Chasing an Evanescent Cosmos

by Shane L. Larson

Humans have looked at the sky for as long as we have inhabited our small, planetary home. For 40,000 generations we have basked in the warmth of our mother the Sun, watched the Moon hurtle across the sky in an ever-changing succession of phases, and fallen asleep as the stars slowly wheeled overhead in their familiar and comforting constellations.

The familiar shape of Orion, with it’s distinct three belt stars, graces the winter skies in the Northern Hemisphere, as it has for all of recorded human history. [Image: M. Larson (iphone!)]

Our perception of the Cosmos is one of steady and dependable clockwork, changing slowly on the timescales of days or months as the patterns of the constellations shift in slow progression with the seasons, but returning to their familiar places in the sky a year later with dependability. Yes, the sky changes, but never dramatically. On the timescale of human history, it is in many ways a dependable constant. The skies I see tonight are just like the skies my grandparents knew, and their grandparents before them.

Comet NEOWSIE (C/2020 F3) graced the skies of Earth unexpectedly in the summer of 2020, during the Coronavirus Pandemic. Comets are one example of rare, transitory experiences humans can have with astronomy. [Image: S. Larson]

There are, of course, events where the sky changes in remarkable and dramatic ways. A bright comet will suddenly brighten and be visible for a few weeks. A distant star will gasp its last breath, and blow off its atmosphere in a cataclysmic nuclear explosion. And sometimes, the Earth and the Moon in their eternal dance around the Sun, will toss their shadows on one another to give us lunar eclipses and solar eclipses.

Any of these events are dramatic enough to have caused consternation in the course of human history, particularly in the days before humans had begun to dissect the patterns of Nature and use that knowledge to better understand our place and purpose in the Cosmos. 

But you and I live in the future. Many before us, both before the birth of modern science and after, have learned the patterns of the sky to such precision that we can predict when the Moon will spin into the line of sight between us and the Sun, and shed a narrow band of shadow on the Earth — a solar eclipse.

A total solar eclipse occurs when the Moon passes between the Earth and Sun. The Moon’s shadow races across the surface of the Earth, blotting out the Sun for those who stand under the racing shadow. [Illustration by S. Larson]

One such moment was this past week, on 8 April 2024. A total solar eclipse was due to roll ashore on the southern coast of Mexico near Mazatlan, then roll northward into Texas, and track along to the northeast through the midwest of the United States, before passing over the eastern seaboard of Canada and out into the north Atlantic.

The path of the 8 April 2024 total solar eclipse, moving from the southwest to the northeast over North America. The centerline is marked in red; everyone in the grey band was within the path of totality.  [Illustration: S. Larson]

Many of us had been fortunate to see a previous solar eclipse that crossed North America from Oregon to South Carolina in August 2017. I saw that eclipse, and wrote about it both the day before, and the day after!  The response that many of us had was a common one that is almost always expressed as a question: “When can I see another one?!”

The April 2024 eclipse was a remarkable opportunity for many of us who had asked that question. We could travel to the centerline pretty easily, and the duration of the eclipse was going to be longer than in 2017 — almost four and a half minutes, longer than the two minutes and 27 seconds from 2017. It doesn’t sound like much, but some deep part of our soul knows that longer matters. 

That’s because standing in the shadow of the Moon, your mind knows something remarkable is happening, but it is far outside your everyday experience. It is difficult to grasp and hold on to and put into words. You have a lifetime of experience with the Sun — it is steady and reliable and is the same every time you see it. You never see it do something unusual, and your brain knows it.  When the Sun disappears, and is replaced by an inky spot of darkness in the sky, you feel it deep down in your soul, and you want to hold on to that memory and feeling — it is precious.

A message from a friend of mine about the experience of standing in the utter darkness of totality during a solar eclipse. [Shown with permission]

The impetus for many adventures are some friends who are keen on the experience, but want to share it with someone else. Sharing gives you the opportunity to talk afterward, a chance to draw out the gossamer threads and cement those few magical moments in your memory. It was no different for us. It began with a simple plan that was just a few people, but rapidly swelled until we were just a tidge more than 30. Old and young, children and parents, millenials and old hippies. A motley crew that was a skein of linked acquaintances, soon to become a band of friends who experienced one of Nature’s great spectacles together.

