Eruvin 56 - East and west

When one draws the rectangular techum boundary around a city, he aligns it with the directions of the world, north, south, east, and west. For this, he can use the stars: Taurus will point out the north, and Scorpio - the south. If he does not know how to find these constellations, he can use the sun. On the longest day of the year the sun rises and sets in two points which together indicate the north, and on the shortest day of the year he can determine the south. The Talmud continues discussing the constellations and their influences on the world.

In measuring the rectangular area of the techum (or a square for a round city), one makes sure to include the corners, thus gaining more walkable distance. When there is a possibility of drawing the techum in two different ways, he should prefer the one that gives more room. In general, the law of the techum follows the Sage with the most lenient opinion. That is because the two thousand steps limitation on walking on Shabbat and Festival is only a safeguard around the fifteen thousand limit prescribed by the Torah.

Art: Sir James Thornhill - Constellation of Taurus,  from Atlas Coelestis

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