Vernal Equinox: Understanding the Yearly Variation

[selfAdjoint]

Today, March 20, "is the Vernal Equinox". This event seems to vary yearly between the 20th and the 22d of March, without any obvious pattern. And I wonder if there is any clear description of the relevant variables that cause the variation. I am not iterested in a formula to calculate it, still less a program or simulation, but some reasoning as to how it varies. I presume it is due to the lineup of the tropical year with the Gregorian calendar date, but just how?

I tell people that the equinox happens when the Sun is directly overhead at exactly Noon at some place on the Equator. That acccounts for the odd times of day published but not for the larger scale wanderings.

 

[Bystander]

Think about leap years.

 

[selfAdjoint]

Think about leap years.

Yes of course, that's why I mentioned the Gregorian Calendar.

That calendar went through one of its long term adjustments six years ago; 2000, although an 'every-four" year, dis not have a leap day; that only happens every 400 years.

So I guess I would be satisfied to know when the Gregorian date and the Tropical day are or last were in sync. Pesumably the Tropical excess accrues at some .22 day through the four years between leap years, achieving something like .88 day by the time the leap day arrives and the difference is reset to -.12 day? This would imply the variation is less than 24 hours, but the actual variation is more like 48 hours.

 

[RandallB]

The actual needed change would be more like .2425 days / year.
You only need a bit more than 24 hours out of sync spread to get three days not 48.
Just a little before midnight on the 21st puts it into the 20th.
A bit after midnight on the 21st puts into the 22nd.
Probably still the 20th in some time zone every year.

 

[tony873004]

The signs of the zodiac are the same. I was born on Jan. 21, on a cusp, so my astrology friends tell me I'm an Aquarius, but about 1 out of every 4 years, I'm a Capricorn.