Our Rabbis taught: If a town is to be squared the sides of the square must be made to correspond to the four directions of the world.
That is, a circular town has a Shabbat limit drawn in a square. A square contains more area just by virtue of adding the corners. Later discussion shows that the square is drawn in such a way as to maximize the area given.
But the square is also to be aligned with the sides facing in the 4 cardinal directions:
Its northern side, [for instance,] must correspond to the North, and its southern side to the South
But how is this to be determined without benefit of a compass?
and your guiding marks are the Great Bear (aggalah – lit. “wagon”) in the North and the Scorpion in the South.
That is, using the constellations to determine North and South.
But there is a daytime option as well:
R. Jose said: If one does not know how to square a town so as to make it correspond with the directions of the world, one may square it in accordance with the circuit of the sun. How? — The direction in which on a long clay the sun rises and sets is the northern direction. The direction in which on a short day the sun rises and sets is the southern direction.
And if that doesn’t work, there’s always scripture:
At the vernal and autumnal equinoxes the sun rises in the middle point of the East and sets in the middle point of the West, as it is said in Scripture: It goeth goeth the south, and turneth about the north; (Eccl. 1:6) ‘It goeth along the south’ during the day ‘and turneth about the north’ during the night. The wind turneth, turneth about moveth (ibid.) refers to the eastern horizon and the western horizon along which the sun sometimes moves and sometimes turns about.
So goes the power of observational astronomy. But there is an astrological component as well. For the hours of the day were each supposed to be under the influence of a particular heavenly body (Mercury, the Moon, Saturn, Jupiter, Mars, the Sun and Venus – in that order). Their influence determined the character of the hour. For example:
Samuel further stated: The vernal equinox never begins under Jupiter but it breaks the trees, nor does the winter solstice begin under Jupiter but it dries up the seed. This, however, is the case only when the new moon occurred in the moon-hour or in the Jupiter-hour.