Tuesday, November 15, 2011

Flying through the Totality

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Have you ever wondered what it would be like to see something 70 miles wide moving over the landscape at almost 1,700 miles per hour?

I think it was a Carl Sagan essay where I first heard the experience described. He had that very experience standing on a hilltop with hundreds of other people. All expected the experience and understood what was happening. But when a shadow stretching from horizon to horizon appeared and swept over the valleys and grasslands and swallowed the assembled people, some involuntarily screamed.

I am, of course, talking about a total solar eclipse.

Solar eclipses come in four flavors. In each case, the moon interposes itself between the sun and Earth and part of the moon’s shadow falls on Earth.

The moon’s shadow has three primary portions: The umbra, the penumbra, and the antumbra. You’ve probably seen this phenomenon in shadows before with everyday objects and familiar illumination sources, but you probably haven’t thought much about them. You’ve probably noticed that shadows have a dark central part and a less-dark outer part. And, if the observation point is far enough from the obscuring object, there’s not much of a shadow at all.

The umbra is the darkest part of the shadow and it is located directly on the other side of the moon from the sun. It's cone-shaped with the base of the cone on the diameter of the moon and the point at some location in space that varies between 228,000 to 236,000 miles from the moon. It varies because the Earth-moon system’s orbit around the sun is slightly eccentric and the distance of the moon from the sun affects the length of the moon’s umbra.

Additionally, the moon’s orbit around Earth is slightly eccentric and the moon’s distance from Earth varies from about 226,000 miles at perigee to about 252,000 miles at apogee.

If you’ve been paying attention so far, you’re figured out that the moon’s umbra doesn’t always touch Earth during a solar eclipse because the umbra is shorter than the distance between the moon and Earth much of the time. And sometimes the umbra is long enough to reach Earth, but it misses and goes north of the north pole or south of the south pole.

To go completely overboard on the details: Because of the action of tides on Earth, Earth is transferring its rotational momentum to the moon as orbital momentum, which lengthens Earth’s day by about 23 microseconds each year and causes the moon to recede about 38 millimeters each year. Absent a major change, the moon’s umbra will, at some point about 425 million years from now, be unable to reach Earth and no further solar eclipses will occur on Earth.

Where the umbra touches Earth’s surface, observers see the sun entirely obscured by the moon. That’s a total solar eclipse and it’s the most common image you’ll see of a solar eclipse.

The part of the shadow beyond the end of the umbra is called the antumbra. In the antumbra, the angular size of the moon is less than that of the sun, the eclipse is said to be of a magnitude of less than one, and the moon obscures only some part of the central disc of the sun. The disc of the moon moves entirely within the disc of the sun and some of the sun’s disc is visible around the entire disc of the moon. This is called an annular solar eclipse.

At every location that is in the broader shadow called the penumbra, viewers see the moon’s disc partially obscure the sun’s disc but don’t see the moon’s disk entirely obscure, or pass entirely within, the sun’s disc. This is called a partial solar eclipse.

There’s also such a thing as a hybrid eclipse where the umbra touches down in mid-eclipse, so observers on parts of Earth see an annular eclipse and observers on other parts see a total eclipse. As you might imagine, hybrid solar eclipses are very rare.

It’s not hard to see a partial solar eclipse. They happen frequently and are visible from broad areas. It’s not uncommon to be able to see the partial eclipse almost from pole to pole. The penumbra is huge. The umbra, on the other hand, is tiny and can’t be more than about 167 miles wide. Thus it’s rare for a particular place on Earth’s surface to see a total solar eclipse.

There hasn’t been a total solar eclipse visible from the continental United States since 1979. It was visible only in Washington, Oregon, Idaho, Montana, and North Dakota.

But on August 21, 2017, a total solar eclipse will be visible from places all the way across the continental US. It’s the first total eclipse to cross American soil since 1991, the first on the mainland since 1979, and the first to sweep across the entire continental US since 1918. And the next one to be visible from the continental US won’t be until April 8, 2024.

The 2017 total eclipse will have a magnitude of about 1.03, which means that the angular size of the moon will be about 3% larger than that of the sun.

The eclipse will make landfall on the Pacific Coast at Lincoln Beach, Oregon due west of Salem and just south of Siletz Bay Airport (S45) at 17:15:58Z and totality will last a little less than two minutes.

The point of greatest eclipse will be N 36.9664° W 87.6639°. That’s near Cerulean, Kentucky, about 60 miles due west of Bowling Green and about and 14 miles northwest of Hopkinson-Christian County Airport (KHVC). At the point of greatest eclipse, the leading edge of the totality will arrive at 18:24:09Z and the trailing edge will arrive at 18:26:50Z for a duration in totality of 2:41.

