This has been bugging me for a while. Does precession of the Earth (~26,000 year period) cause the seasons to change over this course of time? This one I'm pretty sure happens, but the follow-up question is then does the current leap-year rules account for this change, if it exists?

Thanks

Precession makes our seasons occur at a slightly earlier position in Earth's orbit each year; but our calendar, with leap-year adjustment, is tied to the seasons. So we see no seasonal drift year on year. What we see is that the Sun's position during each season is shifted slightly against the background stars compared to its position in previous years.

Grant Hutchison

Oh.
But there is one effect on the seasons. At present, Earth is closest to the Sun in January, so we get through that part of our orbit fastest. This means northern winter (southern summer) is the shortest season; northern summer (southern winter) is the longest. Southern seasons are made more extreme, because we're closest to the Sun during their summer, and furthest during their winter.
But because of precession, that won't always be the case. The Earth twists on its axis once every 26000 years, as you say. Each calendar year starts a little earlier in Earth's orbit than the one before. So it will take 26000 years before the Sun and stars get back to the same apparent alignment at the start of spring. Meanwhile, the closest point to the Sun in Earth's orbit drifts from year to year, going once around the Sun every 110000 years. After the Earth has gone around the Sun once, it has to go a little farther before it catches up with this drift. The net result is that the seasons drift relative to the closest approach with a period of 21000 years.
So in ~10500 years, the seasonal extremes will be reversed from what they are now.

Grant Hutchison

Thanks

The 110000 year period, what is this? It has something to do with Einstein right?

Some tiny part of it is from GR, and the same effect that was used to explain Mercury's precession of perihelion, but most of it is from the gravitational interaction with Jupiter.

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Yes. Our calendar is set up so that the changes asked about in the OP do not occur. The length of time the earth takes to orbit the sun is about twenty minutes longer than what our calendar is based upon.

Since there are about 365.25x24x60 minutes in a year, it takes about 365.25x24x60/20 years for a cycle--which computes out to around 26,000 years.

all rotating bodies display precession. tops spinning on the floor show it dramatically.

the 26000 year precession that was referred to earlier causes the sun's position to be a little different at solstice each year.

the earth's axis points in a different direction this means the sun won't be exactly in the same spot.




Therefore it will affect our seasons, but summer in the North will always be summer in the north (if we calculate only for the earth's precession) The summers may not last quite as long.




as far as the 110000 year cycle, that has not to do with the earth's precession but rather the orbit. that will affect the time of the seasons. the orbit rotates, and that's the other change discussed here.

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The time from equinox to equinox is called a "tropical year". This year is different in length from, say, a sidereal year (the time it takes to go around the Sun using the distant stars as a reference). The tropical year is used to prevent the seasons from slipping through the calendar year, so spring will never be in, say, July.

A great place to read all about this stuff is the US Naval Observatory website. Check the FAQ, for example.

P.S. I made this mistake in the first version of my book, saying that precession will change the dates of the seasons. I was wrong!

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wow...

Ask the "average" individual when they think the earth is closet to the sun, Northern Hemisphere's summer or winter.

My experience has most answering summer. I then try to explain that it's the duration of sunshine due to tilt that causes summer to be warmer, not proximity to earth and that we are actually further away in summer.

But now that this concept, coupled with precession has been presented, it makes me wonder...
How long ago was the situation reversed, where nothern summers were longer?
A quick estimation, looking at my little globe, I'd guess at least 60% maybe as high as 80% of "habitable" land is in the northern hemisphere. (any accurate numbers out there?)
Since civilization arose predominantly in the northern hemisphere, could this have been a factor in the speed (or delay) of our progress?

I wonder if anthropologists have ever considered this.

All,

Thank you for this thread. I just learned something I was wrong about, but I see the BA his own self made the same error, so I don't feel so bad. I thought the seasons would change relative to the calender due to precession myself and I remember telling people this. I'm going to have to "revise and extend".

Although I do have a BS in physics (that I haven't used a whit in real life) and have done my share of "high powered cipherin'", I'm as dumb as rock when it comes to astronomical time keeping -- just get the MEGLO effect trying to visualize how the sky changes as the earth does all it's various cycles. But the thing I do realize is how important it is that our earth time be tied to astronomical "markers" so things "just work out" as they should be (as we want them to be).

