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Does this mean that the seasons also drift with regards to the calendar year over a period of 26000 years?
Good question. I don't believe so, since precession affects the direction of Earth's axis, but not the angle of its tilt relative to the ecliptic. The 23.5º axis tilt is constant, so the seasons themselves continue on just the same
But direction in which earth inclined changing (like orbital node argument - 90 degree). So assuming constant orbital period and ideal calendar, 1 january of 999999 year can be not a winter in northern hemisphere
A year is defined not as once around the sun, but vernal equinox to vernal equinox. So every 26,000 years, the earth moves 26,001 times around the sun. The seasons don't drift by definition.
There's an obliquity wobble between 21.5 to 24.5 degrees over 41,000 year period... but while that might make the seasons somewhat more or less severe, it doesn't change their timing, as you said.
The axis also does change over time also. The Earth has a wobble that varies from 22 and 24.5 degrees over a period of 40,000 years.
I believe there are a total of seven different motions of the Earth- can't remember all the names.
The points where the celestial equator crosses the ecliptic precess too, no? And that determines the time of equinox. So in 13000 years, the Sun will be on the opposite side of of the zodiac at the start of Spring. Whether that translates to a different part of the calendar...
... would depend on what kinds of adjustments future generations decide to make to the calendar.
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