The leading edge of the totality hits the east coast near the Francis Marion State Forest near McClellanville, South Carolina about 40 miles northeast of Charleston and 18 miles south of the Georgetown County Airport (KGGE) at 18:46:25Z and the trailing edge goes feet-wet 2:34 later at 18:48:59Z.

By the time the totality has passed over the continental United States, it will have passed over parts of Oregon, Idaho, Wyoming, Nebraska, Missouri, Kentucky, Tennessee, North Carolina, Georgia, and South Carolina.

The leading edge of the totality will travel just over 2,500 great-circle statute miles across the continental US in a about 90 minutes for an average ground speed of something 1,667 statute miles per hour.

There’s plenty of information available about how to best view an eclipse and I leave it as an exercise for the listener to go out and find that information. Carl Sagan found a hilltop with a valley sprawling out below so that he could see not only the eclipse but the onrushing umbral shadow. And he also looked up at the event in the sky using proper viewing filters or other appropriate means.

The only thing that I’ll say about it that is of general interest is that you should never look directly at the sun without safety glasses or other appropriate equipment. I shouldn’t have to say that. You guys know better. Just like you know not to ram the yoke abruptly forward and scream when you’re flying passengers. Unless you’re flying prisoners and you pull out before getting to Vne.

So all of this eclipse stuff is cool. And there’s an extent to which it’s justified simply because it’s cool science and that’s well within the usual coverage of Airspeed. But I have more than that in mind for this one.

Sometimes it’s cloudy at the site that you have picked out to view the eclipse. Sometimes it’s tough to find a good hilltop. Sometimes there are logistical problems with landowners in getting to your site. And if you really want, as I do, to see that umbral shadow sweeping ominously over the ground, you’re going to have to find some high ground. Probably a couple of different places so that you have a backup if the weather is bad at your primary location.

But these factors are not problems for you and me. We’re pilots, after all.

The total eclipse of 2017 is a long time from now. But, when it happens, I plan to be at about 10,000 feet MSL in an airplane somewhere along the center of the path of totality and have a front-row seat to see the umbral shadow coming at me at something like local Mach 2. I’m going to fly through the totality. And you can, too.

Here’s what I have in mind. You can look at this as a mere thought experiment or as an action plan. It’s fun any way you look at it.

First off, what are you going to see? Out in the penumbral shadow, things won’t get very dark at all. Maybe something like dusk. I’ve been in the penumbral shadows of several eclipses and that’s a pretty fair description.

Sagan said that there was a discernable wall of umbral shadow of the eclipse about which he wrote. But it’ll be fuzzy. It won’t be a brignt-line division. Remember that we’re out in the last 10,000 miles or so of the umbral shadow and it’s going to be fuzzy.

That’s where I think that being in an aircraft at altitude will be helpful. Being a mile or more above where the shadow makes contact with the ground or cloud tops ought to allow me to see the big picture and give me the best chance of seeing the motion of the shadow over the ground.

I plan to fly perpendicular to the path of the totality. There are those who give more than a little thought to flying away from the oncoming totality so as to extend their time within it. But at a maximum true airspeed of something like 110 or 130 knots, you’re only going to extend your time in the totality by a second or two. I’d rather fly perpendicular to the path and see it coming.

By the way, there was a plan afoot to fly the Concorde in the umbral shadow, but the accident involving Air France Flight 4590 put an end to it prematurely. A group did get up in 1992 and viewed the eclipse from at DC-10 at FL410 and there’s another account in Wired Magazine of a 2010 chase at FL 390 that extended the time in the totality to 9:23 (well beyond the theoretical maximum of 7:32 possible from a fixed spot on Earth’s surface.

What about the practical and legal aspects of flying around in the dark?

If the FARs are the same at the time of the eclipse as they are now, you won’t have to be night current in order to fly through the umbra. Generally speaking, FAR 61.57(b) presently provides that “no person may act as pilot in command of an aircraft carrying passengers during the period beginning 1 hour after sunset and ending 1 hour before sunrise, unless within the preceding 90 days that person has made at least three takeoffs and three landings to a full stop during the period beginning 1 hour after sunset and ending 1 hour before sunrise, and (i) That person acted as sole manipulator of the flight controls; and (ii) The required takeoffs and landings were performed in an aircraft of the same category, class, and type (if a type rating is required).”

