People are telling me where they plan to go this August 21. It makes me very happy. Folks are increasingly realizing that an unbelievable experience is about to unfold. You need no special equipment, no telescope; only a $1 filter to watch the partial phase safely. The first mainland-US total solar eclipse in 38 years merely requires that you place yourself in the path of totality.
Go online and Google “2017 total eclipse” and you’ll see a zoomable map. Hotels in the eclipse path are sold out, but that doesn’t matter; you don’t need to sleep there. The eclipse happens at midday, so you can drive into the path that morning, and go to any park or library lawn.
This comes to mind because I just returned from “Greatest Spectacle Number Two,” which is a vivid display of the aurora borealis. I just finished our yearly Northern Lights tour, and this time we had three successive nights of Level Four and Level Five displays, using the classification system from the University of Alaska. I was so glad for our group, which included a couple from Columbia County.
We get nice aurorae right here, but it has now been a while: Our last vivid display was in the autumn of 2001. And we had a bright display one year earlier, in the fall of 2000. Before that, there was a brief overhead dusk display about five years earlier; I don’t remember what year that was. Before that, there were the now-famous all-night, all-sky, mind-boggling Northern Lights of March 13, 1989.
In north central Alaska, there are bright displays every week (also in Iceland and Scandinavia, but it’s often cloudy in those places). Our science tour company, Special Interest Tours, keeps returning to our secret places, and I hope some of you will join us for one of those. If not, or if you’re willing to wait for the next time we get a good one here in the Hudson Valley, here’s a quick guide.
A good aurora is the second-best spectacle the human eye can ever see. But what constitutes a major display? Most folks imagine that it involves lots of vivid colors. But that never happens. One of the world’s leading aurora experts, Neal Brown of the University of Alaska at Fairbanks, once told me that “a deep-red aurora is a once-in-a-lifetime experience.” I think he nailed it; I’ve only seen one in all these decades. And when a red aurora does appear, it’s an extremely high-altitude affair that shows itself as an enormous red blotch, without any shapes, formations or details.
The minimum aurora – and the one we get most commonly around here – is a simple glow in the northern sky. Even in Alaska, most aurorae start out that way an hour or two after sunset. But as we approach midnight, when aurorae typically begin to become full-blown, everything changes.
Here’s a checklist for a truly memorable display: The first criterion is size, or aerial extent. How large is it? Is it merely confined to the north, or is it directly overhead? Does it occupy a huge swath? The best ones spread themselves over vast tracts of sky.
The second criterion is brightness. The better aurorae light up the ground and the surrounding countryside (this assumes you’re away from any city). You never need total darkness. A bright aurora can easily be seen when the Moon is out. Indeed, last Thursday, a gorgeous aurora conspicuously appeared over Fairbanks: a small city with roughly the same population as Kingston. A bright aurora lights up the mountains.
The third criterion is color. Aurorae are commonly a pale green. This glow, from ionized oxygen at 5577 angstroms, is reliable. However, my own repeated surveys of our tour groups reveal that one-quarter of people do not perceive this color, and instead see it as gray. That’s because color perception requires that our retinas use their six million cone-shaped cells, which require a minimum brightness level of around 0.3 lamberts. People apparently have great individual variation as to where their colorblind, low-light, rod-based scotopic vision gives way to full-color photopic vision. Two years ago, during an intensely green display, ten of our 40 people raised their hand when I asked who could not see the color. The other 30 were astonished. “You can’t see that green? Really? Really?” one asked incredulously.
During some powerful displays, the green “curtains” have distinctly pink fringes, which may also appear purple or plum-colored, though these are usually subtle and pastel.
The fourth criterion is detail. A good display is not merely a glow containing blotches, ovals and arcs. They can look like slowly rustling drapes. They have lines, twists and braids. They are highly detailed.
My final criterion is motion. Aurorae commonly have a slow-moving grandeur. To see changes, you often have to look away for half a minute, then look back again. But this past week’s aurorae displayed rapid, jumpy motion. An aurora that is both bright and rapidly changing can even be captured in a video.
Check out one we took two years ago:
Aurora videos are usually created by splicing together separate long-exposure images. By “long exposure,” I mean tripod-mounted six-second shots at around f/ 1.6, using a high ISO or ASA setting. But two years ago we could simply shoot straight videos, with no processing. In all cases, a camera deepens the green color and brings out other colors that the eye does not see.
So there’s your aurora guide. There’s your “Number Two Great Spectacle.” But first things first: Please try to see “Number One” this summer. You’ll be glad you went.