But from where? 

A site was selected based on historical weather averages. April is springtime in North America. Along wide swaths of the eclipse track that means turbulent weather, spring rains, occasional snows, and cloudy days as Spring tries to assert herself over Winter. For many, this is a once-in-a-lifetime event, so you play the odds. We, like many others, descended on Texas.  

The historical cloud cover in North America on 8 April, generated from 43 years of data.

We settled on an RV park in the Texas hill country, west of San Antonio, on the banks of the Rio Frio just outside Garner State Park. If you were to pick an awesome site for the eclipse, this was the one.  Right along the centerline. 

We converged on Saturday, two days before the eclipse, and the forecast was looking grim. Predictions for Eclipse Day on Monday were for 90% clouds, possible rain. Quite against all odds, the northern reaches of the eclipse track, historically cloudy in April, were predicting clear skies! 

The cloud forecast from the Washington Post on 5 April, just three days before the eclipse!

The agony of statistics!

There was a lot of hand-wringing around the firepit that evening, and long into the next day. What should we do? Should we stay put? Should we try and drive to clear skies? It was predicted only 50% cloudy three hours north — were those odds we liked? It was predicted only 10% cloudy nine hours north in Arkansas? Did we want to do 18 hours on the road (there and back)? What if we got caught in traffic and missed everything? 

In the end, we decided to stay put. The magic of the moment was being together, and there was still an experience to be had. Even on a cloudy day, it was still going to get unnaturally dark. Maybe we’d get lucky, and maybe we wouldn’t. 

Eclipse day dawned cloudy, as predicted. But Nature is a tease. The clouds were layered, and moving in different directions, and every now and then there was a fabulously clear blue spot, sometimes in front of the Sun — shadows would sharpen and we could feel the warmth on our faces, tantalizing moments of hope.

We had an eclipse talk before the event, to answer questions and help everyone feel like they were prepared. [Image: M. Mesel]

We gathered about an hour-and-a-half before the eclipse, together with more than a hundred people camping at the RV park, and talked about what eclipses are, and how and why they happen. We reviewed how to see the event safely, and made sure everyone had eclipse glasses. And lastly, we talked about what to expect and experience, clouds or not.

Then we all dragged our camping chairs to a wide grassy spot, and waited. The clouds were thick, but there were gaps that dashed across the sky, and moments where we could make the Sun out through the clouds, and other times it stood in the clear sky.  Eclipse glasses pressed tightly to our eyes, we could see the Moon had started the eclipse run and was dutifully eating more and more out of the right side of the Sun.  

Our eclipse expedition crew, waiting for totality on 8 April 2024.

First we could see just a little tentative nibble, and then a cloud would dash in front and obscure the view. But just a tiny bit later, the Sun would emerge again, and joyous cries of amazement would rise up from the sea of chairs as people witnessed the Sun slowly disappear.

Our views of the ongoing eclipse through eclipse glasses (left) and through the cloud layer (right). [Images: S. Larson]

As the last few moments approached, the Sun had become a thin crescent, a mimic of how the Moon sometimes appears. But we could see dark skies ahead. A massively thick cloud was rolling our way, with all possible ill-timing. It was roiling and dark, no breaks to be seen anywhere. It moved in front of the Sun, barely a minute before totality began. 

We could feel the chill on our skin. Was it the ever fading light from the Sun, now obscured? Or was it the cold embrace of weather carried on our nemesis, The Cloud? 

The Cloud, the nemesis of our expedition. [Image: S. Larson]

Totality was approaching, not a break to be seen. Dutifully linked to the digital world, we had GPS times on our phones, and precise knowledge of when Totality should occur, now on the far side of The Cloud. About 20 seconds before Totality, a swelling countdown rolled across the sea of people…

20 – 19 – 18 – 17 – 16 – 15 – 14 – 13 – 12 – 11 – 10 – 9 – 8 – 7 – 6 – 5 – 4 – 3 – 2 – 1 – 0!