There are other places in the FARs where “night” matters, such as in rules governing visibility, cloud clearances and other matters having to do with operation in various parts of the National Airspace System. FAR 1.1 presently defines “night” as “the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac, converted to local time.”

The closest that it’ll be to night local time during the eclipse will be when the totality goes feet wet on the east coast, where it’ll be a little before 3:00 p.m. local time. The sun won’t even set there until 7:58 pm local. If you think about it, it’s pretty obvious. You can’t fly through the totality of an eclipse after sunset. You need – er – the sun.

If you’re like me, you’ll plan to launch at least 30 minutes prior to the arrival of the umbral shadow and land shortly after the umbral shadow passes. You won’t be taking off or landing any time near “night” as defined by the FARs.

In almost any case, it’s going to be pretty dark in the penumbra around that time. And the partial eclipse, with its penumbral shadow, last a long time. The sun will be in at least partial eclipse for almost three hours. The penumbra is huge. During this eclipse, it’ll as far north as the north pole and as far south as the northern part of South America.

We pilots are (or ought to be) pretty good at knowing the difference between what’s legal and what’s smart. And the difference between current and proficient.

I plan to be both night current and night proficient before launching. If you’re the least bit uncomfortable flying in the dark, don’t launch. In fact, you ought to seriously consider getting some actual or simulated night instrument work in before the event. If you’re a confident VFR night pilot and you live near inhabited areas with lots of lights on the ground, think about how much your comfort derives from having those lights on the ground to use as references. It’s almost a sure bet that the street lights and other outdoor lighting will not be on during the passage of the umbral shadow. All of that stuff is on timers and there’s almost zero chance that anyone is going to think to flip the switch or change the timers for this event.

You also need to be sure that all of the lights inside the aircraft are working and that you know how to operate them. Many of us don’t fly much at night and many of us don’t even know where the light switches or dimmers are.

I ran into this when doing my initial training in a G1000-equipped aircraft. I had just become comfortable with the PFD presentation of airspeed, altitude, and other indications and was feeling pretty good about things. On the flight in question, the sun had already set by the time I was 20 miles out from Pontiac. The PFD and MFD each have an automatic dimming function that dims the display down as the ambient light fades. The displays dimmed much faster than the ambient light demanded and I found myself in the pattern with both displays nearly dark. The light control knobs on the panel only controlled the pedestal and other lights in the cockpit. The G1000 displays can be set manually, but that requires going through the menus to the manual settings – not something you want to try to figure out while you’re in the pattern. I landed using the backup round airspeed indicator. No problem. But I’ve since gone back and figured out where the display brightness lives in the G1000 menu system and I even put the procedure in the little cheat sheet that I use whenever I fly that aircraft.

I’d like to see the event VFR in clear air so that I can see the umbral shadow moving along the ground. But I’ll bet that VFR on top of a cloud deck would be pretty cool, too. In any case, it would be a good idea to be instrument rated, current, and proficient. That will help both in terms of getting on top of a local overcast and in terms of going somewhere else if there’s a high overcast or other adverse weather at the initially-planned interface location.

There’s some chance that your aircraft will not be the only aircraft up there doing exactly what you’re doing. Some nutjob with an aviation podcast got the word out more than five years before the big day and now the skies are darkening with airplanes, to say nothing of the moon.

And, sad, to say, it’s possible and even likely that you’ll be sharing the skies with a lot of people who aren’t anywhere near as prepared as you are.

Assume that August 21, 2017 will be just like that first nice weekend day in the spring in the northern US when every Tom, Dick, and Harry at the local GA airport drags out his airplane and decides that today’s the day to knock off the rust and make you number four to land on a busy CTAF.

Spread out and give everybody some room. The path of totality over the continental US is going to be more than 2,500 miles long. There’s really no reason to do it near Cerulean, Kentucky. The duration of the totality will be only 50 seconds longer at Cerulean than it will be on the west coast. You’re going to get about two minutes of totality at minimum no matter where along the path you fly. Go someplace away from the crowds and get some elbow room.

It would be entirely reasonable to get flight following so that you can obtain traffic advisories. It would also be a good idea to use an aircraft that’s TIS-enabled and fly in an area that supports automated traffic reporting. It might even be a good idea to tell the controller what you’re doing and have a couple of navaids in mind to use in telling the controller where you’ll be. If the controller is handling a dozen eclipse-viewing flights and one has a solid and easily-describable flight profile, who do you think is going to get favors from the controller if they need them?

