Overview
Alos Hashachar (עלות השחר), meaning "the rise of dawn," is the moment when the first light appears on the eastern horizon. This marks the beginning of the halachic day for many purposes, including the earliest time to put on tallis and tefillin, and the start of the prohibition of eating before Shacharis.
Talmudic Source
The Gemara in Pesachim 94a states that the time span between Alos Hashachar and sunrise (Hanetz HaChama) equals the time it takes an average person to walk 4 mil (a mil being approximately 2,000 amos or about 1 kilometer). This same Gemara discusses the parallel time from sunset to tzeis hakochavim.
The Gemara records a dispute: one opinion holds the distance is 5 mil, while Rabbi Yehuda holds it is 4 mil. Most Poskim follow Rabbi Yehuda's opinion of 4 mil.
Since it takes 18 minutes to walk one mil, the total time is 72 minutes before sunrise.
The 16.1° Calculation
The Vilna Gaon (Gra) and others ruled that Alos Hashachar cannot be defined as a fixed number of minutes before sunrise. The Gemara's figures were stated specifically for:
- The equinox days (when day and night are equal)
- The latitude of Eretz Yisrael and Bavel
For other locations and seasons, different calculations apply based on the sun's position below the horizon.
Zmanim Kehilchasam by Rabbi Dovid Yehuda Burstein explains that 72 minutes before sunrise on a perfect equinox day in Jerusalem corresponds to when the sun is 16.1 degrees below the horizon.
Therefore, Alos can be universally defined as the moment the sun reaches 16.1° below the horizon, ensuring consistent appearance of dawn across all locations and seasons.
Alternative Opinions
72 Fixed Minutes
Some communities use a fixed 72 minutes before sunrise regardless of location or season. This is simpler to calculate but may not accurately reflect the actual appearance of dawn at different latitudes.
90 Minutes (18° or 19.8°)
The Magen Avraham and others hold that it takes 22.5 minutes to walk a mil (based on a different calculation), making Alos 90 minutes before sunrise. This corresponds to approximately 18-19.8 degrees below the horizon.
The common practice in Jerusalem (following the Luach Tucazinsky) uses 20 degrees (90 minutes at the equinox).
17.5° vs 16.1°
Some distinguish between:
- 17.5° — When the very first light begins to appear
- 16.1° — When the eastern sky is noticeably illuminated
Practical Applications
Alos Hashachar is relevant for:
- Earliest time for Shema (according to some opinions) — though many wait until misheyakir
- Beginning of the fast on minor fast days
- Prohibition of eating before Shacharis and certain mitzvos
- End of nighttime for purposes of Krias Shema at night
- Calculation of sha'os zmaniyos according to the Magen Avraham