How the seasons are calculated
Each season begins at the moment the Sun’s apparent longitude reaches the start of a sign: 0° Aries at the March equinox, 0° Cancer at the June solstice, 0° Libra at the September equinox and 0° Capricorn at the December solstice, with the other signs every 30° in between. The times come from the same planetary engine as our birth chart calculator, which is checked against the Swiss Ephemeris; for the Sun the difference is a few arcseconds, about two minutes of time at most.
Questions
What is a zodiac season?
A zodiac season is the stretch of about a month when the Sun moves through one sign of the zodiac. Each year the Sun passes through all twelve, so each sign has its season, and people born during it have that sign as their Sun sign.
Why do zodiac season dates change every year?
The calendar year and the Sun’s cycle are not exactly the same length, and leap years shift the calendar, so the Sun enters each sign at a slightly different time each year, sometimes a day earlier or later.