Why Does the Zodiac Start with Aries if the Sun is on Pisces on March 21?

People are used to express their birthdate associated with a sign of the zodiac. The zodiac represents the 12 constellations of the ecliptic, along which the Sun travels throughout the year.

When the zodiac was brought up into common knowledge, it had to have a start date. The chosen date was March 21st or the day of the spring equinox in the northern hemisphere. It is a very precise point of start. At the time this occurred, on the day of the spring equinox, the Sun was located in the region of the constellation Aries. The Greek astronomer Hipparchus 190-120 BCE) introduced the term "First Point of Aries".

Earth has a motion called precession, the "wobbling" of the axis at a rate of 26000 years per complete turn. This means that the constellations of the celestial sphere behind which the Sun travels will change over time. It is said that in the year 68 BCE, on the day of the spring equinox, the Sun crossed into the region of the constellation of Pisces. Some time in the near future, the Sun will cross into the region of the constellation of Aquarius.

So the reason the Zodiac starts with Aries is because is fixed on a celestial sphere more than 2000 years old, which did not account for the precessional movement of the Earth.

Take a look at this article for further information.

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