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 time, the sun reaches one of two places where its rays shine directly on the equator. Then it will shine equally on both halves of the Earth. More precisely, at that time, the sun will shine directly over the equator at a point over the Atlantic Ocean, approximately 790 miles (1,280 kilometers) east of macapa, Brazil.
From the years 1980 to 2102, arrives no later than March 20. In fact, in 2028, for the Western Hemisphere, spring will officially begin on March 19. This change in dates is due to the fact that the Earth’s elliptical orbit does not coincide perfectly with our calendar. The vagaries of our Gregorian calendar, such as the inclusion of a leap day in centennial years divisible by 400, they also contribute to seasonal date change. If the year 2000 had not been a leap year, this year’s equinox would occur on Saturday (March 21), not Friday.
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They are not “equal” at the equinox!
Another complexity involving the spring equinox has to do with the axiom: “equal days and equal nights at the equinox.” However, every year I always get at least one or two questions wondering why this isn’t the case. Perhaps someone flipping through the weather page of their newspaper on the day of the equinox, looked at the almanac chart giving the local sunrise and sunset times, and noticed that the length of day and night is not equal at all. In fact, on the dates of the March and September equinoxes, the length of daylight is actually more extensive than darkness for several minutes.
Look at the situation in Pittsburgh. As the following table shows, the days and nights are not equal on the equinox, but on St. Patrick’s Day:
|
Date |
Sunrise |
Sunset |
length of day |
|---|---|---|---|
|
March 17 |
7:28 a.m. |
19:28 |
12 noon 00 min. |
|
March 18 |
7:26 a.m. |
19:30 |
12 noon 04 min. |
|
March 19 |
7:24 a.m. |
19:31 |
12 noon 07 min. |
|
March 20 |
7:23 a.m. |
19:32 |
12 noon 09 min. |
One factor to consider is that when we refer to sunrise and sunset, we are referring to when the top edge of the sun appears on the horizon. Neither its center nor its lower edge.
This fact alone would cause the sunrise and sunset times to have a difference of just over 12 hours on equinox days. The apparent diameter of the sun is approximately equal to half a degree.
But the main reason this happens is because of our atmosphere; It acts as a lens and refracts (bends) its light over the edge of the horizon. In his calculations of the times of sunrise and sunset, the US Naval Observatory It usually uses 34 arc minutes for the angle of refraction and 16 arc minutes for the semi/half diameter of the solar disk. In other words, the geometric center of the sun is more than eight tenths of a degree below a flat, unobstructed horizon at sunrise.
As a result, we end up seeing the sun a few minutes before its disk rises and a few minutes after it has set. So you can thank our atmosphere for making our days a little longer; The length of daylight on a given day increases by approximately six or seven minutes.
So . . . When you see the sun rise above the horizon at sunrise or descend below the horizon at sunset, you are seeing an illusion. The sun is not really there but is below the horizon!
Now you see it. . . when you don’t!
Joe Rao serves as an instructor and guest lecturer at New York University. Hayden Planetarium. Write about astronomy during natural history magazine, sky and telescope, The Old Farmer’s Almanac and other publications.