For instance, I didn't realize the earth's actual rotation period was shorter than "one day" until it I understood that since the earth moves a little bit in its orbit each day, the earth has to turn around slightly more than one revolution so the sun will be "back to the same position" (and this "same position" is sort of fluid with the seasons as well, isn't it?)

So I take it the year is actually defined in similiar way? That is rather than one orbitial period, it is the time it takes for "something about the sun" to get back to its "original position", so the effect of precession over that one year period is cancelled out??

Is there some simple way to state this defintion of the year?

-Richard

Tentative: "A year is the time the earth takes to make one elliptical orbit around the sun".

As the ellips itself turns (precession), this means that when you have finished one ellips, you will not have exactly the same position with regards to the stars.

I'm trying to visualize it like this:
you have the Sun, and and ellips around it. Let's say that now, we have the long axis of the ellips from left to right, and the short one straight up, and the sun in the left focal point of the ellips. So the autumn and winter are on the left, and spring and summer on the right (I'm turning counterclockwise, and thinking about Northern hemisphere).
After some 5500 years (1/4th of 22000), the long axis of the ellips will be pointing straight up, and the short one will be going from left to right. At this time, spring-summer and autumn-winter are exactly as long (half a year).
Then, they'll start moving apart again, with autum-winter being longer and spring-summer being shorter.
The precession is the movement of the ellips around the sun (seen from a fixed point above the pole of the sun), and the year is the time you need to go from e.g. the tunrning point of the long axis to the same point of the ellips again, even though the ellips has shifted slightly wrt to the stars.

Is this more or less correct (not very concise though)?

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No, what we call a year is the mean time from one vernal equinox to the next.Less correct

There are two types of precession here. The type you are describing is the rotation of the semimajor axis of the orbit's ellipse, which grant hutchison said had a period of 110,000 years. The precession that is mentioned in the OP is the gradual movement of the earth's rotation axis, so that the earth "tilts" in a different direction over a period of 26,000 years or so--which affects the time at which the vernal equinox takes place.

Ah, now I get it. Thanks!

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Okay, we define one (tropical) "year" as the time between vernal equinoxes (and could we also define it as the time between the other equinox or the solsitices if we liked?). Anyway, would someone please tell just what the heck the equinoxes are.

My trouble is visualizing *what they look like on earth*. It is something about the sun's apparent motion, correct. I think solstice = sol-stice = "sun stop", right? And the equinoxes are the midpoints between these "stopping points". Just what is this apparent motion of the sun that we observe?


-Richard

The solstices are the longest/shortest day, equinoxes are when day and night are equal length.
They don't fall exactly 1/4 year from each other because of the elliptical orbit of the earth around the sun.

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Equinox means equal night. The equinoxes are the dates when days and nights are equal in length (Actually, they aren't quite the same length. The days are a few minutes longer due to atmospheric refraction, but they would be if the Earth did not have an atmosphere.). On the equinoxes, the terminator (The line between day and night) is perpendicular to the equator and passes directly over the poles. At the poles, the Sun circles the horizon without rising or setting. On the equator, the Sun passes straight over head at noon.

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Because the earth is tilted relative to the plane of the ecliptic (the path of the sun through the map of the stars), sometimes its north axis tilts towards the sun, sometimes away. When it is tilted towards the sun, the sun can be straight overhead at noon on the summer solstice, at the Tropic of Cancer--but never farther north of that. Before the solstice, the sun climbs higher in the day sky. Then, it stops, and begins to get lower. Whenever it progresses so that it is directly over the equator, we call that the equinox.

Another approach: the plane of the ecliptic and the plane of the equator intersect at two points of the ecliptic. At an equinox, the sun is at one of those two points.

All,

Thanks for the explantions -- I'm think I've got the picture in my head. Again, I can picture all this stuff from the POV of a mind's eye coordinate system well outside where I can see the earth orbiting and spinning and see how the tilt of the axis works. It's just, in the mind's eye, trying to transform that to the POV of an observer on the earth, rotating with it that gets me. :-)

Speaking of the view at the poles, I've always wanted to see the "sun never setting" during the peak of the summer. How far down does this effect go. I think I've read about it in stories about Alaska. There was something about a weird effect this has on a certain fraction of the population -- it has a medical name, but I can't remember. With no night, the internal biologic