Editing Hour (section)

===Middle Ages===
{{main|Canonical hours}}
[[Image:Bishopstone sundial.jpg|right|thumb|200px|A 7th-century Saxon [[tide dial]] on the porch at [[Bishopstone, East Sussex|Bishopstone]] in [[Sussex]], with larger crosses marking the [[canonical hours]].{{sfnp|Wall|1912|p=67}}]]

Medieval astronomers such as [[al-Biruni]]<ref>{{ cite book | author=Al-Biruni | year=1879 | orig-year=1000 | title=The Chronology of Ancient Nations | url=https://books.google.com/books?id=pFIEAAAAIAAJ&pg=PA147 | pages=147–149 | translator-last=Sachau | translator-first=C. Edward | author-link=Al-Biruni}}</ref> and [[Sacrobosco]],<ref>{{ Citation | last = Nothaft | first = C. Philipp E. | date = 2018 | title = Scandalous Error: Calendar Reform and Calendrical Astronomy in Medieval Europe | publisher = Oxford University Press | place = Oxford | page = 126 | isbn = 9780198799559}}</ref>   divided the hour into 60 [[minute]]s, each of 60 [[second]]s; this derives from [[Babylonian astronomy]], where the corresponding terms{{clarify|date=April 2020}} denoted the time required for the Sun's apparent motion through the [[ecliptic]] to describe one minute or second of arc, respectively. In present terms, the Babylonian degree of time was thus four minutes long, the "minute" of time was thus four seconds long and the "second" 1/15 of a second.<ref>{{cite journal |last=Correll |first=Malcolm |journal=The Physics Teacher |volume=15 |pages=476–479 |issue=8 |date=November 1977 |title=Early Time Measurements|doi=10.1119/1.2339739 |bibcode=1977PhTea..15..476C }}</ref><ref>{{cite journal|journal=Journal for the History of Astronomy|author1=F. Richard Stephenson|author-link=F. Richard Stephenson|author2=Louay J. Fatoohi|date=May 1994|doi=10.1177/002182869402500203|title=The Babylonian Unit of Time|volume=25|issue=2|pages=99–110|bibcode=1994JHA....25...99S|s2cid=117951139}}</ref>

In medieval Europe, the Roman hours continued to be marked on [[sundial]]s but the more important units of time were the [[canonical hours]] of the [[Greek Orthodox Church|Orthodox]] and [[Catholic Church]]. During daylight, these followed the pattern set by the three-hour bells of the [[forum (market)|Roman markets]], which were succeeded by the [[church bell|bells]] of local churches. They rang [[prime (liturgy)|prime]] at about 6{{nbsp}}am, [[terce]] at about 9{{nbsp}}am, [[sext]] at noon, [[nones (liturgy)|nones]] at about 3{{nbsp}}pm, and [[vespers]] at either 6{{nbsp}}pm or [[sunset]]. [[Matins]] and [[lauds]] precede these irregularly in the morning hours; [[compline]] follows them irregularly before sleep; and the [[midnight office]] follows that. [[Vatican II]] ordered their reformation for the Catholic Church in 1963,<ref>{{citation |author=Paul&nbsp;VI |author-link=Pope Paul VI |date=4 December 1963 |title=Constitution on<!--sic--> the Sacred Liturgy |url=https://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19631204_sacrosanctum-concilium_en.html |location=Vatican City |at=§89(d) }}</ref> though they continue to be observed in the Orthodox churches.

When mechanical [[clock]]s began to be used to show hours of daylight or nighttime, their period needed to be changed every morning and evening (for example, by changing the length of their [[pendula]]). The use of 24 hours for the entire day meant hours varied much less and the clocks needed to be adjusted only a few times a month.