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I have been intrigued by the word o'clock since I learned English. Although there is an equivalent to this word in my native language (Spanish en punto meaning on point or on the dot) I want to know the origin of the term o'clock, especially why it has an apostrophe. Is it meant to contract on the clock or something similar?

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    Same as O'Brian, O'Reilly, O'Flanagan... son OF Brian, son OF Reilly...
    – OneProton
    Commented Nov 2, 2010 at 16:11
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    It most certainly is not. "O" in Irish names is derived from the Irish word "uaidh" meaning grandson.
    – Colin Fine
    Commented Nov 4, 2010 at 16:26
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    Colin Fine is correct, according to Wikipedia.
    – Jaime Soto
    Commented Nov 4, 2010 at 16:29
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    Flagged as general reference because a Google search for "o'clock" leads directly to several pages explaining it including three dictionaries.
    – Hugo
    Commented Oct 14, 2011 at 8:11
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    Wiktionary suggests that this is the "shortened form of 'of the clock' or 'on the clock'; that is, 'according to the clock'." Commented Jul 25, 2012 at 23:44

4 Answers 4

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According to The Time-traveller's Guide to Medieval England 'of the clock' was used to describe time when it was being sliced in 24 equal parts (hours) of the day.

It was used to differentiate the practice, used equally as much, of using solar time, whereby the 7th hour would shift in actual time, however would always be 7/12ths of a solar day after sunrise, and the length of an hour would increase in summer and decrease in winter.

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    Ooh, I need to get that book.
    – Marthaª
    Commented Nov 3, 2010 at 14:11
  • I would recommend it, it's a great read (and, no I have no affiliation with it, the author or the publisher :) )
    – johnc
    Commented Nov 3, 2010 at 18:46
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    We should totally go back to solar time, at least for work hours. ;)
    – devios1
    Commented Jul 10, 2011 at 22:27
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    The last sentence is simple physics: heat expands and cold contracts, so of course the hours get longer in summer and shorter in winter.
    – Jay
    Commented Dec 20, 2011 at 16:30
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    Note that daytime hours expand in summer and contract in winter. Nighttime hours do just the opposite: they're longer in winter and shorter in summer.
    – Marthaª
    Commented Jul 26, 2012 at 21:49
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I believe it's "of the clock".

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    Thanks! I found this among many links explaining the o'clock contraction.
    – Jaime Soto
    Commented Nov 2, 2010 at 15:32
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    Yes, this is what they told me in elementary school.
    – GEdgar
    Commented Jul 26, 2012 at 21:34
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Yes, o’clock is a contraction of the phrase “of the clock”. Online Etymology Dictionary traces the phrase back to the 1640s; the contraction first appears around 1720.¹

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Etymonline says it’s an abbreviation of of the clock, which makes sense phonetically—both the f in of and the th in the are reduced to null in some contexts, and it’s a very common phrase.

The earliest citation they give is 1720, which is fairly late; prior to that, the phrase was spelled out in full—even if it was pronounced the same.

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  • And it continued to be spelled out in full well into the 19th century (source: just search for something like "ten of the clock" in google books or in some corpus). Commented Jan 10, 2016 at 19:37

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