The next word on our list is יום [YOM] which means ‘day’. Like the English word ‘day’, יום has two different meanings. It means the period of time it takes for the earth to make a complete rotation on its axis – 24 hours. It also means the part of that period of time during which it is light. Thus, the 24 hour day is divided into two parts – ‘day’ and ‘night’. In Hebrew these are called YOM and LAILA.
In our civil calendar the day begins in the middle of the night. This is convenient as typically there is little activity at night. In the Jewish calendar the new day begins at the beginning of the night, near sunset. This is based on the biblical verse which describes the creation – ‘and it was evening and it was morning the first day’ – ‘evening’ came first, and therefore the evening is the beginning of the day.
Having the transition from one day to the next while people are awake causes many complications. If you are writing a contract at the end of the day you may accidently put the wrong date on it. Starting a day at the beginning of the night is further complicated by the fact that the transition from day to night is gradual. There is a whole series of events: the sun moves towards the west, the sun sets and the sky gradually darkens until it finally turns to black. There is no precise point when day turns to night.
Jewish law accommodates this gradual transition by marking different periods within it. Any time before the sun has set is definitely day. Once it is dark it is definitely night. Between the two, there is a period of time called ‘twilight’, which is neither day nor night.
So how do Rabbis decide when a day actually begins? Rabbinic sources give different definitions. For example, the Talmud says (Shabbat 34b): “From sunset on, as long as the eastern sky has a reddish glow, then, when the lower horizon is dark, but not the upper horizon, it is twilight; but when the upper horizon is as dark as the lower, it is night, so says Rabbi Yehudah. Rabbi Nechemiah says: [Twilight begins at sunset and lasts] as long as it takes a person to walk half a mil (.35 miles).”
The length of twilight changes according to the place and time of year. For practical purposes, the Rabbis assume that the sources speak about an average day in Jerusalem. Once this was established, it is relatively simple to determine the exact start of night. You simply need to determine how dark it would be in Jerusalem at the times indicated by the Talmud, and use this level of darkness to define night.
From a scientific point of view, darkness is determined by how far the sun is below the horizon. While there are different opinions and customs, the beginning of night (i.e. the end of Shabbat) is usually defined as the time when the sun is 4.9 (or according to some 5.4) degrees beneath the horizon. That is the time you will find on your Jewish calendars.
Day and Night are basic life categories and serve a central role in the smooth running of many aspects of society. It is surprising that Jewish tradition has left the border between them so poorly defined. But it is not surprising that the ingenuity of the Jewish people has succeeded in making order out of that chaos, and has created a system that works.
Rabbi Chaim Weiner
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My Hebrew Word thanks the World Zionist Organisation and Masorti Olami for their support of this Project.