How do they know what the dates of the zodiac are?

Answer by  fig (71)

The dates of the zodiac are determined by the position of the sun along the ecliptic at any given time during the year. The cusps of Aries, Cancer, Libra, and Capricorn correlate exactly with the vernal equinox, summer solstice, autumn equinox, and winter solstice respectively, and then the remaining cusps are found by trisecting the four arcs between these points.

Answer by  Jonah14 (640)

Since ancient times (Greek Antiquity, to be precise) people have noticed trends or shapes of stars and also personality variations according to months or year times.