Right on cue, the skies descended to darkness. What was moments before a cloudy day illuminated by dim filtered sunlight, plunged into darkness. People erupted in applause and cries of joy.

Totality descended on us, right on schedule. Dark as night, but it swept over us in mere seconds. [Image: S. Larson]

We strained to see a break in the clouds, but there were none. We looked hither and thither, drinking in the darkness around us. Everyone around was atwitter with things they were noticing, calling them out to everyone else, so we could all share in the moment.

The automatic streetlight at the entrance to the RV park came on. The birds got quiet. Bats started flitting overhead. The humans were still taking pictures. We craned our heads around, and could see that near the horizons, it was still light — realms outside totality, where the light still fell. But where we were, it was dark.

For 4 minutes and 28 seconds, we were in the Shadow of the Moon.

We stood in totality, and could not see the Sun nor the gossamer aura of its corona. But still we stood in the darkness in wonder. [Image: S. Larson]

Together, we basked in  a few moments of darkness. For a few precious minutes, the world did something completely unusual — something our rational minds could process and understand, but something deep down we knew was unusual and different and ephemeral. To share it with friends is an essential part of experiencing the event as humans, so that you can together process and understand what the event emotes within us.

And then, it was over as quickly as it had started. The light rose, and we were once again sitting in a grassy patch on a cloudy spring day, the soft light of the Sun filtered by clouds above. Except we were different. For a long while, we stayed put. Comparing experiences, taking selfies, laughing, and sharing the joy.

Post-eclipse reverie, with friends and song. [Image: Z. Mesel]

Later that night, after normal darkness fell, we sat in a great circle in the middle of the camp, and engaged in another time-honored human tradition of sharing songs and music. Songs of the sky and moonshadows, songs of sharing and family, and songs of learning and travelling home. Picked out on the strings of a guitar, with some voices well-trained and other voices simply happy to be together, we bonded just a little bit more to ensure that we each remembered how special that day had been.

For some of us, there will be other eclipses. Perhaps with clouds, perhaps without. But for all of us, there were four and half ephemeral minutes of darkness that we shared, on the middle of a cloudy day in southern Texas.

Total Eclipse: On the Far Side of Totality

by Shane L. Larson

How do you describe the indescribable?

I’ve been a skywatcher for more than half of the years of my life. I’ve literally spent thousands of hours with my telescope, watching the sky, making sketches to remind me of what I saw, keeping notes about appearances, and making lists of favorites to look at again. I’ve seen the rings of Saturn, and the Great Red Spot. I’ve observed supernovae, seen comets stretch across the sky, and watched the aurora borealis storm overhead. I’ve seen a transit of Venus, a once in a lifetime event.

But nothing prepared me for what I saw on Monday, standing on a hillside in Casper, Wyoming. For a brief two minutes, the Moon covered the Sun in the total solar eclipse of 21 August 2017.

Our morning started early, arriving at 5:30am. We weren’t tired now; that was later, after the eclipse was over and the adrenaline wore off! [Image by S. Larson]

My family and I, together with eight of our long time friends, set up on the grounds of Casper College, together with a vast number of our amateur astronomy colleagues who had been in Casper for the  2017 Astrocon Convention. The eclipse was due to start at 11:42am MDT, so we arrived on site early to set up: 5:30am!

By 6:00am the horizon began to glow with the scarlet tones of sunrise, and at 6:19 the Sun rose slowly over the distant horizon. We couldn’t see it, but we knew the Moon was right there too, steadfastly churning along its orbit, its shadow streaming through space, like a needle waiting to poke the Earth.

Sunrise and the appearance of the Sun. This was the moment when we knew in our cores that we were going to witness the total solar eclipse, without fail. [Images by S. Larson]

We had a row of tripods — some set up to image, some with binoculars, some with telescopes, all with solar filters. Every one of us had eclipse glasses that we constantly watched the Sun with, waiting for the first moment when the Moon would slowly begin to move between us and the Sun.

