Shock and Awe (part three)

It all happened so fast. The sun became a tiny sliver as totality approached and then slipped, liquid-like into a shining ring around the moon. Then it slithered into mercury-like beads and in a few seconds was gone. Shouts and whoops came from the group as the darkness increased and we could all look directly at the spectacle without our viewers. I was transfixed by the filamentous corona extending into space. It’s something I’ve seen pictures of but had never seen in real life. Much like seeing wildlife in the wild instead of a zoo, the thrill was palpable. I was short of breath and my heart was racing. I kept thinking “wow” and I think I kept saying it, too. The silvery whorls and plumes of light extending from the eclipsed sun were subtle and beautiful and extended out at least a full solar diameter.

Everyone was excited. I hugged my bride and told her I loved her. She’s faithfully joined me on many nutty adventures over the decades and this one was particularly special. I was a bundle of nerves all month beforehand, worrying over trip details and wondering if we’d get skunked by crowds, chaos, or the weather. But none of that mattered now. An odd calm came over me and I started to feel a little giddy. All the worries and stresses of daily life seemed so silly standing there on the mountainside with the moon covering the sun.

And we were looking, unaided, at the sun. The eclipsed sun. One doesn’t go around looking at the sun. But with the moon blocking its rays we could. That was a weird feeling, I must say. Totality was much too brief. The image of the usually-hidden solar corona was etched into my mind, for good I hope. I had a feeling that I’d been let in on some special occult knowledge, that I was now a member of some secret society, that I’d been allowed to view what was forbidden. But that was transitory. The two minutes were up before we could fully grasp the event and the process began to repeat itself in reverse.

There were cries of delight and not a little bit of anguish as the sun began to emerge again. On one hand, my reptilian brain was thrilled that our life-giving orb was still there to shine down upon us. On the other, my modern mind was sad that the great conjunction was waning away. All of us wanted it to last longer so we could revel in its novelty. How many of us will experience another totality? I thought again about how lucky I was to live in a place where I could play in the shadow of the moon. I had the luxury of both time and means to pursue this adventure. I marveled too at the ancients who could, many centuries ago, watch and learn the patterns of the moon and the sun and grasp the rhythm of the saros and predict the re-occurrences of this celestial marvel.

As the twilight began to fade and the daylight re-emerged we all began to relax and chat and mingle. I noticed that I was both exhausted and exhilarated. I’d lived a whole day in a few minutes! The coronal features had faded away and we got another fleeting look at the diamond ring and then it was back to a partial eclipse. The day began to warm up and the landscape began to look normal again. I used to joke that whenever I did any interesting demonstration in chemistry or physics class that the students would never really see it the first time because they unfailingly demanded that I “do it again!” which of course was not always practical or even possible. I understood them now. I did my best to be open to this amazing experience but I longed for a re-do so I could look and feel again, and see it and feel it with a sharper eye and more finely-tuned heart.

But it was not to be. The sun and the moon had done their dance and that was that. There was still an hour to go until the final contact, and we kept putting up the filters to our eyes and scanning the sky, but the main event was over. All we had now was our memories and impressions. We began to talk and share among ourselves and re-live the experience. I developed a sudden thirst and was thankful that our party had prepared itself with coolers of beverages and refreshments. Of course I had my water bottle but what I really needed was a cold beer and I sucked one down greedily. I remember being a little unsteady on my feet and sat in my trusty fold-out nylon camp chair, alternately looking up and waiting for the eclipse to be completely over and staring wordlessly out across the forest all around me and the naked ridge to the west. I was, for a time, bereft of thought.

The approach of totality and totality itself seem to race by unnaturally fast. Time had sped up just when we all wanted it to slow down. Now time came to a stop. The last limb of the moon took, it seemed, forever to be free of the sun. Those final minutes stretched agonizingly. Oddly, we all wanted it to be over! Now that it was done let it be done. But whenever you wish for something to happen faster it never does, it always slows to a crawl. But nature can’t be bothered with our perceptions and the moon and sun continued on their imperceptible separate marches and soon all was back to the way it was before.

