REVACH L'DAF
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SUMMARY
1. People or animals that live in an area where they must constantly go uphill and downhill age faster.
2. The Gemara qualifies the known fact that the sun rises in the east and sets in the west.
3. The solar year always starts at either the beginning of the day or night, or the halfway point of the day or night.
4. The border of a city is measured by making a square border.
5. The Techum Shabbos of a city is done measured by making a square border.
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A BIT MORE
1. Rav Huna mentioned the names of some cities that indeed had this effect on him.
2. The Gemara says that the sun never rose [due] northeast and set [due] northwest; it never rose [due] southeast and set [due] southwest.
3. This is because the solar year is 365 and 1/4 days long. Since one quarter of a day is six hours, the start of the solar year (or Tekufas Nisan) occurs either at the beginning of a day (0:00) or night (12:00), or the midpoint of the day (6:00) or the midpoint of the night (18:00).
4. For example, a city that is in the shape of a circle has an imaginary square placed around it. This is considered the border of that city for Shabbos. (For a person who resides in that city, the entire city is considered to be his four Amos.)
5. Once a city is measured into a square (see #4), a second square is measured to make the Techum Shabbos for the city (that is, the limit until which the residents of the city may walk on Shabbos). The square is made by placing its four corners at a distance of 2,800 Amos away from the corresponding corner of the city. (That is, the top right corner of the Techum is 2,800 Amos away from the top right corner of the city's border, because the diagonal is longer. Consequently, a straight line from the city limit to the Techum is 2,000 Amos long.)
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