Eclipse glasses. This is pretty much how we looked all morning once the Sun came up. [Images by S. Larson]

We knew we were in for a wait, as the eclipse wasn’t due to start until 10:22am, and totality wouldn’t start until 11:42:42am!  So we paced back and forth restlessly. We took selfies with each other. We joked around. We checked the traffic, wondering if the slug of red between Denver and Cheyenne would make it to the path of totality on time. We played games. We make cookie art to track the eclipse.  You know — normal nerd herd stuff.

Just passing the time, tracking the eclipse by making art with cookies. [Images by S. Palen (L) and K. Larson (R)]

The folks down the row from us had an old Astroscan telescope, outfitted with a DIY homebuilt funnel projection screen (here are instructions from NASA) that was ideal for letting everyone see what was going on, and for taking pictures of what the Sun looked like during the partial phase of the eclipse. There was an excellent group of sunspots arcing across the middle of the Sun, and another small group down near the limb.

Our observing neighbors had an Astroscan fitted with an observing funnel (top left). The Sun had some great sunspot groups that day (top right). The screen made it easy to see the progression of the partial eclipse (lower 4 photos). [Images by S. Larson]

We also watched the progression with pinhole projections, looking at the shadows of anything with tiny holes in it. Every one showed a dazzling array of small crescent Suns, slowly being eaten by the Moon.

“Pinhole projection” is an easy way to enjoy the progress of the partial eclipse. We used a cheese grater and a Ritz cracker (Left), and also watched the dappled light in the shadows of the trees (Center and Right). [Images by S. Larson]

And then, we knew the moment was coming. We were all watching. The light got really flat and dim all around. It started getting darker, quickly. At the last moment before the Sun was completely covered, the left side burst out in a brilliant flare we call “the diamond ring.” Simultaneously, I remember the right side illuminated with a sharp edged circle, rimming the edge of the Moon. And then the Sun was gone. The total solar eclipse had begun.

We were on that hillside with maybe a thousand other people, and it erupted with cheering, screaming, whistling, and shouts of joy and astonishment. There are a million photos on the web of these precious few moments of darkness, suffused with the effervescent, gossamer glow of the Sun’s corona. They are fantastic pictures, but none of them capture everything I remember now in my mind’s eye.

The shapes are right, with two streaming horns pointing up and to the right and one down and to the left. Many of them show the bright red spot of a solar prominence peeking out from behind the limb of the Moon. A few that have been processed show the amazing interlaced structure in the corona.  My lifelong observing buddy (Mike Murray) and I agreed that the word we would have used to describe the view is translucent.

The best representation of the color I remember is the translucent blue of this plastic, in the middle of this cup (above the dark blue in the bottom). [Image by S. Larson]

But even more what I remember was it did not look white to me. It looked kind of pale blue. I’ve observed a lot of objects in the sky, and the diaphanous light of the corona reminded me all the world of the color in some planetary nebulae or bright, hot stars. I’ve been struggling to find something this same color, or a better way to describe the color. That night at dinner with my wife and daughter, I stumbled on a drinking cup whose translucent plastic comes close to being right.

But in the end, all of these attempts to describe the event fall far short of what I remember. There is nothing quite so profound as standing in the shadow of the eclipse, and it’s over before you even know it. The memory, while powerful, feels slippery — I want to cement it somehow, because it would be horrible to forget what I felt in those few moments. I immediately wrote down my notes (images of them are included at the bottom of this post), but they are pale by comparison to what is in my head. I recently met a psychologist named Kate Russo who studies and writes books about people who watch and chase solar eclipses. She names this feeling we all have about our solar eclipse experiences: ineffable.

I had resolved not to make a considerable effort to set up equipment and try to photograph the event. It was my first total solar eclipse, and I didn’t want to be distracted by the equipment and miss it. I did throw my phone up and snap a couple of pictures, but no more. It is small and grainy, but it looks like an eclipsed Sun. Knowing how fast it went, and how much more I wanted to just LOOK at it, I’m not sure I’ll ever go the route of taking pictures.