The moon, despite eclipsing the sun, was thoroughly invisible, as it always is during its new phase, and what evidence we had seen of its presence in the sky was gone. The sun climbed upward and westward as it does every morning, heating the air and the ground and casting the shadows that mark the progress of the day. It was like the thing had never happened!

But it had, and we were there. I’m sure I’ll be playing that movie again and again in my mind in the days ahead, and I’ll remember the feelings even if I won’t be feeling them again. It’s hard to recapitulate awe. Suffice to say it was indeed something special, and I’m thankful that I got to be a tiny part of it.

Shock and Awe (part two)

Sunday, the day before the eclipse, featured high clouds in the morning that strengthened during the day. It was worrisome. We had the perfect spot to camp: along a forested creek in the mountains. We had the perfect viewing site: an old quarry a few hundred yards up the road. But the weather had to cooperate. I went to sleep on Eclipse Eve with a bit of trepidation. We’d left smoky skies behind us. The John Day River basin was the clearest place we’d driven through and the air got even better with the increase in elevation. The days we’d been in camp featured bright, blue skies.

The sun didn’t come up over the treetops until about seven in the morning but I was up and out of the camper an hour before that. My friend Robin told me later she’d heard my feet crunching on the gravel and was tempted to stick her head out of her tent and yell “Christmas morning!” It sure felt like that. I was all anticipation. I marched up the hill to our view spot, spooking a grouse in the process, curious if anyone had come to camp there. Sure enough, it was empty. More important than that, the sky was clear.

I knew we had a few hours before the event and that smoke or clouds could come rolling in but somehow I knew they would not. The morning was cold and very still. The air was dry and fresh, like a breeze had come through in the wee hours and given our little area a quick cleaning-out. I had to stop and deal once again with this feeling of shock. It was going to work out! The preparation and planning was going to pay off! We were going to get a perfect morning with which to experience the eclipse! I felt really lucky to be where I was and to be surrounded by the people I’d hoped to share the space with.

The road flattened out and the benches cut into the hillside gave us a broader view of the landscape than we had down in our riparian campsite. The rocks were a loose, brittle, shale-like stuff that I had a hard time figuring why anyone wanted to quarry. I suppose they might have made road fill, or riprap, but I sure would not have built anything on them. It didn’t look like material had been mined there in many years. Pumice was interspersed among the big plate-like pieces and angular chunks, it was sharp-edged and bits of volcanic glass were embedded in it. It wasn’t exactly a place to lie down, but it was a nice contrast to the flour-like soil in camp which oozed its way into the pores of my skin, not to mention coating everything we had with a fine dust.

Our camp neighbors joined us, it was they in fact who’d scouted the spot and suggested it. There were sixteen humans and two canines in our combined parties: thirteen adults, one teenager, two children, one old dog, and one puppy. Eighteen individuals poised to be in the shadow of the moon that would race across this eastern Oregon mountainside. Suffice to say all were eager and excited. Well, I can’t speak for the dogs, but I suppose they were happy to be running around in the sunshine.

The weather was crisp and cool at half-past eight but the bright rays of the rising sun warmed up the rocks beneath our feet in a hurry and we soon peeled off a layer as the air temperature started to catch up. First contact was a little after nine and the whole event, until final contact, was to last almost two-and-one-half hours. The sun would gain over twenty degrees in altitude and swing westward almost thirty-five degrees during the eclipse. We were feeling good about our spot, and you can see why:

site

The centerline is in red, you can see we were quite close, effectively on top of it as the duration was predicted to be the same, just a few seconds over two minutes.

We had our eclipse glasses ready and we settled in to watch and wait for totality. Partial eclipses are certainly interesting, but unless they are near-total you don’t get the light and color changes that happen when the sun is fully obscured. It takes very little sunlight to seem normal, a lot of the sun can be covered and yet not be noticeable. But this event was different. We knew the darkness was coming but it was still not something you could prepare for. I suppose the first thing most people noticed as the second contact approached (a little after ten) was the drop in temperature. The air got quite cool, perhaps ten degrees lower on Fahrenheit’s scale. At this point I was rooting for the moon as I’m a shadow, shade, and cold-weather fellow. Anything that reduces the heat of the sun on me makes me happy.

