "midnight" is replaced with "11:59 p.m." for the end of a day or "12:01 a.m." for the start of a day. That has become common in the United States in legal contracts and for airplane, bus, or train schedules, though some schedules use other conventions. Occasionally, when trains run at regular intervals, the pattern may be broken at midnight by displacing the midnight departure one or more minutes, such as to 11:59 p.m. or 12:01 a.m.
   The 12-hour clock in speech often uses phrases such as " ... in the morning, ... in the afternoon, ... in the evening," and "...at night". "Rider's British Merlin" almanac for 1795 and a similar almanac for 1773 published in London used them. Other than English-speaking countries, the terms "a.m." and "p.m." are seldom used and often unknown.
  
  Section::::History and use.:Computer support.
   In the modern 12-hour clock, counting the hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11. Solar noon is always close to 12 noon (ignoring artificial adjustments due to time zones and daylight saving time), differing according to the equation of time by as much as fifteen minutes either way. At the equinoxes sunrise is around 6 a.m. (, before noon), and sunset around 6 p.m. (, after noon).
   The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin "ante meridiem", translates to, before midday) and p.m. (from Latin "post meridiem" translates to, past midday). Each period consists of 12 hours numbered: "12" (acting as zero), "1, 2, 3, 4, 5, 6, 7, 8, 9, 10," and "11". The 24 hour/day cycle starts at 12 midnight (may be indicated as 12 a.m.), runs through 12 noon (may be indicated as 12 p.m.), and continues to the midnight at the end of the day. The 12-hour clock has been developed from the middle of the second millennium BC to the 16th century AD.
   The word is derived from the Latin ', from ' (equal) and ' (genitive ') (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality, primitive cultures noted the day when the Sun rises due East and sets due West, and indeed this happens on the day closest to the astronomically defined event.
   BULLET::::- a.m./p.m. – These are abbreviations for the Latin adverbial phrases "ante meridiem" ("before noon") and "post meridiem" ("after noon"). Some argue that they thus should not be used in English as nouns meaning "morning" and "afternoon"; however, such use is consistent with ordinary nominalization features of English. AHD4 lists adjectival usage with "an A.M. appointment" and "a P.M. appointment". RH gives "Shall we meet Saturday a.m.?" without comment. Also, the National Institute of Standards and Technology contends it is incorrect to use 12 a.m. or 12 p.m. to mean either noon or midnight.
   Since the Sun's altitude is the same at times equally spaced about noon (e.g., 9am and 3pm), the user had to know whether it was morning or afternoon. At, say, 3:00 pm, that isn't a problem. But when the dial indicates a time 15 minutes from noon, the user likely won't have a way of distinguishing 11:45 from 12:15.
  
  Additionally, altitude dials are less accurate near noon, because the sun's altitude isn't changing rapidly then.
   Section::::Contents.:Chapter 10. The Road Back.
   Midnight marks the beginning and ending of each day in civil time throughout the world. As the dividing point between one day and another, midnight defies easy classification as either part of the preceding day or of the following day. Though there is no global unanimity on the issue, most often midnight is considered the start of a new day and is associated with the hour 00:00. Even in locales with this technical resolution, however, vernacular references to midnight as the end of any given day may be common.
   The Latin abbreviations "a.m." and "p.m." (often written "am" and "pm", "AM" and "PM", or "A.M." and "P.M.") are used in English and Spanish. The equivalents in Greek are and , respectively, and in Sinhala () for (, – fore, pre) and () for (, – after, post). However, noon is rarely abbreviated in any of these languages, noon normally being written in full. In Portuguese, there are two official options and many other used, for example, using 21:45: 21h45 or 21h45min (official ones) or 21:45 or 9:45 p.m. In Irish, "a.m." and "i.n." are used, standing for "ar maidin" ("in the morning") and "iarnóin" ("afternoon") respectively.
  