The Autumnal Equinox is upon us.
Okay, we know that this means that there are equal amounts of daylight and nighttime. We know that winter is coming to the Northern Hemisphere and that summer is coming to the Southern Hemisphere. But there are other Fun Sun Facts worth mentioning…
At the risk of sounding like a geocentrist, it is easier to describe the movement of the Sun as if the Earth were stationary and the Sun revolved around it. On the Autumnal Equinox, the sun is directly above the equator for this day. If you were standing on the equator at the Equinox, at noon, you would see no shadows, since the sun is directly overhead. The same thing happens on the Vernal (Spring) Equinox. If you want to check this out yourself, catch a flight to Quito, Ecuador, which is right on the equator. The official name of Ecuador, by the way, is the República del Ecuador, which translates into “Republic of the Equator.”
The sun is now making its visit to the southern Hemisphere, and has been doing so since the Summer Solstice (June 22 this year), that day when the sun is as far north as it ever travels. Its “equator” that day is the 23.5°N latitude, also called the Tropic of Cancer. Six months later the sun is as far south as it goes, and—you guessed it—the “equator” on that day, the Winter Solstice (December 21), is 23.5°S latitude, the Tropic of Capricorn.
These Solstice days make for interesting times in those communities lying at or above the Arctic Circle 66°N latitude. On the Summer Solstice, they receive 24 hours of daylight; the Winter Solstice brings them 24 hours of darkness. Here is a good representation of the Sun and the Earth on the Summer Solstice.
If you happen to travel on an East-West road and you are traveling East early in the morning, you may soon be blinded by the rising Sun. On the Equinox days—no matter where you live—the Sun rises due East, and sets due West. As you continue your day-to-day commute, you will notice that the Sun is rising further and further south of the East-West horizon line (which is that East-West road on which you are traveling). The Sun will rise furthest in the South on the Winter Solstice, after which it begins its journey northward, blinding you again on the Vernal Equinox, and rising north of that east-west road, until it reaches is furthest sunrise point on the Summer Solstice.
I happen to work in an office building which has a hallway facing due East. On September 22, I suspect that the white ceramic tile floor will be ablaze with reflected light coming from the rising. My own little Stonehenge!
So, what’s all this have to do with coil coatings?
Most of our products end up facing the sun, so it is helpful and important to know something about how the sun interacts with our products. The industry standard for weathering is South Florida. The weathering sites there are at 25-27°N latitude, just a little north of the Tropic of Cancer. This area is referred to as being “subtropical.” In this usage, “sub-” means “nearly,” not “less” or “below.”
One last Sun Fact: On September 22 this year, check the local paper to see how many hours of daylight (also called “daylength”) there are. Twelve hours, right? Wrong! It will be twelve hours plus a few minutes. How can this be? I thought that “Equinox” meant equal daylight and darkness. Why the difference? Here’s why: Official sunrise is when the upper tip of Sun breaks the horizon. Sunset is when the upper tip of the Sun descends below the horizon. Using this method of calculating the length of a day, you get the maximum number of daylight hours. But we know better!
-David A. Cocuzzi, NCCA Technical Director