I noticed that the shadows around us were strange, with distinct penumbras and other odd features. We played with our splayed fingers, casting multiple crescent-sun images on the ground at our feet. Straw hats were in abundance and their multiple tiny holes gave us a panoply of pinhole projections. Colors began to get weird, like someone had dialed back the saturation, the hues graying and smearing, the objects losing their crisp edges. It’s hard to describe. It wasn’t like anything I’d seen before and I’m not sure I can put words to it. It was eerie, I can tell you that, and everyone was feeling a strange mix of excitement and wonder, even a little fear. The sun is one of those things we count on everyday—when the moon comes to cover it up you think, just for a moment, that it might not reappear!

Soon totality was seconds away and all of us were riveted by what was happening. This is the “awe” part of the story. When we were in Mexico in 1991, on the beach at Mazatlan, our view of totality was covered by fog. The sun was higher in the sky as the eclipse was at noon and it was also a long one, almost seven minutes. That time we could see for miles along the beach and out over the Sea of Cortez. We saw what looked like an enormous thundercloud hurtling towards us. It was the shadow of the moon! The sky was very dark and sunset colors were on the horizon in all directions. Our site this time did not give us the same vantage, and the short duration (and clear sky!) let us focus on the sun itself. Darkness, perhaps to the level of nautical twilight, came quite suddenly, eliciting gasps and cheers.

And we got to experience those special totality-only features: the corona, the diamond ring, and Baily’s Beads. But that’s for the next post.

 

Shock and Awe (part one)

I spent several weeks trying to find the perfect eclipse site. I wanted to avoid the I-5 and US-97 corridors as I figured they would be the busiest. I figured the Oregon coast might have a chance of summer fog and did not want to go through that again. We got “fogged out” on the Mexican coast in 1991 during totality so I did not want to be anywhere near a marine environment. I wanted to stay in Oregon and not go further east so the drive times would be manageable. I thought US-395 would be our best bet and I noted that the centerline was just north of the little burg of Prairie City which was just a few miles along OR-26 from the 395 junction at Riley. The Malheur National Forest had many excellent roads and opportunities for dispersed camping, some within a few miles of the centerline. We found such at spot only five miles south of highway 26. It was a large primitive hunters’ camp with many flat areas and access to Clear Creek. We decided that was the place. I organized material for my friends who said they wanted to join us for the event and I made sure they knew exactly where to go as we would be out of cell range. Then I fretted for the remaining weeks, worrying that my secret spot would not be so secret and that hordes of campers would be jockeying for it and for my fallback spots as well.

We got there on Thursday the 17th. The eclipse was Monday morning the 21st. We encountered very little traffic on our drives and saw almost no one once we left Prairie City and headed into the Southern Blue Mountains. Shaking with anticipation, we pulled our camper off the highway and on to Clear Creek Road (FR 2635). We encountered no vehicles or people. At the sign for Looney Springs we saw our turnout and entered the camp area. For the first few seconds all seemed perfect. Too perfect. Sure enough, at the very end of the site, a big blue Sprinter van was parked. Someone else had found “my” spot!

I wasn’t upset. Really. I was too anxious and too excited to fret about “losing out” and I knew the campsite was large enough for us and them. They turned out to be very nice people from Seattle (a couple and their young daughter) and they had been there for five days. Five days! That’s planning. I had to tip my cap to them. Sure enough they had worked out that this place would be perfect for the eclipse using a similar logic to my own. How could I be upset about that? Not to mention that their drive (about 420 miles) was identical to mine. They were also expecting another party, or perhaps two, to join them.

