Image: Graphic on Equinox
Sky & Telescope
The sun rises due east and sets due west on the equinoxes in March and September. At other times of year it comes up and goes down somewhat to the north or south.
By Night sky columnist
updated 3/18/2005 2:09:33 PM ET 2005-03-18T19:09:33

Avid "Seinfeld" fans might remember the episode when Jerry’s friend, George, was desperately trying to find a way to postpone his impending Christmastime wedding with his fiancée, Susan. He finally comes up with a solution:

"Have the wedding on March 21 — the first day of spring!"

Unfortunately, if George had gone through with the nuptials (and Seinfeld aficionados know why he never did), he would have been a full day late. You see, in America, spring no longer falls on March 21. In 2005, for instance, the vernal equinox, the first day of spring for the Northern Hemisphere comes on Sunday, March 20, at 7:33 a.m. ET.

Now this doesn’t seem right. I mean, when we were all growing up, the first day of spring was always on March 21, not March 20, right? Now, all of a sudden, spring comes on March 20.

How did that happen?

While it’s true that we’ve traditionally celebrated the beginning of spring on March 21, astronomers and calendar manufacturers alike now say that the spring season starts one day earlier, March 20, in all time zones in North America. Unheard of? Not if you look at the statistics. In fact, did you know that during the 20th century, March 21 was actually the exception rather than the rule?

The vernal equinox landed on March 21 only 36 out of 100 years. And from 1981 to 2102, Americans will celebrate the first day of spring no later than March 20.

In the years 2008 and 2012, those living in Alaska, Hawaii and the Pacific, Mountain and Central time zones will see spring begin even earlier, on March 19. And in 2016, it will start on March 19 for the entire United States.

There are a few reasons why seasonal dates can vary from year to year.

  • A year is not an even number of days, and neither are the seasons. To achieve a value as close as possible to the exact length of the year, our Gregorian calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for Earth to complete one orbit around the sun. It eliminates leap days in century years not evenly divisible by 400, such 1700, 1800 and 2100, and millennium years that are divisible by 4,000, such as 8000 and 12000.
  • Another reason is that Earth’s elliptical orbit is changing its orientation relative to the sun (it skews), which causes Earth’s axis to point progressively in a different direction — a phenomenon called precession. Since the seasons are defined as beginning at strict 90-degree intervals, these positional changes affect the time Earth reaches each 90-degree location in its orbit around the sun.
  • The pull of gravity from the other planets also affects Earth's location  in its orbit.

The current seasonal lengths for the Northern Hemisphere are:

As you can see, the warm seasons, spring and summer, combined are 7.573 days longer than the colder seasons, fall and winter (good news for warm weather admirers).

However, spring is currently being reduced by approximately one minute per year and winter by about one-half minute per year. Summer is gaining the minute lost from spring, and autumn is gaining the half-minute lost from winter. Winter is the shortest astronomical season, and with its seasonal duration continuing to decrease, it is expected to attain its minimum value — 88.71 days — by about the year 3500.

Another complication revolving around the vernal equinox concerns the length of day versus night. We have been taught that on the first days of spring and autumn, the day and night are equal to exactly 12 hours all over the world.

Yet, if you check the calculations made by the U.S. Naval Observatory or the sunrise/sunset tables in any reputable almanac, you will find that this is not so. In fact, on the days of the spring and fall equinox the length of daylight is actually longer than darkness by several minutes.

Check out New York City. As the table below shows, days and nights are equal not on the equinox, but on St. Patrick’s Day:

One factor is that the moments of sunrise and sunset are considered when the top of the sun, and not its center, is on the horizon. This alone would make the time of sunrise and sunset a little more than 12 hours apart on these days. The sun’s apparent diameter is roughly equal to half a degree (0.50 degrees).

But the main reason that this happens can be attributed to our atmosphere; it acts like a lens and refracts (bends) its light above the edge of the horizon. In their calculations of sunrise and sunset times, the U.S. Naval Observatory routinely uses 34 minutes of arc for the angle of refraction and 16 minutes of arc for the semi diameter of the sun's disc. In other words, the geometric center of the sun is actually 0.83 derees below a flat and unobstructed horizon at the moment of sunrise.

Or, put in another way, when you watch the sun either coming up above the horizon at sunrise or going down below the horizon at sunset, you are actually looking at an illusion — the sun is not really there, but actually below the horizon.

As a result, we actually end up seeing the sun for a few minutes before its disk actually rises and for a few minutes after it has actually set. Thus, thanks to atmospheric refraction, the length of daylight on any given day is increased by approximately six or seven minutes.

For Europe, spring will begin on March 21 in 2007. That, however, will be the last time until 2102.

For places much farther to the east, such as Tokyo, Japan (9 hours ahead of Greenwich), spring will fall on March 21 in two out of every four years from 2006 through 2023 (2006, 2007, 2010, 2011, etc.), then once every four years from 2027 through 2055. But then that’s it until 2101.

Sorry, George!

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York. The equinox illustration appearing with this report is courtesy of Sky & Telescope.

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