With regard to the measurements of a city’s boundaries, the Sages taught the following baraita: If, in order to measure the Shabbat limit, one comes to square a city, i.e., to extend the city’s boundaries to include all of its protrusions within an imaginary square, he squares it so that the sides of the square align with the four directions of the world. He sets the northern side of the square to align with the north of the world, and its southern side to align with the south of the world. And your sign by which you can recognize the directions of the world is as follows: The constellation of Ursa Major is in the north and Scorpio is in the south. The directions of the city are determined by these constellations.
The Great Bear (ursa major) – referred to by the Gemara as agala, “wagon” – will always be seen in the north. In the south, the Gemara appears to referring to the constellation Scorpius. While the northern constellations are constant and will always be visible in the Northern Hemisphere, Scorpius can only be seen during the summer months, and it does not always appear due south. It is likely that for this reason the Gemara gives other suggestions for establishing the directions.
Rabbi Yose suggests that the directions can be ascertained based on the rising and setting of the sun. His suggestion is based on the fact that the earth spins at an angle as it rotates around the sun. Therefore, the seasons are not equal to one another in the length of days and nights, or where the sun will rise and set. The dates mentioned by the Gemara are as follows:
Date Sunrise Sunset
September 23 Due East Due West
December 22 (shortest day) 27 55′ SouthEast 27 55′ SouthWest
March 21 Due East Due West
June 22 (longest day) 27 55′ NorthEast 27 55’NorthWest
Rav Yose’s suggestion is to follow the sun on the equinox (September 23 and March 21) to learn the directions of east and west and on the solstice (December 22 and June 22) to learn the directions of north and south.
The Jerusalem Talmud also suggests making use of the rising and setting sun, but suggests a much simpler approach. If you track sunrise from the shortest day of the year to the longest day of the year, the place between those two angles is East. Similarly the place between the setting of the sun in summer and winter is West.
Shmuel presents his understanding of the seasons, based on a perfect 365.25 solar year. A more precise approach to this matter is that of Rav Adda, who believed that the solar year is slightly shorter than that.