Is the constellation calculated by birth date or birthday every year?

According to the date of birth in the solar calendar. The dates of the twelve constellations are calculated according to the orbits of celestial constellations. The sun is always in motion. When the sun moves, it will pass through the position of the constellation, and the person born in the area where the sun passes is what constellation.

The orbit of the sun is called the ecliptic, and it is divided into 12 zones, each of which is 30 degrees. These 12 regions are what we call solar constellations. The twelve constellations begin in spring, that is, when the sun runs to the vernal equinox around March 20 every year, they will be sorted in order: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Extended information Mesopotamian civilization astrologers in order to facilitate the study and observation of many stars in the sky, people divide the starry sky into several regions, and each region is a constellation. It's hard to say clearly when humans began to have the concept of constellation. This astronomical knowledge was known long before there was a historical record. The name of the constellation probably comes from the sailors who sailed early.

The first Babylonians made a horoscope of the four seasons according to the horoscope. Because ancient humans lived according to the change of the sky, they found that the movement of the sun, the moon and the stars in the universe affected the rise and fall of everything.

Reference source: Baidu Encyclopedia-Constellation