Public Service Announcement

[kengr]

Due to 2000 *being* a leap year, the equinoxes and soltices tend to be about a day later than they were in the 20th century.

This year's Autumnal Equinox occurs on Sep 23rd and 4:03 am PDT (3 minutes after noon GMT)

Edited to correct leap year status of 2000

Comments

[aurictech.livejournal.com]

Um, 2000 was a leap year, as 2000 is evenly divisible by 400.

[kengr]

Arrgghhhh!

I know better, too.

Astronomical events

[morgan-gw.livejournal.com]

have been getting later all along, not just because of yet another leap year. I remember them being closer to the 21st when I was younger, but they're almost always the 22nd/23rd nowadays.

But if you need to check on one, the USNO site has times for solstices and equinoxes from 1992-2020. Good resource to bookmark :-).

Re: Astronomical events

[morgan-gw.livejournal.com]

Oh, and it's 0403 GMT, which is 9:03pm tonight PDT :-).

Re: Astronomical events

[kengr]

Ah. The site I use http://www.archaeoastronomy.com/ gives all 8 points for the year. I hadn't noticed that they'd given up trying to do it in the user's local time, and switched to GMT.

As for getting later, not exactly. The *purpose* of leap years is to keep the calendar from getting too out of synch with the sun.

But that means they merely have an "average" position over a 400 year cycle.

The 2000 leap day shoved them to one end of the range, they'll get earlier over the next 400 years. But mostly on average. :-)

Re: Astronomical events

[morgan-gw.livejournal.com]

But precession doesn't mean that the calendar is getting out of synch with the sun, it affects the movement of the sun in relation to the sidereal background.

The cross-quarters -- the midpoints between the solstices and equinoxes for those who don't know *g* -- used to happen around the 1st of their respective months. That's why the cultural celebrations still center on the 1st of Feb, May, Aug and Nov (or the eves of those days). But the actual 15 degree points of the signs now happen several days later.

I still recall that the quarters used to always happen nearer the 21st than they have in a long time. I suppose someone with the time and inclination could do the research and find out how the dates have actually changed, but I have other things I prefer to spend my time on :-).

Re: Astronomical events

[kengr]

The requirement for leap years has nothing to do with the precession of the equinoxes (which is on a many thousand year cycle anyway.

Leap years exist because the "tropical year" (ie the time between a given solstice and it's next occurence, so called because on the solstice the sun is directly overhead on either the Tropic of Cancer or the Tropic of Capricorn) is not an even number of days.

It's roughly 365.2422 days. Which means that if we went with a 365 day year, the date of the quarter and cross quarter days would be about one day later every four years.

That's what led to the Julian calendar. Add a leap day every 4 years and things match fairly well. Instead of gaining a day every 4 years, you lose a day every 128 years.

Which is how the vernal equinox (and Easter which is tied to it) crept earlier and earlier in the year. Leading to the Gregorian calendar. Which has the 400 year rule. It only loses one day in around 3333 years.

Check the Calendar FAQ for far more info about this sort of thing than you ever want to know. :-)

Re: Astronomical events

[rolandx.livejournal.com]

The requirement for leap years has nothing to do with the precession of the equinoxes (which is on a many thousand year cycle anyway.

That was my original point -- that the dates of equinoxes wasn't related to leap years.

Re: Astronomical events

[kengr]

And I explained why your point was wrong.

The dates of the equinoxes depend on the *calendar* and it's relation to the length of the tropical year.

"precession of the equinoxes" is an entirely seperate thing that has to do with the shape and orientation of Earth's orbit, not with the period of the orbit.

Without leap years, the date of the equinox would drift thru the entire year on a roughly 1500 year cycle. Which it actually *does* in the Mayan calendar.

Leap years are an attempt to keep things like equinoxes on the same calendar date. They always occur at the same point Earth's orbit (and thus the same point in each tropical year). The days just don't line up well with this.

Since we can't do fractional days well, every leap year shifts the date forward a day while the non-leap years shift it *back* roughly 5 hours, 48 minutes, 45 seconds.

The *date* is affected by leap years because they aren't prefect adjustments. We have to adjust the length of the *calendar* year in one day increments. And sometimes that causes a bigger shove than other times.

http://en.wikipedia.org/wiki/Vernal_equinox_year might bear some reading.

Precession of the equinoxes is only 20 minutes a year.

So it's sort of like cionfusing the fact that the tilt of the earh's axis causes the seasons with the fact that the orientation of the tilt changes due to the precession of the earths axis (and degree of tilt changes on yet another cycle).

Yeah those factors affect the seasons, but the position of the earth in its orbit overwhelms them until you start dealing with timeframes of 10,000 or more years.

Short form. precession of equinoxes affects the length of the tropical year. Very small effect. Difference between length of tropical year and calendar year shifts *date* of equinox. Hundreds of times larger effect.

Leap years don't affect the *interval* between equinoxes. But the affect what the dates involved are.