Rif Shabbat 14b

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14b

{digression to Gittin 6b}
One who creates an atmosphere of excessive fear in his house in the end comes to three sins: illicit sexual relations, bloodshed, and desecration of the Shabbat.

MISHNA:
WHEN IT IS DOUBTFUL WHETHER IT IS NIGHT OR NOT, THAT WHICH IS CERTAINLY [UNTITHED] MAY NOT BE TITHED, UTENSILS MAY NOT BE IMMERSED, AND THE LIGHTS MAY NOT BE KINDLED.

BUT DEM'AI {that which there is a doubt whether it was tithed} MAY BE TITHED, AN 'ERUV MAY BE PREPARED, AND HOT FOOD MAY BE STORED AWAY {insulated so at to retain their heat}.

Gemara:
We establish the Mishna as referring to eruvei chatzerot {created to allow carrying into the otherwise Rabinically prohibited joint courtyard} but not eruvei techumin {created to establish the Shabbat "dwelling" from which one may travel 2000 cubits in each direction}.

Rava said: If two men said to one person, 'Go forth and place an eruv {techumin} for us', and he placed an eruv for one while it is yet day, and for the other he made the eruv at twilight, and the eruv of him for whom he placed it by day was eaten at twilight, and the eruv of him for whom he placed it at twilight was eaten after nightfall, both acquire [their] eruv.
How will you have it?
If twilight is day, the second should acquire
{because the eruv lasted from day on Friday - that is, twilight, into the night of Shabbat, since it was eaten after nightfall},
but not the first
{since it lasted from the day of Friday into the day of Friday - that is, twilight, and was consumed before Shabbat entered};

while if twilight is night, the first should acquire
{because it lasted from Friday day into twilight which is the night beginning Shabbat}
, but not the second
{because it lasted from the night - twilight - which was Shabbat, into further of the night of Shabbat, but it had not been established while it was yet day}
?

It {twilight} has the status of doubt as regards a Rabbinic law, and a doubt within Rabbinic law is judged leniently.

Rava said: Why was it said that one must not put away [food] in a substance which adds heat, even by day {before Shabbat enters}? As a decree lest he first {heat} it up {before insulating it, on Shabbat - our gemara has a different girsa - see there}.

By way of explanation, this that we learnt in the Mishna: "WHEREIN MAY WE STORE [FOOD], AND WHEREIN MAY WE NOT STORE [IT]? WE MAY NOT STORE [IT] IN PEAT, FOLIAGE, SALT, LIME, OR SAND" - all of these substances are ones which add heat, and we do not store {insulate} in them even while it is still day {and not yet Shabbat} as a decree lest he heat it up {on Shabbat}.

Abaye said to him: if so, during twilight let us also decree?
He said to him: During twilight, the default pots are {already} boiling.

Rabba said: For what reason did they say that we may not insulate with something that does NOT add heat once it gets dark?

By way of explanation, items such as fruits, and clothing, and the wings of a dove. And they did not permit to cover with them unless this was done while it was still day, or else when it is doubt whether it is dark {nightfall} or not, as we learnt {tnan} "if it is a case of doubt whether it is dark or not, we do not tithe, etc., but we do insulate." From which we learn that if it is dark for certain, we do not insulate the heated items, as a decree lest he insulate it with ashes and come to stoke the coals.

{Shabbat 34b}
The Sages learnt {in a brayta}: As to twilight [period] it is doubtful whether it is partly day and partly night, or the whole of it [belongs to the] day, or the whole of it night: [therefore] it is cast upon the stringencies of both days. And what is twilight? From sunset as long as the face of the east has a reddish glow: when the lower [horizon] is pale but not the upper, it is twilight;