Overview
A Sha'ah Zmanis (שעה זמנית), meaning "proportional hour" or "halachic hour," is a fundamental concept in Jewish timekeeping. Unlike fixed 60-minute hours, a halachic hour is 1/12th of the daylight period. This means that in summer, when days are longer, each halachic hour is longer than 60 minutes, and in winter, they are shorter.
Many zmanim are calculated using these proportional hours. For example, "the third hour" doesn't mean 3:00 AM, but rather three halachic hours into the day.
Why Proportional Hours?
The Torah and Talmud describe times relative to the day's length rather than fixed clock times. The Gemara speaks of events happening "at the third hour" or "at the sixth hour," understanding that the day was naturally divided into twelve parts based on sunlight.
The Mishnah Berachos 1:2 discusses the time for Shema using these proportional hours, and Shabbos 9b explicitly discusses the concept of sha'os zmaniyos.
This system ensures that halachic times correspond to the actual position of the sun and the natural rhythm of daylight, regardless of season or latitude.
The Two Main Calculations
The critical question is: what defines the "day" that we divide by 12?
Vilna Gaon (Gra)
The Vilna Gaon (Rabbi Eliyahu of Vilna, 1720-1797) holds that sha'os zmaniyos are calculated from sunrise (הנץ החמה) to sunset (שקיעה).
His reasoning:
- The "day" for mitzvah performance is properly from sunrise to sunset
- These are clearly observable astronomical events
- The Torah says "from the rising of the sun to its setting" (Malachi 1:11)
Calculation:
Sha'ah Zmanis = (Sunset - Sunrise) ÷ 12
Magen Avraham (M"A)
The Magen Avraham (Rabbi Avraham Gombiner, c. 1635-1682) holds that sha'os zmaniyos are calculated from alos hashachar (dawn) to tzeis hakochavim (nightfall).
His reasoning:
- "Daytime activities" are permitted from dawn until nightfall
- The halachic day for many purposes begins at alos
- This provides a longer "day" and earlier times for morning obligations
Calculation:
Sha'ah Zmanis = (Tzeis - Alos) ÷ 12
Practical Difference
The Magen Avraham's calculation always produces an earlier time than the Gra's for morning zmanim (since his "day" starts earlier):
| Season | Gra Hour | M"A Hour | Difference |
|---|---|---|---|
| Winter Solstice | ~45 min | ~54 min | ~9 min |
| Equinox | ~60 min | ~72 min | ~12 min |
| Summer Solstice | ~75 min | ~90 min | ~15 min |
For a zman at "the third hour," the M"A time might be 30-45 minutes earlier than the Gra time.
Example Calculation
On a day when:
- Alos is 5:30 AM
- Sunrise is 6:30 AM
- Sunset is 7:30 PM
- Tzeis is 8:30 PM
Gra calculation:
- Day length: 7:30 PM - 6:30 AM = 13 hours = 780 minutes
- Sha'ah Zmanis: 780 ÷ 12 = 65 minutes
- Third hour ends: 6:30 AM + (3 × 65) = 9:45 AM
M"A calculation:
- Day length: 8:30 PM - 5:30 AM = 15 hours = 900 minutes
- Sha'ah Zmanis: 900 ÷ 12 = 75 minutes
- Third hour ends: 5:30 AM + (3 × 75) = 9:15 AM
Which Opinion to Follow?
For Torah Obligations (Shema)
Most authorities recommend being stringent and following the earlier M"A time for Krias Shema, which is a Torah obligation.
For Rabbinic Obligations (Tefillah)
Many communities follow the later Gra time for Sof Zman Tefillah, since the Amidah is a rabbinic obligation.
Common Practice
The prevalent custom is:
- Try to complete Shema before the M"A deadline
- At minimum, complete Shema before the Gra deadline
- Use the Gra time for Tefillah
Zmanim That Use Sha'os Zmaniyos
The following times are calculated using proportional hours:
| Zman | Hours |
|---|---|
| Sof Zman Shema | 3 hours |
| Sof Zman Tefillah | 4 hours |
| Mincha Gedola | 6.5 hours |
| Mincha Ketana | 9.5 hours |
| Plag HaMincha | 10.75 hours |
Historical Note
The concept of proportional hours was used throughout the ancient world. The Romans, Greeks, and Egyptians all divided daylight into twelve parts. Sundials naturally measure these proportional hours.
The innovation of fixed 60-minute hours came later with mechanical clocks. Halacha retained the original sun-based system, which is more aligned with the natural patterns of human activity and agricultural life that the Torah addresses.