Kansas Snapshots by Gloria Freeland - March 29, 2024

It's about time

 "Wait, isn't it on the 20th or 21st?" I asked.
 This was March 19. My question had been directed at my engineer, all-around science guy and husband Art. It was prompted by the TV meteorologist on the late news saying spring had arrived just minutes before.
 "It's because this was a leap year," Art answered.


The weatherman said spring had arrived at 10:06 p.m. How can a season begin at some odd-ball time? A little digging was called for!

The seasons are the result of the earth's tilted axis. Half of the year, the sun shines down directly on areas north of the equator, making where I live warmer than usual. In the other half of the year, it is over the southern hemisphere and so we are cooler than usual. But for an instant twice each year, the sun is directly over the equator. For those of us living in the northern hemisphere, the March occurrence has been selected as the point where winter gives way to spring. Later this year, when the sun is again directly over the equator, we say summer becomes fall.

Lunar and Planetary Institute's website drawing illustrates how the sun's light concentrates on different parts of the earth during different seasons.

Summer's start in the northern hemisphere - the summer solstice - is precisely halfway between the March crossing and the following September's crossing. Solstice is Latin for "stopped sun" and is the day when the sun is directly over its northern-most extreme and then begins moving southward. This is the winter solstice in the southern hemisphere.

The equator crossing days are called equinoxes and imply that the night and days are equally long ... 12 hours. But in Manhattan, Kansas, daylight on March 19 began at 7:31 a.m. CDT - and hung around until 7:38 p.m. CDT - seven minutes longer than half of a day!

Daytime is defined as the time between when the sun just peeks over an unobstructed horizon in the east until the last remnant disappears in the west. The "peeking" part arrives a minute earlier than the sun's center and the "disappearing" part leaves a minute later than the center. So the "day" is two minutes more than the night on the equinoxes because of the sun's width.

In addition, the earth's atmosphere acts like a lens and bends sunlight. So we see the leading edge of the sun in the morning about two and a half minutes before it actually reaches the eastern horizon. Because of that same effect, we can see the last of the sun for two and a half minutes after it has dropped below the horizon.

So the sun's width plus the lens effect stretch the "daylight" by seven minutes, making the equinoxes the almost equal night and day days!

Moving east or west changes when the sun appears. Labrador is two time zones east of where I live. Spring arrived there at 12:06 p.m., which would be March 20. So while my spring arrived on the 19th, in eastern Canada and those living farther east in places like London, it appeared on the 20th. It was the same instant everywhere, but not the same time or date!

While these season-starting dates move around - spring on March 19, 20 or 21 - it's not the universe that cannot make up its mind. It's we humans attempting to make something fit into a box that doesn't really fit.

The earth requires five hours, 48 minutes and 46 seconds more than 365 days to complete its journey around the sun. When midnight on December 31 arrives, our calendar year has ended, but the earth still needs almost a quarter of a day more to reach the same place in the solar system it was at when the previous New Year's Day began.

We could ignore it, but eventually the accumulated error would make "Christmas in July" sales seem like the real deal, complete with snow, low temperatures and long nights. Envision Santa arriving on December 25th not wearing a heavy suit riding a sleigh, but in a Speedo on a waterski.

To fix this, once every four years we add a day at the end of February. That shifts all the days that year after leap day, making astronomical events occur on the calendar one day earlier.

This once-every-four-year "correction" would be perfect if the earth's sojourn was six hours longer than a 365-day year, but it is about 11 minutes less than that. We have overcorrected! Correcting the correction" involves skipping the leap day on three century years - such as 1700, 1800 and 1900 - and then having one on the fourth - as we did in 2000. Again, this isn't perfect, but the overcorrection is now less than 3 hours in 400 years!

Most celebrations are tied to a day of the month - Independence Day, Christmas Day, New Year's Day, birthdays, etc. But there are oddballs, such as election day and Thanksgiving Day. Part of the reason I began thinking about this is because the winner of the oddball trophy - namely Easter - is on our doorstep. Easter is the first Sunday after the full Moon that occurs on or after the arrival of spring. If the full Moon falls on a Sunday then Easter is the next Sunday. To make life easier for the poor monks in Western churches who were running the calculations, the religious community decided spring's arrival was always March 21.

Easter is this coming Sunday, March 31. That seemed early to me, but it and April 16 are actually the most common dates. Easter can't be earlier than March 22 or later than April 25. In the 500-year span from 1600-2100, March 22 and April 24 are the most rare, each occurring only once!

None of this information will make the daffodils any prettier or the Easter eggs taste better. But it just seemed it was about time to clear up some of these things I have wondered about all my life.