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During the year the Sun moves along the ecliptic passing all the signs of the zodiac belt, in my understanding. This means that it takes ~1 month to go from one zodiac sign to the next.

The equinoxes are approximately on 20th of March and 23rd of September. This means the vernal equinox Sun is in Pisces (or Aries) and the autumnal equinox Sun is in Libra (or Virgo).

But also I found that there is the observation of precession of the equinoxes, where the equinox Sun passes through all zodiac signs in a calendar round that lasts ~25,000 years.

How is this possible?

If my understanding about the Sun's yearly round across the zodiac is correct, the equinox Sun should always be at the same zodiac signs. How can this precession happen?

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As you said, the Sun takes a year to cycle the Zodiac, and it takes a year between two spring or autumn equinoxes. Both statements are true, but "year" in them means two different things. It takes a sidereal year to cycle the Zodiac, but it takes a tropic year to cycle between equinoxes. Since the tropic year is about 20 minutes shorter, the equinoctial points move across the Zodiac cycling it in about 25,000 years.

The calendar year we use in the Gregorian calendar is designed to approximate the tropic year to have it synchronised with seasons.

The reason for all it is that the Earth's axis is not fixed but its orientation slowly rotates.

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  • $\begingroup$ @Pere: Could it be possible to expand/elaborate a bit more in your answer. To be honest I don't understand it. E.g. this Since the tropic year is about 20 minutes shorter, the equinoctial points move across the Zodiac cycling it in about 25,000 years. I don't understand it at all $\endgroup$ – Jim Apr 29 at 19:50
  • $\begingroup$ Your question stated that since the time it takes the Sun to move across the Zodiac is the same time it takes the Sun to return to the equinoctial point (1 year), the equinoctial point can't be moving across the Zodiac. My answer just tells that it's not the same time, and therefore there is no contradiction with the fact that the equinoctial point is moving across the Zodiac. $\endgroup$ – Pere Apr 29 at 21:26
  • $\begingroup$ Yes but I would like to understand the topic better. E.g. what do you mean by 20 minutes shorter makes the zodiac cycling it to be 25000 years? $\endgroup$ – Jim Apr 30 at 16:24
  • $\begingroup$ Not an expert but I just calculated that a year is $60 \times 24 \times 365.25 = 525960$ minutes. Divide that by $20$ and you get $26298$. So, in that many sidereal years, there will be an extra tropic year. $\endgroup$ – badjohn May 1 at 22:12
  • $\begingroup$ Divide 525960 by "about 20" and you will get "about 25.000". You can get more precise figures from Wikipedia, although I think the question and the answer focus on why both years are different and exact numbers have never been intended to be there. $\endgroup$ – Pere May 1 at 22:28
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The equinoxes are where the ecliptic crosses the equator. As the Earth's axis and equatorial plane pivot under precession, the equinox points migrate westward along the ecliptic, 360° in 25800 years or 1.4° per century. This animation shows the vernal equinox drifting 28° in 20 centuries of precession:

Vernal equinox movement 100-2100 CE Images generated by Stellarium

The Sun moves eastward (right to left) 360° per sidereal year relative to the stars along the ecliptic. Meanwhile the vernal point moves 0.014° per year the other way, so the Sun moves that much less between vernal equinoxes - a tropical year. This makes a tropical year 20.4 minutes (1/25800 year) shorter than a sidereal year. The modern calendar approximates the tropical year to keep the average equinox and solstice dates from drifting.

While some traditions define a sidereal zodiac anchored to the stars, the Western zodiac is tropical, linked to the equinoxes and solstices and migrating along with them. The sign of Aries is defined as the first 30° along the ecliptic east of the vernal equinox, even as that point moves through the constellations Pisces and Aquarius. Similarly, the sign of Libra remains the first 30° east of the autumnal equinox despite its present alignment with the constellation Virgo. We can blame the resulting confusion on Ptolemy, who understood precession yet named the 12 equal signs after the nearest constellations in his own lifetime anyway. For more about the distinction between constellations and signs, see this answer.

Given a tropical year and a tropical zodiac, the Sun is always in the same sign at the same time of year. However, as the equinoxes and solstices drift westward over the centuries, so does the Sun's position among the constellations at a given time of year.

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Terminology is critical. As mentioned in other answers, precession causes the equinoxes (the points where the ecliptic intersects the celestial equator) to move from one constellation to another. For example, the March equinox was in the constellation Aries in the time of the ancient Greeks, is in the constellation Pisces today, and will be in the constellation Aquarius around the year 2600.

A zodiac sign, also called an astrological sign in Western astronomy, is defined as starting at the March equinox with Aries, then Taurus, and so on until the 12th sign, Pisces. Each sign occupies 30 degrees, even though the actual constellations are a variety of different angular widths.

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"We say equinox is at same place" - This is an approximation. Due to precession,equinox keeps on shifting every year by a fraction of 360 Deg/26000 year=0.0138 Deg/year. In fact based on certain references in Indian scripts the Year of particular incidence say- "Epic Mahabharat" is estimated 5000 years back. If interested I can provide more details

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