The campsite was large enough, as I said, for several vehicles and had sufficient spacing and tree cover that more than one party could inhabit it comfortably. We grabbed the big flat area right the entrance on the opposite end from our neighbors. I pitched a tent that we didn’t intend to use in order to make the site look more full to discourage other campers. It worked—the few folks that pulled in to check it out (three other vehicles that day) saw us at one end and them at the other and decided to drive on to somewhere else.

The next day my buddy Otto arrived in his camper and we placed it strategically to block off some flat camping spots. Only a handful of others came by to check out the site, I talked with some and gave them suggestions about other places to go. No one was frantic or bothered, there was still time to get settled for the weekend and the crowds predicted had not yet materialized. After darkness fell and we were ready to hit the sack a vehicle came by and I had to bullshit a bit to discourage them from coming down the road any further. There were places to camp that would not have been too disruptive to us or our neighbors, but I was trying to save spots for at least three more vehicles that I expected the next day. Part of my line was to whine about “those people from Washington who came last week and took the spot I had scouted” and that generated some sympathy. The poor guy even said “sorry you lost your spot” as he reluctantly backed out and went exploring down the road. I did not feel guilty, there were plenty of places to camp all over the forest, denying him one place would not deny him a chance to experience the eclipse. The early bird gets the worm, man.

Saturday came and we had some early morning traffic on Road 2635 but our little ruse held out long enough for the rest of our group to arrive. We “circled the wagons” at our end of the camp and all was good. Our neighbor’s second party arrived as well (I saw Washington plates and just pointed when they pulled in) and by the evening all were settled and happy. There were ten of us in six vehicles, and one of my pals used the tent we had pitched. Our neighbors at the other end had two vehicles and six people. Several dozen yards separated our sites so we all felt good about the space. Over the weekend we visited with them and got to be quite friendly and we wound up sharing the same viewing space for the eclipse on Monday morning.

But that’s the next post. My first reaction? Shock. I still can’t believe I picked a perfect spot out of the the many possibilities. I did my homework and it paid off. And my friends all came and we had a great time together. We had no complications. We even had perfect weather for the event. But like I said that’s the next post. I’m still in shock, really. Things worked out so well it almost doesn’t seem real. I expect it will take a while for the shock to wear off. Then I can re-live the awe of the eclipse itself.

Stuff

It takes a hell of a lot of stuff for two humans to take a road trip to the mountains. Today I’m on stuff-management duty, otherwise known as packing. Sleeping bags, flashlights, shovel, saw, sun shower, camp chairs, binoculars, radio, rope, hammock, tent and stakes, mats and pads, tarp, crate, and bucket. That’s a few of the items. Plus water—LOTS of water—and a water filter. Food, of course, in bags and coolers. Ice is always the limiting factor of a camping trip. Beer, actually, is the limiting factor, but that’s what the ice is for! Clothing and footwear for five days at the campsite and a day on either side in the motel room. Not to mention reading and writing material, cards and games, the all-important maps, plus eclipse information. I’ve got a three-page checklist just for the stuff I keep in the camper and another page for the stuff I’m bringing. I know I’ll forget something, and I already can’t find something I know I’d like to have with me.

It’s a dizzying array of crap, I have to say. Who knew it was this hard to “get closer to nature?” Speaking of crap or the-call-of-nature we have to take care of our own needs. We won’t be in a developed campground so there won’t be a pit toilet. In the old days you dug a latrine. These days they frown on that what with all the impacts on well-visited sites. Veteran campers all know places where the toilet paper is still sitting, half-buried, trying to decompose. We’ve got a portable loo solution that works pretty well and we’ve used it successfully before. (Check out ‘wag-bags’ if you have not heard of them.) We’ll also have to pack out all our own trash. The campgrounds don’t have garbage bins and we only found one place in town (at the ranger station) that had a dumpster. We may have to drive home with all our refuse and debris, which I am prepared to do. It’s a small price to pay for a clean camp. We are actually bringing large heavy-duty bags to collect the existing trash that people left behind from previous visits to the spot. It’s disgusting what people leave behind and bewildering that they would do so. We figure we’ll pick up what we can, it will make our stay more pleasant, but we’ll leave the tied-off bags behind with a note for the Forest Service. Something like “you can thank us for cleaning up after the low-life assholes who were here before.”