Totality. The best photo my iPhone could take? I dunno — the best photo I could take for the time I was willing to spend looking at the screen and not the eclipse! [Image by S. Larson]

In the day since the eclipse, I’ve seen many fantastic pictures of the eclipse from friends. Every single one, no matter how technically awesome it was shot, is spectacular. Why? Because they capture that ineffable moment that every person was trying to capture for their mental review later. Each one is a tiny memento, small and bright, captured on silicon and in digital pixels, that reminds the photographer what it was like to stand, just for a moment, in the shadow of the Moon.

During that same day, I’ve been looking at all those pictures, scrolling through what I had on my phone, talking with my wife and daughter, and talking with friends on social media. Always trying to cement the experience in my head. But quite by chance, I did something that I am thankful for. I set up my video camera (a little Sony Handycam) pointed at the Sun for about a half hour around totality. I didn’t know what it would capture on video, but what I wanted it for was audio.

I couldn’t have asked for any better record and memory of the event than that audio record. I’ve listened to it many times now, and each time I’m transported back to that moment on a hillside in Casper, surrounded by friends and a thousand other of my fellow humans, gazing up at the sky in stupified awe. Nothing shocks my memory with better clarity than this audio. A good friend of mine who is a psychologist in Colorado told me listening to this audio fired off mirror neurons in her brain. Mirror neurons are neural responses in your brain that respond from observing something going on as if you were there doing it yourself.

So I close this meager recounting of my experience with the audio of me and my friends, immersed in the moment. I hope you glean from it some of the joy and awe and ineffable wonder that we all felt standing there for those two minutes, and reminds you of similar moments you may have experienced and shared.

Mirabilis sole!

————————————-

I’m a fastidious note-taker. Here are images of my notes I took in the hours immediately after the event.

Notes Page 1. When big things happen, I often have my friends who are with me sign my observing log, so I remember they were there. [Image S. Larson]

Notes page 2. [Image by S. Larson]

Notes page 3. [Image by S. Larson]

Notes page 4. [Image by S. Larson]

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Here is the previous post I wrote leading up to this solar eclipse: Total Solar Eclipse: Anticipation (20 Aug 2017)

Here is a previous post I wrote about the astronomy behind a total solar eclipse: Stand in the Shadow of the Moon (25 Aug 2014)

Total Eclipse: Anticipation

by Shane L. Larson

There are many amazing sky events that happen to pique the interest of amateur astronomers. I went out in 1994 and watched the shattered fragments of Comet Shoemaker-Levy 9 plummet into Jupiter, scarring the giant planet’s atmosphere with dark swaths of discolor larger than the Earth.  In 2001, I sat up all night in a campground in Cloverdale, CA counting Leonid meteors in one of the best storms in recent memory.  I’ve sat cross-legged in an empty field just off of I-80 in Kearney, Nebraska, peering through a small spotting scope at a bright supernova in spiral galaxy M101, just detectable at the edge of my vision.

All of these events, and many more like them, are important and interesting to those of us who forego sleep regularly to stand out under the dark and look at the night sky. There is something exhilarating about hunting for a few photons that just happened to cross the great gulf of space and fall to Earth at the exact moment I was looking up. But it is not for everyone.

Total solar eclipse progression. [Image by Justin Ng]

Tomorrow, people living in North America will be witness to one of the most profound spectacles the Cosmos has to offer: an eclipse of the Sun. Everyone in North America, if their skies are clear enough, will be able to see something. For a couple of hours, on 21 August 2017, the Moon will pass between Earth and the Sun, partially blocking the view of the Sun.  If you stand along a pathway roughly 70 miles wide that stretches from Oregon to South Carolina, there will be about two minutes during your day when the Sun is completely hidden behind the Moon. The skies will get dark, and the day will feel cooler — it will, for two minutes of your day, feel like night. You are standing in the shadow of the Moon.

Standing in the shadow of a total solar eclipse is one of the most profound personal experiences with the Cosmos you can have. Those comets and supernovas I mentioned before are cool to witness, particularly if you know what you’re looking at and can be reflective about the profound distances those little photons of light have travelled.  But a total solar eclipse is different.

