Do solstices and equinoxes shift over time?
Right now, I think the winter solstice is around December 21. Has it always been around this date constantly?
Since the earth's axis's 23.45° tilt will be on the exact opposite side in 13000 years (or was 13000 years ago) due to precession, it'd seem like the seasons would flip every 13000 years. So, +/- 13000 years from now, it would've turned summer around December 21 in the Northern Hemisphere, right?
Yes. I could type a long answer, but it's already been done: https://en.wikipedia.org/wiki/Precession_of_the_equinoxes
The 2003 and 2007 December solstice occurred on December 22. The 2044 and 2048 December solstice will occur on December 20. Source: http://www.timeanddate.com/calendar/seasons.html
@mickael-caruso you've got 3 answers now, how about ticking the one you think best answers the question?
@Marc except "yes" isn't the correct answer to the question of the seasons inverting every 13000 years. Perhaps you want to edit your comment?
@david-h the time and date site you point to shows the dancing around of the solstice date, but it doesn't show a seasonal drift because the calendar is regularly readjusting for it. http://en.wikipedia.org/wiki/Gregorian_calendar#Calendar_seasonal_error
The Gregorian Calendar was created so that annual astronomical events, specifically the vernal equinox (used to determine when Easter is), would on average keep their places in the calendar year over time. It is the best official approximation to the definition of the tropical year, which is defined as "the length of time that the Sun takes to return to the same position in the cycle of seasons". Because this calendar describes 97 leap years out of every 400 years, it defines the average year as exactly 365.2425 solar days, or exactly 365 days, 5 hours, 49 minutes, and 12 seconds.
However, the mean tropical year is in reality about 365 days, 5 hours, 48 minutes, and 45 seconds, or 27 seconds shorter.
Because the Gregorian Calendar is based on the tropical year, the calendar dates of the year will keep up with the solstices and equinoxes, and thus the seasons. If this calendar were exactly the length of the tropical year, then the calendar would keep the vernal (northward) equinox around March 20th for all time.
But because of the slight inaccuracy, it will take about 3,200 years (60 s/min * 60 min/hr * 24 hr/day / 27 s/year) for these 27 seconds to add up every year to be 1 full day, and that will result in the solstices and equinoxes marching backwards in the calendar by 1 day every 3,200 years or so, depending on the accuracy of the 27 seconds difference. This very slow shift is due to the slight inaccuracy in the Gregorian calendar in, on average, matching the tropical year, not because of the precession of the equinoxes.
3,200 years from now, if the Gregorian Calendar is still used, the date of the vernal (northward) equinox will be on average one day earlier in March. The precession of the equinoxes will still occur, so the Earth's axis tilt will be significantly different from today. The Earth will be at a noticeably different position with respect to the Sun on the vernal (northward) equinox from where it is today, in 2014, on the vernal (northward) equinox, but it will still be in March.
This inaccuracy may very slowly increase over time, because according to the same Wikipedia page for the tropical year, the tropical year is very slowly getting shorter, and the mean solar day is even more slowly getting longer. But for 10,000 years to come, the Gregorian Calendar will keep the vernal (northward) equinox in March, even if it slowly shifts earlier in the month.
This is in contrast to the scenario that you imply, where the calendar date would correspond to the relative position of the Earth in its orbit around the Sun. That describes the sidereal year, the time taken for the Sun to reach the same spot in the sky relative to the stars, which is 365 days, 6 hours, 9 minutes, and 10 seconds. A sidereal calendar would explain why you might think that precession would cause the dates of equinoxes and solstices to change in the calendar year. That would result in a shift in the calendar of one full month in 1/12th the cycle length of the precession of the equinoxes, or about 1 full month about every 2,000 years.
Re: your last paragraph... Where does he talk about a correspondence to the position of the earth around the sun?
@Jeremy If precession changed the calendar dates of solstices and equinoxes, then that implies that the calendar is based on the Earth's position relative to the Sun (a sidereal calendar) instead of a tropical calendar. It was implied, but not talked about directly, but that's root the question: on what is the calendar based?
Ok, I see what you're saying... A slight rework of that paragraph could make it clear that there is an implied presumption in the question that the seasons relate to orbital position that is incorrect. Do you want to give it a crack, or shall I?