Thursday, March 22, 2012

Equinox and Leap Year

Grau and I have had discussions in the past about the first day of Spring. He said it always falls on his father's birthday and I said that can't be because sometimes it falls on my father's birthday (both gentlemen were born in March two years apart).

Fast forward a few weeks and suddenly it's how time flies.
And I wondered yet again, how it is that I remember the first day of Spring being on my father's birthday, but it hasn't been for a few years.
So I looked up Vernal Equinox.

And a friend posted about the equinox (which, for those of you wanting to know, actually means "equal night") and how it snuck up on her this year. And I posed the question to her- how could the equinox be on the 21st every four years but not have been on the 21st for the past five?

It's a math problem, apparently.

She didn't know, and tried looking up the answer before responding to my question post to her.
Then today, another lady chimed in with some info I knew, but couldn't correlate to my query.

Here's the basics- Vernal Equinox occurs every year, usually March 20th or 21st.
To quote the commenter-

The Gregorian calendar, which most of the world uses today, is imperfect. Its days and months don’t precisely represent the position of the Earth in its orbit around the sun. In other words, the Earth isn’t in the same spot every year on a given date.
The equinox would fall on exactly the same day each year if Earth completed its orbit in exactly 365 days. But it actually takes about 365.25 days. The extra quarter of a day means that the equinoxes occur 6 hours later each year, which pushes the date the equinox falls on around from year to year…..
I knew this- as leap year only happens 97 times every 400 years. To be a leap year, the year must be divisible by 4, except on hundred years (1900, etc) where it must be divisible by 400. And as we all know, leap year was added in by Caesar around 45AD to make the calendar match the orbit.

So I tried meshing the two pieces of information...and it took the hour of the equinox to make it make sense.

The last time Vernal Equinox occurred on March 21st was 2007. It should have occurred again last year, but did not. This is because not only of leap year, but also because of the TIME of the equinox.
You see, if equinox occurs at 0007 on the 21st- as it did in 2007- the next time it should fall would be approx 6am on the 21st of 2008. Except 2008 was a leap year, losing a day the month before the equinox.
Hence, vernal equinox occurred at approx 6am on the 20th, not the 21st, which set in motion a perpetual loop of March 20th equinoxes.

It is only after a complete leap-year cycle of four centuries that these dates will be repeated. In the present century the times of the equinoxes have ranged between the latest dates (March 21, 1903) to the earliest dates (March 20, 2000).
The next time Vernal Equinox will be NOT on March 20th will be March 19, 2044.  It is then that we start the loop between the 19th and 20th every 4 years.


  1. Great Job of explaining the math on this. Mystery solved. :)

  2. Thanks. Although in re-reading my post, I thought I should clear something up-

    When I say that 'on leap year, we lose a day' some may find that confusing. Yes, we GAIN a day during leap year, but for the purpose of the Vernal Equinox, since there is an added day, it actually pushes BACK the equinox on the calendar. The time of the equinox stays, the date changes.

  3. I plan to design, build, and deploy orbital realignment boosters on the moon so the first day of spring is always on my Dad's birthday. It's in the project queue right behind the orbital deathray platform :)