Now that Spring has sprung

As we went through the spring equinox, I couldn't help but notice that there were some questions about the day and night not being the same exact length, which is the actual meaning of the word equinox, after all. It was a spring "not completely equal night".  Space.com offers some explanations of that. 

The reason is "the usual", if you'll allow that short summary. The world isn't exactly perfect, orbits can be not exactly symmetrical, and imperfections are everywhere. I should note that when you look up the date and time of the equinox, you get a specific time of day - to the minute. 

Saturday (March 20) at 10:46 a.m. Eastern Daylight Time (7:46 a.m. Pacific Daylight Time) the vernal or spring equinox was expected to occur. At that moment, the sun comes to one of two places where its rays shine directly down on the equator. It will then shine equally on both halves of the Earth. More precisely, at that moment, the sun will be shining directly down on the equator at a point over the Atlantic Ocean, roughly 790 miles (1,280 kilometers) east of Macapá, Brazil.

When I was learning these things, I filed away that seasons tended to change on the 21st of month. Spring started on March 21, Summer was June 21, Fall was in September and Winter was December. In all cases, that was the first day of "new season." 

Space.com says that's not right. They say that during the 20th century, March 21 was the exception rather than the rule, with the equinox landing on that day only 36 times out of the 100 years in the 20th century. 

From the years 1980 through 2102, it comes no later than March 20. In 2028, in fact, for the Western Hemisphere, spring will officially begin on March 19. This shift in dates happens because the Earth's elliptical orbit doesn't match our calendar perfectly. The vagaries of our Gregorian calendar, such as the inclusion of a leap day in century years divisible by 400, also help contribute to the seasonal date shift. Had the year 2000 not been a leap year, the equinox would be occurring this year on Saturday (March 21), not Friday. 

The Space.com author goes on to note:

One factor to consider is that when we refer to sunrise and sunset, it refers to when the very top edge of the sun appears on the horizon. Not its center, nor its bottom edge.

This fact alone would make the time of sunrise and sunset a little more than 12 hours apart on the equinox days. The sun's apparent diameter is roughly equal to half a degree.

The main reason for this difference is that our atmosphere refracts (or bends) light above the edge of the horizon. Because of that refraction, we end up seeing the sun for a few minutes before its disk actually rises and for a few minutes after it has actually set.

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

Earth's seasons diagram. (Image credit: NASA/Space Place)