Precession of the equinoxes and why the zodiacs drift
Precession of the equinoxes is the slow backward drift of the equinox points along the ecliptic, and it is the reason the tropical and sidereal zodiacs differ.
What it is
Earth's rotational axis wobbles like a spinning top. The point where the Sun crosses the celestial equator in spring moves westward along the ecliptic at about 50.3 arcseconds a year, or about one degree in 72 years.
How it is worked out or read
A full cycle takes roughly 25,800 years. Tools in this site apply a linear yearly rate from a base value at J2000 (January 1, 2000 at noon). The rate is a standard approximation and is accurate over the few centuries that matter for a chart.
Where it appears on this site
The ayanamsa on the method page is built from this drift. See what the ayanamsa is.
A way to picture it
Imagine a spinning top that is slightly tilted. Its axis does not stay still; it traces a slow circle. Earth's axis does the same, completing one circle in about 25,800 years. As it moves, the point where the equator crosses the ecliptic, which is the equinox point, slides backward along the zodiac. Each year it moves about 50 arcseconds, which adds up to roughly one degree every 72 years. Nothing dramatic happens to the stars themselves. Only the reference point we use to measure the zodiac is moving, which is why the tropical and sidereal zodiacs drift apart over time.
What precession does to a chart
In a birth chart the effect is the ayanamsa. A person born in 1900 and a person born in 2020 are measured against references that differ by roughly two degrees, so a planet that falls at the edge of a sign can land in different signs depending on the year. Precession does not change the planet's motion, its house or its relationship to other planets. It changes only the offset. For that reason, the site computes the ayanamsa for the exact date of birth, not for the present day, and states the formula on the method page.
Why the number is an approximation
The yearly rate of about 50 arcseconds is a good approximation over the centuries that matter for a chart, but it is not exact. Over very long periods the rate itself changes slightly, and the planets also cause small periodic variations in the equator's orientation. These effects are tiny for the purposes of a birth chart, which is why the linear rate is used. They would matter for historical work that spans millennia, which is one reason the site presents its ayanamsa as a convention for the chart and not as a claim about ancient astronomy.
Precession in the calendar
Precession affects the calendar as well as the zodiac. Over long periods, the seasons and the festival dates tied to the solar year shift against the sidereal positions, which is one reason why the sankranti dates and the festival rules are discussed with care in different calendars.