These are the abbreviations of two major Rabbis from the XVIIth and XVIIIth centuries, who stated important decisions regarding the way to compute the daylight duration (among others).
Their approaches have direct consequences in the Zmanim of the prayers and other daily observance in Judaism.
GR”A
The Vilna Gaon, Rabbi Eliyahu ben Shlomo Zalman (1720 – 1797).
Sunrise and Sunset are two observable astronomic events, when the first and last ray of Sun are visible above the horizon line, from a given standpoint. According to GR”A, the Daylight is the duration that spans between the Sunrise and Sunset of a given date at a given location.
The Proportional Hour (in Hebrew: Sha’a zmanit שעה זמנית, also called Seasonal Hour) measures one twelfth of the daylight duration.
M”A
The Magen Avraham, is the title of a famous book by Rabbi Abraham Abele Gombiner (c. 1635 – 1682). We often refer to Rabbis by the main piece of work of theirs.
According to the M”A, the Proportional hour is defined by one twelfth of the time spanning from dawn to nightfall. In other words, it is the time between sunset and sunrise, plus the morning and evening twilight.
This duration is longer than the one calculated by GR”A.
There are also a number of rabbinical opinions regarding the accurate definition of dawn and nightfall. Therefore, unlike with GR”A, the definition of the Proportional hour itself, may vary.
Consequences
Some important milestones along the Jewish day relate to the measure of the Proportional hour, such as:
- the final time to recite the Shma’ Israel is the end of the 3rd proportional hour after Sunset ;
- the earliest time to recite the Mincha prayer is six and a half proportional hours after Sunset ;
and so on.
Therefore, the calculated Zmanim differ according to the selected opinion regarding the daylight duration.