Monday, August 21, 2017
Pertinent Facts To Know Concerning Today's Total Solar Eclipse
After reading an AP account of the eclipse in the Friday Denver Post, to the effect this "is the first total solar eclipse in 99 years" I knew it was time for another post to clarify. It turns out assorted year periods have been tossed around: "38 years", "99 years" etc. without providing a context.
First, let's note that - on average- there are in fact two total lunar eclipses and two total solar eclipses every year. It's just that they mostly don't occur in or near populated areas. Second, the "first in 99 years" refers to the first coast to coast passage of the umbral shadow in that time. E.g. Our current track looks like this, from the Pacific coast of Oregon to the Atlantic coast of S. Carolina:
The "first solar eclipse in 38 years" refers to the first total solar eclipse to occur in the U.S. in 38 years. Let's return to this general depiction of the solar eclipse (graphic below), and as can be seen, there is a central dark shadow cone (the umbra) and a lighter shadow cone (the penumbra). Most Americans today will be in the cone of the penumbra and will have to travel to be within the umbra to see totality. That is, the total blocking out of the Sun by the Moon as seen in the top image.
Many would-be viewers as well as media, also fail to grasp that the durations of viewing times, both for the partial eclipse and total, are variable. For example, at Casper Wyoming, barely 350 mi. north of us, the partial phase of eclipse begins at 10:20 a.m. and totality begins at 11:43 a.m. Thus, there is an interval of 1 h 23 m from first contact to totality. The totality itself will last for a duration of 2m 26s. But at Hopkinsville, KY, the partial eclipse begins at 11:56 a.m. local time and totality at 1:25 p.m.. It can be seen here the complete eclipse duration is longer in Hopkinsville, nearly 3 hrs. from first to last contact. The totality phase alone will be 2 m 40s (Carbondale, IL comes in a close second at 2 m 37 s).
What gives? Well, the speed of the umbral shadow moving across the country from Pacific to Atlantic coast, varies. (The total time to cover the distance of 2,600 mi. is 1 h 33m.) Bear in mind- as the eclipse diagram above indicates - the moon's shadow is being projected not onto a flat surface, but onto a sphere (Earth). Near the middle of the totality path, the shadow is moving at its slowest along the surface of the planet. But at the very beginning of the totality path, and at the very end, the shadow is striking Earth at a very oblique angle at these points. Indeed, given the fact the lunar shadow cone is tangent to the curved surface of the Earth one can say it is moving at "infinite speed". In between, not so much. For instance, at Madras OR, the shadow speed will be about 2, 200 mph. At Hopkinsville, KY it will be about 1, 450 mph. Thus, because of the significantly slower shadow speed, observers in Hopkinsville will enjoy a longer total phase for the eclipse, 14 seconds longer than at Madras. (Though people in Madras can brag about being among the first to see it.)
What about those special eclipse-viewing glasses? Basically, you will need them for all but the totality phase (assuming you can be in the totality band). At that point (total eclipse) you can remove them because they become useless. Besides, you'd miss one of the most spectacular sights, the solar corona. Obviously, if you live outside the totality band area you will need to wear your glasses for whatever duration you're viewing.
During the first hour of the eclipse phase (i.e. from "first contact") few people would recognize anything unusual happening. No surprise given that not much of the Sun is being covered. But pay attention to the last 15 minutes or so before totality (again assuming you're lucky enough to be in the totality band and have clear weather) and note the sharpening shadows, changes in the light and sometimes cooling winds. This is also the phase where the 'Globe Observer' group wants to get temperature measurements - going through the total phase. (See the end of my last post).
Without any doubt the most awesome period begins in the final minute before totality, when the Sun transitions from being 99 percent obscured to total coverage. By some estimates, the sky gets about 10,000 times darker. It's absolutely eerie, as even assorted animals fall sway to the sudden change in light intensity: bees return to their hives, spiders take in their webs, birds cease their bird song etc.
Just before totality those lucky to be in the total band will behold the interplay of sunlight and mountains in profile on the lunar surface. These produce a phenomenon called "Bailey's beads.' Note how the last bead gleams brilliantly just as the Sun's corona becomes visible.
I hope everyone will be able to enjoy this event to the max, and maybe learn a bit about astronomy from it, even enticing further exploration. For myself, I will have to be content with watching it from televised accounts on the TV. Alas, wifey is still recuperating in the hospital after hip replacement surgery on Friday. Also she seems to be suffering from a rare complication that will make her recovery longer: injury to the lateral femoral cutaneous nerve. It makes any kind of move on the right leg almost unbearable. So we will likely watch totality from her hospital room.
For the rest of you, get out there and enjoy this (almost) once in a lifetime experience!