|Many observances in Jewish law are performed at specific times during the day. The calculation of these halachic times, known as zmanim (Hebrew for "times"), depends on the various astronomical phenomena of the day for the specific locale.
Sunrise, sunset, the amount of time in between, and the sun's angular position before rising are all factors that determine the halachic times and "hours" of the day.
(The hour has special meaning in Jewish law. When we say that a certain mitzvah may be performed three hours into the day, this doesn’t mean at three in the morning, or three clock-hours after sunrise. Rather, an hour in halacha means 1/12th of the day. Thus, if the sun rises at 5 am and sets at 7:30 pm, one sha'ah zemanit, or proportional hour, will be 72.5 minutes, and all calculations will use that number.)