If you’re going to make use of your instrument ticket to get on top to see the shadow, it might be best to pick a nice holding spot on an airway and then ask for a hold with nice long legs. Alternatively, it might be a good idea to file two IFR flight plans: One to get on top (which you’ll then cancel and pick up a convenient 500-foot-increment cruising altitude) and one to open when you’re ready to go back through to land. Again, if there’s a lot of traffic on top and everybody wants to air-file for return, who do you think is going to get the best handling? The guy or gal with the IFR flight plan already on file? Yep. I thought so.

Without adding yet another factor into the age-old high-wing/low-wing debate, the kind of aircraft that you fly is going to be important. And it’ll depend on what you want to see.

If, like me, you want to see the umbral shadow, a high-wing aircraft is going to be your best bet. Unless your high-wing aircraft has a good overhead window, you’re probably going to miss the eclipse itself altogether. At local solar noon at the point of greatest eclipse in Kentucky (about 1:49 pm local), the sun will be about 64 degrees above the horizon. And it won’t really be a lot lower than that during the eclipse. You can pretty much forget about banking steeply enough to see the eclipse itself. Night acro is not my idea of fun and it’s probably not yours either.

If you want to see the eclipse itself and you care less about what’s going on on the ground, I suppose a low-wing aircraft would better. If you fly at an angle to the oncoming umbral shadow wall and stay wings-level, you’re probably get a good view of the ground and could still look up and see the eclipse itself. But, when you add all of the fussy stuff involving safety glasses and other concerns about safely viewing the eclipse itself, you begin to get overly preoccupied with something that’s going on a quarter million miles away and aren’t paying sufficient attention to the conduct of the flight. Especially if the pilot flying wants a look.

Frankly, if it’s really important to get a look at the eclipse itself, you’re better off not being in an airplane in the first place. Stay on the ground and enjoy. There’s nothing wrong with that.

I’ve tried to identify accidents or incidents associated with prior total eclipses in the continental US. This is not by any means the first total eclipse to be visible from the continental US since general aviation has been widespread. There have been five since 1950. I’ve tried to identity accidents or incidents associated with flying through the totality, but I haven’t found any. That could have something to do with what you can imagine happens when you try to do a Google search for “eclipse” and “accident.” There’s just too much noise in the search data.

Viewing an eclipse itself from the ground is underwhelming. You can't really look directly at it. Indirect viewing is okay, but it's remarkably like the pictures that you'll see on the 'net the next day. Ho-um. I've never seen a total eclipse on the ground, but I've seen several partials. The cool part for the partials was the relative darkness. Thus, I'm about chasing the darkness.

There is one thing you'll miss that you might care about. At totality, if there are enough naked-eye planets within view, you can get a real sense of the ecliptic of the solar system because you can see the sun and planets at the same time in the same hemisphere of sky. For the 2017 eclipse, you should be able to see Venus and Jupiter fairly clearly. Mars will technically be visible, but it will be very close to being on the other side of the sun (conjunction occurs July 26, 2017 and Mars will be something like 2.4 AU away). Saturn will be below the eastern horizon. But, in any case, you can sweep your eyes across the sky and see the sun, Jupiter, and Venus. I've heard that, once you see the solar system's ecliptic during an eclipse, you realize that you're standing at an angle and leaning 45 to 55 degrees out from the planet's surface over the ecliptic. And I've heard that it'll induce vertigo and that it's a memorable experience. But it requires very clear skies and a broad field of view.

So that’s my plan. Even if I don’t end up doing it, it’s a great exercise that combines the sciences of aviation and astronomy. But I’m planning to do it. I wonder if the local FBO will take a reservation for the RG for 2017 . . .


The Wikipedia entry on the eclipse.

A NASA page that includes a Google map
An animated sequence of images taken by the Eumetsat satellite during the total eclipse of the sun on 29 March 2006. You can see the umbral shadow move across Earth’s surface.

The lead image shows the moon's shadow on Earth, as seen from the International Space Station at an altitude of 230 miles on March 29, 2006. NASA photo used per NASA’s photo policy.

Many thanks to John Walker and those who built Your Sky, the interactive planetarium of the Web (http://www.fourmilab.ch/yoursky/). The sky map showing the planetary locations at the time of the 2017 eclipse comes from that fine web resource.


thelaker said...

Sounds like an excellent plan. Say, KJXN isn't too far out of your way, so if you need an extra passenger, it'd be great if you could stop by and pick me up on the way to Kentucky (assuming, of course, that I'm still living nearby in 5 years).

Anonymous said...


I mean, wow.

That is the most batshit insane, totally awesome thing I have heard so far this year.

The fact that you have thought about it in such depth, this far out, is both really scary and inspiring.

And I hope that we'll get a podcast on it. Or a movie.