Our lives, whether we think about it or not, are ruled by the Sun.

It is impossible to be unaware that our lives on Earth are acutely connected to the Sun. We bask in its warmth, play in its dappled rays, and soak up its energy every day. But during totality it will completely vanish from your view — you can’t help but notice that it is just gone from its normal place in the sky and our lives. People struggle to explain the ephemeral and visceral reaction they have to the sight and raw beauty of these singularly moving events.

For this eclipse, there are more people who live near the path of totality, and more people that can get to the path of totality, than possibly any eclipse in history. The eclipse tomorrow will be one of the most viewed natural events in the history of our civilization.  You owe it to yourself, no matter where you are, to at least take a moment and #lookUp and share in the spectacle with your fellow humans.

What can you expect?  In your local media you should be able to find the times when you can see something going on in your area. There are great online tools; I like the Astronomy Magazine widget (here is the link!) that gives you eclipse times for any place you click on a map.

For most people, the partial eclipse will last a couple of hours, and any time during those couple of hours you can see the Sun looking like a cookie with a bite taken out of it.

Eclipse glasses are a cheap, convenient, and easy way to be #EquippedToEclipse. [Image by Adler Planetarium]

During the partial eclipse, the Sun will be too bright to look at! You wouldn’t normally look at the bright Sun, so you won’t feel compelled to now!  If you want to see the partial eclipse, then a pair of the ubiquitous eclipse glasses are needed to look at the partially covered Sun. There has been a lot of concern about safety of glasses found from various outlets and vendors, and the debacle with Amazon has not helped. The American Astronomical Society (our professional society) has produced a page with vetted sources of glasses — this page includes BRANDS OF GLASSES that are certified, and also has a list of stores (e.g. Lowes etc) that are selling glasses that are safe for the event. That page is here: https://eclipse.aas.org/resources/solar-filters

But what if you never got some glasses, you lost your glasses, or your dog ate your glasses?  Well never fear. Eclipse glasses are not the only way to enjoy this!

The dappled light streaming between the leaves on trees will make thousands of little eclipse images in the shadows. Watch for them! [Image by John Armstrong]

During the partial eclipse you can see what is going on by projection. Look in the shadows of trees – the dappled light will be mini eclipses. Hold up a spaghetti colander – the light in the shadow will be mini eclipses. Hold up a Ritz cracker – light thru the holes will show mini eclipses in the shadow. People have used straw hats and lacy sweater sleeves! Be creative, and enjoy the eclipse.

Also — the partial phase lasts almost 3 hours. If your friend next to you has eclipse glasses, you can share. 🙂

My image of the Ring of Fire on 20 May 2012, taken with my iPhone held up to a filtered telescope. [Image by S. L. Larson]

I have never seen a total solar eclipse (though I had the good fortune to observe the Annular Solar Eclipse on 20 May 2012 in Cedar City, UT). We watched that event in a city park, surrounded by maybe a thousand residents of the town who were watching with us. Everyone had eclipse glasses, there were projections with spoons and meshes, and we had our filtered telescopes there and talked to hundreds of people who just happened to be walking by and took a look.

What I remember most, was at maximum when there was a perfect ring of fire visible through your glasses in the sky, there was a tremendous swelling mass of cheering and shouting and joy. There was no big sporting event, no blockbuster music stars inciting that reaction.

Just a thousand humans, witnessing together one of the most beautiful spectacles the Cosmos has to offer, unable to control their joy and emotions.  It was awesome to be standing there shoulder to shoulder in that crowd.

My daughter was in kindergarten when she saw the annular eclipse in 2012, and still remembers it. Now she’s going into 6th grade, and I think she’s going to become an eclipse chaser. 🙂 [Image by S. L. Larson]

I wanted to just get a few thoughts down here about what it is like leading up to the event, musing on how it will feel on the far side.  Will I feel compelled to travel the world for the next possible one I could view (2 July 2019, over the Southern Pacific, Chile and Argentina)? Will I become an eclipse chaser, racking up 10 or 20 total solar eclipses over my lifetime? Or will I just be like, “cool, put it in the log book; when’s the next cool something to happen?” I honestly don’t know.  But we’re about to find out.