But this is an adventure expedition and I’ve no time for negative thoughts. It could be cloudy or smoky or otherwise poor viewing. There could be a forest fire and road closures. The crush of visitors could overwhelm the infrastructure and cause foul-ups or delays. The small towns along the path could run out of gas! Lots of bad things can happen. I’ve decided this is one of those create-your-own-reality moments. I’m going to see—to visualize—a thoroughly successful outing. Easy travels, a happy camp, and great weather. The plan, such as it is, coming together just the way we want it to. That’s where I’m going to put my mental energy. I am banishing fear and doubt and putting courage and faith in their places.

We leave tomorrow and drive to Burns in our VW Eurovan camper. Thursday we are at the campsite. Friday a buddy is coming in his Eurovan. Saturday we expect one couple in their RV and another later in their VW bus. Sunday another buddy should arrive. A few other folks I know have threatened to crash the party and that would be just fine but I don’t expect them. Monday morning is the eclipse. If you think all these people coming to  Oregon this weekend is a cluster-fuck, just wait until they all try to leave afterwards! We will stay one more night in the camper and then it’s back to the motel in Burns for Tuesday and home on Wednesday. (Yes, we have reservations.)

There will be no campfires on this trip. This is an extremely high fire danger time all over the West and the USFS has already issued its restrictions. I’m guessing most people will comply, one hopes that eclipse-chasers have some outdoor savvy and that the idiots will mostly stay home. One hopes.

There’s a full tank of gas in the rig. We’ll stop for ice on the way out of town tomorrow morning. Thursday we’ll make one more stop for supplies in Burns, then head for the hills. Wish us luck!

Weather

Wildfire smoke didn’t blot out the sun last week but it did give the moon a red-orange cast. It’s part of life here in the high country—hazy skies from forest fires. Some are local, some are hundreds of miles away. We get smoke in the valleys every summer and it matters not where it comes from. All you can hope for is the wind to shift and push the smoke somewhere else.

Saturday evening altocumulus clouds emerged from the southwest and gradually covered the entire sky. Sunday morning and early afternoon were overcast and eventually rain clouds formed and we got evening thundershowers. It was nice to get a break from the smoke and be able to open the windows and let in some fresh—well, fresher—air. This morning the altocumulus were back and they looked like a big smear of frosting overhead and to the east. Sure enough the sun was obscured until almost noon.

Right now it is clear overhead but a big heap of clouds still covers the eastern sky. The smoke is back, not as bad, but persisting. This is pretty common stuff all over the West. Mostly summer days are clear, dry, and hot. Rain, other than from thundershowers, is unusual this time of year. If an eclipse were happening in two weeks over Yreka I would tell people that odds are excellent there will be bright blue skies.

We are going north in two weeks to the zone of totality in eastern Oregon. It’s a lot like here. Most of the time it is clear and dry. Sometimes there are thundershowers, but they are typically in the afternoon and mornings are usually could-free. Wildfires are a problem as they spew their smoke all about. It’s not so bad when you can get indoors and stay indoors. But we will be camping for at least five days. Right now the skies over where we want to go are described as “hazy” from smoke. Overcast skies and possible thunder, lightning, and rainfall are expected over the next few days. The National Weather Service graphics for this week up there look just like they did for last week down here.

What can you do? Unless a forest fire closes the roads or otherwise impacts local services we are still going to our planned camping spot. It’s a risk, I know. We could get “skunked” for the eclipse. Our viewing site could be overcast or fouled by smoke. If today, a Monday, is just like Monday the 21st, we’ll be watching the eclipse behind clouds!

But I don’t think that’s going to happen. It’s mostly hot, dry, sunny, and clear in this part of the world during this time of the year. So I’m banking on that. But if we do suffer the misfortune of totality being blocked by some other natural phenomenon then we’ll just have to make the best of it. Sure, I’ll be disappointed. And I’ll pursue my next chance to see a total eclipse with much more vigor and enthusiasm.

