This is a very "niche" question...Obviously, the accepted practice is to calculate halakhic times by שעות זמניות. However, a minority of Rishonim and Acharonim disagree with the whole concept and maintain that שעות שוות should be used. Over time, this opinion has been generally disregarded, except by Komarna Chassidim, who even today follow one of their Rebbes who was adamantly opposed to שעות זמניות. This is the background to my question. The (totally hypothetical) question itself: According to those who would propose the use of שעות שוות for halakhic times instead of שעות זמניות, when exactly would the day begin? When would you start counting the 60 minute hours to determine the deadline for Shema, Tefillah, eating hametz on Erev Pesah, etc.? I am assuming that beginning from an arbitrary point in time like 6AM makes no sense at all. And I know that with regard to Erev Pesah, the Terumat HaDeshen says to count two hours backwards from חצות, seemingly implying that we would have use the frame of reference of daylight hours for some things. But how would this be done in a way that is consistent?
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I'm sure you've realized this, but just to be explicit, the difficulty you are asking about is just one of the reasons nearly everyone disregards this as a reasonable option.– Double AA ♦Commented May 10, 2022 at 13:04
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Tangent: Rav Moshe Feinstein uses a fixed time only for chatzos. And there is an argument that the 30 min safety margin after chatzos is clock time, not solar. So I have seen software that defines minchah gedolah as the later of noon and the location's average noon plus either half a shaah zemanis or 30 min, whichever is greater.– Micha BergerCommented May 10, 2022 at 14:47
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A fixed chatzos also raises the possibility of a different shaah zemanis in the morning as in the afternoon: sunrise (or dawn, acc. to the Magein Avraham) to average noon divided by 6 in the morning, but 1/6 of average noon to sunset (MA: dusk) in the afternoon.– Micha BergerCommented May 10, 2022 at 14:50
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@MichaBerger That possibility exists even with a proper solar noon so long as your morning start point isn't perfectly symmetrical to your evening end point (eg. dawn to three actual stars)– Double AA ♦Commented May 10, 2022 at 15:32
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@MichaBerger Those are 20th century "chumras". No one prior thought mincha or noon was different than any other time.– Double AA ♦Commented May 10, 2022 at 15:36
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1 Answer
Rav Yisroel Dovid Harfenes has a summary in his Yisrael vehaZemanim, vol. I pp. 55-59.
He brings down opinions that seem to hold that day always starts at 6am, and you count sixty-minute hours from there. However, as he points out, it's possible that those opinions stem from before clocks became standardized, when 12noon in every place was simply defined as the actual time of noon (when the sun is overhead).
Alternatively, there are those who say you count backwards from whenever noon is on a particular day. This seems to me like it could be done in a consistent manner.
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1They have to be local time, not stardard time. Standard time only became a thing in the 19th century, and the sjitah dates back to rishonim. E.g. in NY, a day runs from 6:09 to 6:09. Commented May 10, 2022 at 14:44