Editing Hebrew calendar (section)

===Hours===
{{See also|Zmanim#Relative hours|Relative hour}}
Judaism uses multiple systems for dividing hours. In [[Biblical and Talmudic units of measurement#Day|one system]], the 24-hour day is divided into fixed hours equal to {{frac|1|24}} of a day, while each hour is divided into 1080 ''halakim'' (parts, singular: ''[[helek]]''). A part is {{frac|3|1|3}} seconds ({{frac|1|18}} minute). The ultimate ancestor of the ''helek'' was a Babylonian time period called a ''barleycorn'', equal to {{frac|1|72}} of a Babylonian ''time degree'' (1° of celestial rotation).<ref name=Neugebauer1>{{cite journal|first=Otto |last=Neugebauer|title=The Astronomy of Maimonides and its Sources|journal=Hebrew Union College Annual|volume=23|date=1949|pages=321–363 |jstor=23506591}}</ref> These measures are not generally used for everyday purposes; their best-known use is for calculating and announcing the [[molad]].

In another system, the daytime period is divided into 12 [[relative hour]]s (''sha'ah z'manit'', also sometimes called "halachic hours"). A relative hour is defined as {{frac|1|12}} of the time from sunrise to sunset, or dawn to dusk, as per the two opinions in this regard. Therefore, an hour can be less than 60 minutes in winter, and more than 60 minutes in summer; similarly, the 6th hour ends at [[solar noon]], which generally differs from 12:00. Relative hours are used for the calculation of prayer times ([[zmanim]]); for example, the [[Shema]] must be recited in the first three relative hours of the day.<ref>Mishna Berachot 1:2. Note that the mishna specifies that the Shema may be recited "until three hours"; this is understood to mean "until the end of the third hour".</ref>

Neither system is commonly used in ordinary life; rather, the local civil clock is used. This is even the case for ritual times (e.g. "The latest time to recite Shema today is 9:38 AM").<ref>See e.g. [https://www.myzmanim.com/day.aspx?askdefault=1&vars=27526341&q=jerusalem Zmanim: Jerusalem]</ref>