But chasing the moon’s shadow is an adventure. The journey is as much a part of it as the destination. The eclipse, in all its phases, is only a small percentage of the time that we will be out and about. So, we expect to enjoy ourselves regardless of the outcome. The part of the country we will be visiting is both beautiful and bizarre and I know I’ve a lot more to learn about it. It’s always fun being in the mountains. Our site is forested and there’s a big creek running close by. It’s remote, but accessible. We don’t have to go four-wheeling or ford a stream or winch ourselves out of some gully. So there’s little or no danger. It should be mostly relaxing. No phone, no computer, no TV. Lots of hammock time. The only uncertainties that matter are how long the ice and beer will last!

We are ready for whatever Mother Nature throws at us. I’m expecting a fastball right down the middle but if I get a curve or a change-up I’ll still put a good swing on it.

Saros

The moon is a better timekeeper than the sun. At least as far as eclipses go. A solar eclipse can only occur on a new moon. So the NEXT solar eclipse has to be an integral multiple of new moons away. The sun can only be eclipsed when the moon it at its orbital nodes. And the type of eclipse—be it total or annular—depends on the moon-earth distance. The moon appears about 11% larger at its nearest distance to us in its elliptical orbit—this is called lunar perigee. All of these events are on different time scales.

The moon moves from new moon to new moon (the synodic month) in about 29.5 days. The moon moves from node to node (the nodical  or draconic month) in about 27.2 days, and the moon moves from perigee to perigee (the anomalistic month) in about 27.6 days. It turns out that 223 synodic months, 242 nodical months, and 239 anomalistic months are all about 6,585 days. That’s about 18 solar years.

Ancient astronomers were aware of this eclipse cycle and the word that’s come down from antiquity to describe it is saros. The saros is a group of related eclipses, all about 18 years apart. Eclipses separated by a saros have similar characteristics. They have the same geometry: they occur on the same node (ascending or descending), at approximately the same time of year, and when the moon is at the same distance from the earth. The 6585 days is really more like 6585-1/3 days, so every third saros (54 years) the eclipse is roughly on the same place on the earth. That 1/3 day means each eclipse is shifted about 1/3 of the way around the globe, so it takes three cycles to get back to the same area.

The saros was first noticed for lunar eclipses because a lunar eclipse is visible over half the earth at the same time. Ancient peoples did not have the travel and communication capabilities that we have today so they may have predicted a solar eclipse but were unable to know if it occurred. It may have taken place thousands of miles away!

Eclipses happen every year so there are multiple saros series going on at the same time. A saros lasts over a thousand years and is comprised of dozens of eclipses. The upcoming total solar eclipse is part of the same series as the 11 July 1999 total solar eclipse that was seen across Europe and was possibly the most-viewed eclipse in all human history. Eclipses in a saros begin with the most fleeting of partial contacts, pass through the annular/hybrid phases, peak with total eclipses, and then fade back again. They reflect, in depth and degree of the eclipse event, all the motions that have to coincide and overlap for us to be able to experience an eclipse. People mostly live in a narrow temperate-equatorial band on the planet, but the shadow path of a solar eclipse can brush the huge polar regions or plunge miles of empty oceans into darkness.

Humans had to view, record, and study eclipses for generations in order to learn about patterns like the saros. Take a look at the path of the upcoming 21 August 2017 total solar eclipse:

path17

Now take a look at the path of another eclipse in the same saros series, this one 54 years away on 23 September 2071:

path71

I don’t think I’m going to make it to 2071. Adding 54 to my current age pushes me well past the century mark. But it is interesting to see that the third eclipse in the saros after this one is pretty close to the same part of the earth’s surface. (GE means Greatest Eclipse where totality is longest.) The 02 September 2035 event passes over China, South Korea, and Japan. The next one after that on 12 September 2053 sweeps across North Africa and the Arabian peninsula. Finally the third in the group returns to the Americas. It amazes me that astrologers and astronomers of yore were able to predict eclipses, or at least create eclipse-predicting algorithms, of such precision. Obviously they lived in sufficiently stable societies that could preserve the records and educate succeeding generations about the measuring and calculating methods needed.

