Today (March 20) at 10:46 a.m. Eastern Daylight Time (7:46 a.m. Pacific Daylight Time) spring or spring equinox occurs. At that moment, the sun comes to one of two places where the 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 shine directly down on the equator at a point over the Atlantic Ocean, about 1,280 kilometers east of Macapa, Brazil.
From the years 1980 to 2102 inclusive, it arrives on 20 March at the latest. In 2028, for the Western Hemisphere, spring will officially begin on March 19. This shift in dates occurs because the Earth’s elliptical orbit does not perfectly match our calendar. The vagaries of our Gregorian calendar, such as the inclusion of a leap day in century years divisible by 400, also contributes to the seasonal date shift. Had the year 2000 not been a leap year, the equinox would have been this year on Saturday (March 21), not Friday.
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Not “equal” on the equinox!
Another complexity involving the vernal equinox concerns the axiom, “equal days and equal nights on the equinox.” But every year I always get at least one or two inquiries asking why it isn’t. Perhaps someone skimming through the weather page of their newspaper on the day of the equinox looked at the almanac box showing the local time of sunrise and sunset and noticed that the lengths of day and night are not equal at all. In fact, on the equinoxes in both March and September, the length of daylight is actually further than darkness by several minutes.
Check the situation for Pittsburgh. As the table below shows, days and nights are not equal on the equinox, but on Saint Patrick’s Day:
|
Date |
Sunrise |
Sunset |
Length of day |
|---|---|---|---|
|
March 17 |
07:28 |
19:28 |
12 hours. 00 min. |
|
March 18 |
07:26 |
19:30 |
12 hours. 04 min. |
|
March 19 |
07:24 |
19:31 |
12 hours. 07 min. |
|
March 20 |
07:23 |
19:32 |
12 hours. 09 min. |
One factor to consider is that when we refer to sunrise and sunset, it refers to when the top edge of the sun appears on the horizon. Not the middle or bottom edge.
This fact alone would make the time of sunrise and sunset slightly more than 12 hours apart on the equinox. The Sun’s apparent diameter is roughly equal to half a degree.
But the main reason this happens is because of our atmosphere; it acts as a lens and switch (bends) the light over the edge of the horizon. In their calculations of times for sunrise and sunset are US Naval Observatory routinely uses 34 arcminutes for the angle of refraction and 16 arcminutes for the half/half diameter of the solar disc. In other words, the geometric center of the sun is more than eight tenths of a degree under a flat and unobstructed horizon at the moment of sunrise.
As a result, we end up seeing the Sun for a few minutes before the disc actually rises and for a few minutes after it actually sets. So you can thank our atmosphere for making our days a little longer; the length of daylight on any given day is increased by about six or seven minutes.
So . . . when you see the sun either rising above the horizon at sunrise or setting below the horizon at sunset, you are looking at an illusion — the sun is not really there, but is actually below the horizon!
Now you see it. . . when you don’t!
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural history magazine, Sky and telescope, The old farmer’s almanac and other publications.
