Background
Time has a special significance in Judaism, since so many observances are associated with specific times.
Let us take the recitation of the Shema as an example. The Torah commands us to recite the Shema both during the day and at night. How can we determine exactly when day begins and when night ends? What about the time in between the two—where does it belong?
Throughout the Jewish nation’s history, our Sages have dealt with this issue and have established methods for calculating time whereby the day is divided into “seasonal hours,” based on which we can determine the appropriate times for each ritual with exacting calculations. In this way, we can determine the exact times when we can recite the various prayer services, when we can recite the Shema, when we can wear tefillin, when we must cease eating chometz before Passover, when Shabbat or a holiday begins, and so forth.
Of course, in addition to the basic distinction between day and night, there are additional, more delicate distinctions that must be made, such as the various stages of sunrise and sunset, as we shall explain.
Nowadays, it is unnecessary for the average layman to perform any of these calculations. All of the halachic times are readily available on various calendars and on the Internet. Simply search for the area where you live, and you can determine the exact time of sunrise, sunset, nightfall, and the like in your location.
The following is a brief overview of the concepts pertaining to time in Jewish law.
Seasonal hours
“Seasonal hours” refers to a concept of hours of relative proportion, whose length varies throughout the year, based on the length of the daylight hours. All of the halachic decisors accept the use of seasonal hours, and this system serves as the primary means of determining the various times of halachic significance throughout the day.
The calculations are performed by determining the amount of time between sunrise and sunset and dividing it into twelve equal portions—the seasonal hours. In the summer, when the day is longer, the hours are longer; in the winter, they are shorter.
There are two principal schools of thought regarding these calculations; they differ regarding whether the beginning of the day is considered to take place at sunrise or at dawn, and whether the end of the day is at sunset or slightly after. The system that places the beginning of the day at dawn results in a slightly longer seasonal hour.
Alot Hashachar – The Rising of the Dawn
Alot hashachar (the rising of the dawn) is the earliest time that represents the beginning of the day. It is the moment when sunlight begins to illuminate the sky, although the sun itself is still hidden beneath the horizon at this time.
Haneitz Hachamah – Sunrise
Sunrise is the moment when the sun itself becomes visible over the horizon. It is equivalent to the astronomic definition of sunrise.
Zerichah
Zerichah is the time when the first portion of the sun rises above the horizon of the east and it becomes clearly visible.
Chatzot Hayom – Midday
Midday, or halachic noon, is the midpoint between sunrise and sunset. When the day is divided into twelve equal portions, midday occurs exactly at the end of the sixth hour. Midday and midnight are always separated by exactly twelve standard hours.
Shekiat Hachamah – Sunset
Sunset represents the time when the sun is no longer visible to the average human eye.
Bein Hashemashot – Twilight
Bein hashemashot is an “in between” period that begins with sunset and continues until nightfall. The sun has already set and is no longer visible, but its light continues to be somewhat visible.
There are a number of different opinions regarding the status of this problematic time period. It is not clear whether it is considered a part of the day or a part of the night.
Tzeit Hakochavim – Emergence of Stars
Tzeit hakochavim marks the official, definite beginning of the night. From a halachic standpoint, this period of time has special significance, since Judaism considers the day to begin from the night before. Thus, nightfall marks the beginning of the coming day.
There are a number of different opinions regarding how to determine the exact time of nightfall. In general, it varies from about 18 minutes after sunset to as much as 72 minutes or more after sunset.
Chatzot Halailah – Midnight
Halachic midnight represents the midpoint of the period between sunset and sunrise. If the night is divided into twelve equal portions, midnight occurs exactly at the end of the sixth hour.
Kabbalistically speaking, the night is considered to consist of two parts. Before midnight, the Attribute of Judgment is considered to be in force, but after midnight, the Attribute of Mercy is dominant.
