# Solstice dates and the past. **URL:** \ **Category:** Factual Questions\ **Created:** [March 26, 2003, 4:05pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301 "2003-03-26T16:05:40Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Edward\_The\_Head](https://avatars.discourse-cdn.com/v4/letter/e/ce73a5/32.png) [@Edward\_The\_Head](https://boards.straightdope.com/u/Edward_The_Head)\ **Post date:** [March 26, 2003, 4:05pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301/1 "2003-03-26T16:05:40Z")
I have a question that I never thought about until a few weeks ago after reading **Bad Astronomy** , even though it may have been answered in there I don’t remember it being so. Anyway I am trying to understand this from an Archaeology stand point so maybe that will help make the question clearer. I understand that over time, as in thousands of years, the solstice dates change because the Earth slowly wobbles. After 26,000 years or so what is now summer in the North will become winter, at least as I understand it. Now comes the real question I’m asking. Thousands of years ago people built places like Stonehenge, Newgrange and other stone circles. Some of these, Stonehenge and Newgrange, show in some way either the summer or the winter solstices. So while the dates may change on the solstice does the angle that the sun rise and set on those dates change? Or am I missing something about how the solstice dates work? I always hear the archaeologists say that every December 21 for the last 5000 years the sun has shown down the passage in Newgrange, or that for the last few thousand years at Stonehenge on June 21, my birthday, the summer sun rises over one stone. In all the reading and shows I’ve seen I’ve never heard them talk about changing dates so now I’m confused. or maybe it’s the aliens telling is when they’re coming back! ---
**Author:** ![brianmelendez](https://avatars.discourse-cdn.com/v4/letter/b/65b543/32.png) [@brianmelendez](https://boards.straightdope.com/u/brianmelendez)\ **Post date:** [March 26, 2003, 4:40pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301/2 "2003-03-26T16:40:11Z")
I don’t know about the 26,000-year wobble. But according to _The World Almanac and Book of Facts 2003_, p. 673, > [@](#): > > The **tropical year** , upon which our calendar is based, is the interval between 2 consecutive returns of the Sun to the vernal equinox. The tropical year consisted of 365 days, 5 hours, 48 minutes, and 46 seconds in 1900. It has been decreasing at the rate of 0.530 second per century. If that decrease is consistent over time, then the equinoxes will move only one day in 16.3 million years. ---
**Author:** ![elfje](https://avatars.discourse-cdn.com/v4/letter/e/e19b73/32.png) [@elfje](https://boards.straightdope.com/u/elfje)\ **Post date:** [March 26, 2003, 4:42pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301/3 "2003-03-26T16:42:23Z")
> **[The Millennium Matters...](https://www.mm2000.nu)** > > You are about to enter The Millennium Matters..., a repository of source material, links, books, articles, and information about the new millennium from multiple points-of-view. try this, I think this has the answer you’re looking for ---
**Author:** ![Edward\_The\_Head](https://avatars.discourse-cdn.com/v4/letter/e/ce73a5/32.png) [@Edward\_The\_Head](https://boards.straightdope.com/u/Edward_The_Head)\ **Post date:** [March 26, 2003, 6:12pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301/4 "2003-03-26T18:12:26Z")
> [@](#): > > \*Originally posted by elfje \* > \*\*[http://www.mm2000.nu/sphinxaa.html](http://www.mm2000.nu/sphinxaa.html) > > try this, I think this has the answer you’re looking for \*\* Yes I think it does from here: > [@](#): > > Stellar alignments (alignment between an object on earth – such as a pyramid – and one or more stars) are affected by precession, but solstice and equinox markers remain accurate because the angle of the earth’s axis does not change, only the orientation. Another way of saying this, is that if an object on Earth is aligned so that you see “X” during the vernal equinox, you will continue to see “X” during the vernal equinox regardless of precession, althought the actual date of the vernal equinox may change – which is generally how precession of the equinoxes is detected, by the drift in date of a solstice with respect to a stellar year. So it is as I thought the date may change but the alignment does not. I will still have to look more into this as it is intersting. I’m guessing that they don’t say anything about date changes to confuse people. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [March 26, 2003, 7:24pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301/5 "2003-03-26T19:24:51Z")
The tilt of the Earth’s axis _does_ vary, through a cyclical process called **nutation**. The tilt is currently about 23.44 degrees, but varies from a minimum of 22 degrees to a maximum of about 24.5 degrees over a cycle of 41,000 years. We are currently in a “downward” cycle, so the tilt is less now than it was in the time of Stonehenge, although I’ve never read that the difference has been enough to cause a noticeable mis-alignment of the monument. Our calendar is based on the tropical year, and is designed (to the extent practicable) to keep the vernal equinox on the same date, so as long as we maintain our present calendar it will never be “summer in winter”. The date of the winter solstice will bob around by a couple of days due to changes in the length of the seasons as perihelion cycles through the year. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [March 26, 2003, 7:31pm UTC](https://boards.straightdope.com/t/solstice-dates-and-the-past/164301/6 "2003-03-26T19:31:37Z")
The tilt of the Earth’s axis _does_ vary, through a cyclical process called **nutation**. The tilt is currently about 23.44 degrees, but varies from a minimum of 22 degrees to a maximum of about 24.5 degrees over a cycle of 41,000 years. We are currently in a “downward” cycle, so the tilt is less now than it was in the time of Stonehenge, although I’ve never read that the difference has been enough to cause a noticeable mis-alignment of the monument. Our calendar is based on the tropical year, and is designed (to the extent practicable) to keep the vernal equinox on the same date, so as long as we maintain our present calendar it will never be “summer in winter”. The date of the winter solstice will bob around by a couple of days due to changes in the length of the seasons as perihelion cycles through the year.