More Discussions for this daf
1. Taurus and Scorpio 2. North-South Determination 3. The Vilna Gaon's calculation
4. Rav Ada 5. Tekufos - Halachic vs Secular 6. Vernal Equinox
7. Gra on Tosfos D"H Abaya 8. Rashi's Siman 9. Rashi's inexact calculation
10. Solistice? 11. Rashi, D"H v'Ein Tekufos Tamuz.. 12. Rashi DH Mipnei She'Mafsid - Where does the 28 come from?
13. Bach on amud beis 14. Constellations 15. Akrav is in the South
DAF DISCUSSIONS - ERUVIN 56

First, from Yeshiva Shaare Chaim, asked:

Thanking you very much - in advance --for helping me with these questions

Rashi DH Mipnei she'Mafsid writes that the Techum on each side of the city is 1428

Amos from the city. Where did he get the extra 28 Amos from?

First, from Yeshiva Shaare Chaim

The Kollel replies:

Take a look at the attached picture (also viewable at www.dafyomi.co.il/eruvin/discuss/eruv-056.qa6.jpg). Rashi's intention is to calculate the length of the Techum of a city (in a direction perpendicular to the city's border), that is, the length of lines AB & AD, had we measured the Techum of a city using a diagonal line extending 2000 Amos from the corner of the city (that is, AC = 2000 Amos).

Since AC is the diagonal of square ABCD, and Chazal calculate the length of the diagonal of a square as 7/5 times the length of its sides, then AC = 7/5 of AB, or AB = 5/7 of AC.

5/7 of 2000 Amos (that is, AC), is 1428.57 Amos. That is why Rashi mentions the figure of 1428. (As for why Rashi does not mention the remainder, that is discussed by the Maharshal and Maharsha.)

Best wishes,

M. KORNFELD

Kollel Iyun Hadaf