Fortunately for those who would like to view a total solar eclipse we have all these overlapping saros series to choose from. We don’t have to wait 18, 36, or 54 years to see another one. In fact, there is a total eclipse in July of 2019 if you want to go to Chile or Argentina. Hmm, it’s winter then, maybe I can go skiing! Actually I’m trying not to think too far past the trip to see THIS eclipse. But there is another one I want to plan for, it’s in 2024 when I’ll be almost Medicare age:

2024texas

Anyone up for a trip to Austin, Texas? I hear it’s a really fun town.

Syzygy

Partridge says that this word comes from the Greek zeugnumi which means “yoke” or “join” and that gives us the modern sense of the Late Latin word that emerged of “linked” or “paired together.” In astronomy a syzygy is a conjunction of two bodies, actually three, because you have to include the body the observer is on. In the case of an eclipse we have the lining up of Sun, Moon, and Earth for the solar variety; and Sun, Earth, and Moon for the lunar variety.

This happens monthly. Perhaps you had one of those teachers who tried to show you moon phases using a flashlight and a tennis ball. When the moon is between the earth and the sun the near side is in shadow and hence can’t be seen. We call this a new moon. When the earth is between the moon and the sun the near side is fully sunlit. We call this a full moon. But the moon and earth are offset slightly, otherwise we would have a solar or lunar eclipse every two weeks. A solar eclipse can only occur during the new moon. A lunar eclipse can only occur during the full moon.

But we only get a handful of eclipses per year, not two per month. This is due to the inclination of the moon’s orbit about the earth. The earth and other planets lie roughly in the same plane if looked at from outside the solar system. That is why planets appear in the sky very close to the sun’s path. The apparent path of our sun through the sky is called the ecliptic. If we project this line infinitely into space in all directions we get the ecliptic plane. The orbits of our celestial neighbors lie along this imaginary plane:

Ecliptic_plane_side_view

The moon, though, has its own ideas. Its orbital plane is tilted about five degrees relative to the ecliptic. Here’s a way to visualize that:

Moon-inclination

As you can see, the moon only puts itself in a blocking position when it is at the so-called nodes where the two planes intersect. Thus eclipses can only happen then, when the moon is at those points. One is called the ascending node and the other the descending node reflecting our earth-bound sense of up and down. Here’s another look:

MoonEarthShadowsNodes

Most of the time the moon is “above” or “below” the ecliptic and cannot eclipse or block the sun. Thus most new moons don’t produce solar eclipses. The same with lunar eclipses—most of the time the moon is not in the same plane as the earth and so the earth cannot cast its shadow on the full moon’s face. In order to have eclipses the moon must be ascending or descending through one of its two orbital nodes.

The moon is at or near its nodes twice per year. So we ought to have two solar and two lunar eclipses per year. And we do, usually. But it’s not that simple. The nodes are not fixed. This “regression of the nodes” is due to precession, a phenomenon that all non-uniform spinning bodies exhibit. Think of the wobbling of a spinning top—that’s precession about the spin axis. If the nodes were fixed, eclipses would happen at the same two times per year, half a year (six lunations) apart. But they don’t, they can happen during any month. This is because the eclipse half-year is about nine days short of half a solar year and thus two such eclipse half-years are about 18 days short of a full solar year. Consequently conditions for an eclipse (moon at the nodes) move “backward” through the calendar. This was known to the ancients and formed the basis of some of the first eclipse predictions.

This month we will have a syzygy of epic grandeur, that of a total solar eclipse. The moon and the sun and the earth are all in the right places at the right time and observers in the United States will get a chance to experience the turning of day into night. Although the path of totality restricts the viewing of the total eclipse to a narrow band, all fifty states will experience a partial eclipse somewhere within their borders.