Catch you on the other side of totality….

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Here is a previous article I wrote about the astronomy behind a total solar eclipse: Stand in the Shadow of the Moon (25 Aug 2014)

Stand in the Shadow of the Moon

By Shane L. Larson

Imagine a late August morning on the Oregon seashore. Around 10 AM the morning breezes whip your hair around your face. You can hear the crying call of gulls over the relentless crash of waves on the packed sand of the beach. Rocky hills and cliffs rise up to the north and south, giving way to green forests that race eastward toward the seaside town of Lincoln Beach, Oregon.  The water is cold here — the currents flow south along the Oregon coast after skirting the shores of Alaska far to the north; you’re thankful for the warm Sun on your back, drying you off after a brave leap into the chilly water.

Photograph of the 1919 eclipse captured by Sir Arthur Stanley Eddington on an expedition to test general relativity. With the face of the Sun occulted, the outer corona is visible to the naked eye.

But the warm respite provided by the Sun on this morning is soon interrupted, because to the west, beyond the distant horizon, a vast darkness is racing across the Pacific toward you. Around 10:15am, you will experience the darkness  yourself — it will race inland across the Pacific coast, and for 1 minute and 58 seconds, the Sun will vanish, its once brilliant disk replaced by an inky black orb surrounded by ghostly streamers, silently dancing in the sky where moments before you had seen blue skies and that seemingly eternal friend that has warmed and comforted you since childhood. The skies will darken as if night had fallen, the temperature will noticeably drop, and nighttime birds and insects will suddenly become active.  You will be experiencing one of the great spectacles of Nature: a total solar eclipse. But your two minutes of darkness will pass quicker than you can imagine, and the Sun will suddenly appear, as the darkness that you once stood in races eastward toward inland North America.

You will be able to experience this, almost exactly as I’ve described it, on 21 August 2017, when a total solar eclipse crosses the United States from Oregon to South Carolina. It will be the first total solar eclipse visible in the United States since 1979. This is your chance to see what I’ve described, with one notable exception: you will probably be surrounded by thousands of others who have traveled to the centerline to witness this spectacle and stand in the darkness. Never-the-less, it is well worth the experience, and I suggest you start making travel plans now!

What is a total solar eclipse? A total solar eclipse happens when the Moon passes between the Earth and the Sun. When that happens, along a thin path on the Earth, you can see the Sun completely covered by the Moon. For a couple of minutes along the eclipse line, day becomes night — the bright face of the Sun is hidden, making a halo of blazing streamers known as the corona.

A total solar eclipse occurs when the Moon passes between the Earth and Sun. The Moon’s shadow races across the surface of the Earth, blotting out the Sun for those who stand under the racing shadow. [Illustration by S. Larson]

Because the Earth is spinning and the Moon is moving in its orbit, the shadow is moving along, just like your shadow does when you are moving. The combined motion of the Moon in its orbit with the spin of the Earth means the shadow will rocket across the North American continent at 1600 miles per hour. From the moment total shadow first occurs on the Oregon coast (10:15:58am MDT) to the last moment of total darkness on the South Carolina coast (2:49:01pm EDT), only 1 hour 33 minutes and 3 seconds will have elapsed. After that, the event will be all over for those of us rooted to the land.

Track of the 21 Aug 2017 total solar eclipse across North America. The red line is the centerline; anyone standing between the blue lines at the right time will witness the entire Sun being hidden by the Moon. Map from NASA’s eclipse page.

Can you imagine what it was like, hundreds or thousands of years ago, before we had blogs and newspapers and radios to tell us all of upcoming astronomical marvels? The astronomical know-how to predict (and explain!) eclipses has been around for sometime, understood in the Middle Ages by priests and astrologers and in more recent eras by scientists and astronomers. But there was no way (or reason?) to tell anyone what was going to occur. If you and I were peasants in medieval times, we should have been dumbstruck with terror to witness the Sun being eclipsed, yet today we know it is a great marvel and wonder to behold.

