For the source text click/tap here: Shekalim 20
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If an animal that is fit for the altar was found straying, from Jerusalem and as far as Migdal Eder, and similarly if it was found within that distance from Jerusalem in any other direction, it is presumed that the animal came from Jerusalem.
We first learned of this place from Gen 39: 19-21
כ וַיַּצֵּב יַעֲקֹב מַצֵּבָה, עַל-קְבֻרָתָהּ--הִוא מַצֶּבֶת קְבֻרַת-רָחֵל, עַד-הַיּוֹם.
20 And Jacob set up a pillar upon her grave; the same is the pillar of Rachel's grave unto this day.
כא וַיִּסַּע, יִשְׂרָאֵל; וַיֵּט אָהֳלֹה, מֵהָלְאָה לְמִגְדַּל-עֵדֶר.
21 And Israel journeyed, and spread his tent beyond Migdal-eder.
Targun hints at the location as being the site of the revelation of the Messiah:
which leads us to the connection between this site and the birth of the Christ, as described by Alfred Edersheim a 19th Century Jewish Christian who ended up in England.