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The hour has a special meaning in Jewish law. "The third hour of the day" doesn't mean 3:00 a.m., or three sixty-minute hours after sunrise. Rather, an hour in halacha is calculated by taking the total time of daylight of a particular day, from sunrise until sunset,1 and dividing it into twelve equal parts. A halachic hour is thus known as a sha'ah zemanit, or proportional hour, and varies by the season and even by the day.

For example, on a day when the sun rises at 5 a.m. and sets at 7:30 p.m., one sha'ah zemanit, or proportional hour, will be 72.5 minutes long. The third hour of the day will come to a close at 8:37:30 a.m.

This information is important because many observances in Jewish law are performed at specific times during the day. The calculation of these halachic times, known as zmanim ("times"), depends on the length of the daylight hours in that locale.

For more information regarding the various halachic times of the day, as well as some of their associated mitzvot, see About Zmanim.

To find out the halachic times for any location, see Zmanim-Halachic Times.

FOOTNOTES
1.

According the other opinions, from dawn ("alot hashachar") until three stars appear in the sky ("tzeit hakochavim").

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Discussion (8)
January 13, 2014
RE: correspondence between Jewish and Roman calendar
The Jewish day begins at nightfall and the secular day begins at midnight. Since there is the (approximately) six hour lag, technically every secular day corresponds to two Jewish days. However, I do not see how it can span any more than that.
Menachem Posner
January 11, 2014
correspondence between Jewish and Roman calendar
Is it possible for a given day in the Gregorian/Western calendar to span two days or more in the Jewish calendar? If so, why? Thank you.
Deborah Jurkowitz
November 18, 2013
24 hours in the day
How was the determination made that the day should be divided into 12 and the night into 12 it could have been 8, or 9, or any other number? Why 12? Why 60 minutes seconds?

I have read that the Egyptians used the joints of the fingers excluding the thumb, that there is an "earlier" Kabbalistic reason but am unable to find an English source and clear explanation.
Winston Shaer
March 30, 2013
Is the first day of the month a holiday. Also is this day not included in the first week which would make the Sabbath day the Eighth Day
James
New Zealand
August 6, 2012
To Anonymous, Mesquite, TX
You are on the ball. When calculating nighttime hours, we do so by dividing the time from nightfall to dawn into 12 parts. And when we calculate the daytime hours, we do so by dividing the daylight into 12 parts. This means that most times of the years the day hours and night hours are different lengths.
Rabbi Menachem Posner
August 5, 2012
If a day starts at evening, should not daylight of a particular day be calculated by taking the time of daylight from sunset to sunrise?
Anonymous
mesquite, tx/usa
May 27, 2012
proportional hour
what a concept! exciting new information.
Anonymous
Springfield, MO USA
September 14, 2009
The information that you have posted is very helpful to me in teaching about the keeping of the Jewish Holidays in the furture
Anonymous
phila, pa
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