The fact that we have such beautiful eclipses on Earth is a matter of complete, cosmic happenstance. It just so happens that the Moon and the Sun appear to be about the same size in the sky, so the Moon can almost perfectly cover the Sun. But this will not always be the case. At some point in the distant future, there will be one last, perfect total solar eclipse, and then none ever again.  There will come a day when there are no more eclipses on planet Earth.

The reason is that the Earth and the Moon are locked together in an inexorable gravitational dance. These two worlds have been locked together almost since the beginning of the solar system. You can see evidence of their sensuous cosmic tango every day. The relatively large size of both bodies, and their proximity, means that each has a profound effect on the other, largely through tides.

Most of us are familiar with the notion of ocean tides — the daily rise and fall of the seas on the Earth’s coasts. These cycles of high and low tides are caused by the effect of the Moon’s gravity on the Earth. The Moon’s gravity pulls on the parts of the oceans that are closest to it, causing a “rise” in the sea, a “tidal bulge” that points toward the Moon. There is a second bulge, on the far side of the Earth, caused by the fact that the strength of the Moon’s gravity grows weaker the farther you are from the Moon. The oceans on the far side of the Earth from the Moon get “left behind” as the Earth and near side oceans are pulled more strongly toward the Moon.

(A) The Moon’s gravity raises ocean tides on the Earth. (B) The rotation of the Earth pulls the tides out of alignment with the Moon. (C) The Moon tries to pull the tides back into a line, slowing the rotation of the Earth.

The Earth is constantly rotating, so the interaction of the Earth’s crust with the bulged oceans drags the bulge in the direction the Earth spins, so the bulges don’t point directly at the Moon, rather they point in a direction off to one side of the Moon.

The gravity of the Moon is still pulling on the bulges, so the Moon tries to pull the bulges back in line. Since the Earth is simultaneously trying to move the bulge ahead, the net effect of this gravitational tug-o-war is to slow the spin of the Earth down, ever so slowly (between 15 and 25 millionths of a second every year).

Despite this diminutive change in the Earth’s spin speed, it represents a substantial amount of energy. Where does all that energy go? The only place it can — into the Moon’s orbit. When you dump energy into an orbit, the orbit gets larger. In the case of the Moon, it is getting farther and farther from the Earth every year, at a rate of about 22 mm/yr.

As the Moon gets farther from the Earth, it appears smaller in our sky. Eventually, it will be so small in the sky that it will not be able to cover the Sun, and we will see no more total solar eclipses. At the current rate of 22 mm/yr, the last solar eclipse will happen in about 733,200,000 years.

An annular solar eclipse happens when the Moon is too far away, so it does not appear big enough to cover the entire Sun. [Illustration by S. Larson]

Filtered image of the annular eclipse on 20 May 2012 as seen from Cedar City, Utah. [Image by S. Larson]

While those days are in the very far future, we sometimes get a hint for what they will be like, because the Moon’s orbit is not perfectly circular, so sometimes it is a bit too far away to completely cover the Sun, and we see an “annular eclipse.”  When this happens, the entire Sun is not covered, and the daylight does not fade. To the unaided eye, the Sun appears much like the Sun always does, but through a filtered telescope you can see that The Moon has clearly occulted the Sun — 733 million years in the future, that is all our descendants will ever see.

But that is not your fate — you live in a time when you CAN witness the spectacle of the eclipsed Sun, and you should.  Take a look at NASA’s eclipse page, and start planning where you might go.  But be quick; before too long, hotels are going to start filling up. Look at that — another one just got sucked up in Casper, Wyoming. 🙂

PS: If you for some reason miss this one, you’ll only have another 7 years to wait. There will be another North American solar eclipse on 8 April 2024, running from Texas to Maine. It may not be as easy to get to for most people, but it will be worth the effort. My advice: see the eclipse in 2017, because after your first eclipse you immediately ask, “When is the next one?!“ 🙂  Here is NASA’s page for the April 2024 eclipse.