Galileo -- Our most famous early sunspot observer and theorist.
It continues to be a very hot summer on the ground here in the US and in many other parts of the world. Of course the Sun is ultimately responsible, but the Solar Constant (=1.361 kw/m^2/sec) goes up just 0.1% at solar minimum, and we’re closer to solar maximum. It’s our infrared light trapping that’s keeping us warm(er). That the overall climate temperature has been increasing is basically our fault. But that’s not what this blog is about.
This entry is about the fact that in the next handful of years — through 2028, the Sun is going to be a very “hot” topic in popular astronomical observing. Over this period there will be a total of four major total solar eclipses we might consider observing:
North America on April 8, 2024;
Greenland, Iceland; Spain on August 12, 2026;
Spain, Africa and Saudi Arabia on August 2, 2027;
Australia and New Zealand on July 22, 2028.
There will be an annular eclipse on October 14 of this year in North America. An annular eclipse is when the Moon is slightly too far away and the full disk of the sun cannot be eclipsed. If you can manage to be in Spain on January 26, 2028, you can see an annular eclipse fresh from South America too. Spain will be the place where much of the action occurs after next year.
And — you guessed it — smart telescopes can get in on the action and help us experience these events. But really what can we see and how can we best take advantage of it? I’ll tell you, but first let me untangle the astronomy from the astrophysics.
ASTRONOMY vs ASTROPHYSICS
(See the glossary at the end for any terms you might not recognize.)
The first time I saw a partial solar eclipse was when I started working at the Adler Planetarium. We rigged up the Adler’s coelostat so that it could reflect an image of the Sun onto the ceiling of one of our galleries. I still remember how impressed I was that the first contact of the Moon occurred exactly on time. Of course I knew it should, but there’s something about seeing it happen when there are a few hundred people standing around to witness your prediction.
I’ve seen many partial eclipses and just one total eclipse — the one in 2017. Totality was amazing. But, you know, I consider most of these eclipses just demonstrations that show we can get the celestial mechanics of basic astronomy correct. In these days of flat earthers and science deniers, I guess I should be grateful we can predict precise celestial events using just Newton’s law of Universal Gravitation. That's just astronomy, however. I’m more interested in astrophysics.
Astrophysics is the application of laboratory spectroscopy and physics to the understanding of the stars. George Ellery Hale, though well known for his eponymous telescope, was first and foremost a solar astronomer and maybe also the godfather of American astrophysics. He took over the University of Chicago’s astronomy department; launched Yerkes Observatory, and then went on to build Mount Wilson Observatory. (I feel I must always be true to my University of Chicago roots.)
Hale was long gone before I showed up at grad school; however E. N. (Gene) Parker was with me from day one. Parker was famous for many things, but in particular he predicted that the Sun should generate a solar wind of particles. That prediction, and his lifelong leadership in heliophysics, led NASA to name a space probe for him. Gene Parker counseled me at times during grad school and mentored me through the rigors of third quarter graduate E&M. He also taught an amazing 500 level course on stellar magnetic fields. I managed to keep up, but just by the seat of my pants. The class helped me fully appreciate how magnetic fields rule the atmosphere of the Sun.
So, when I observe the Sun, I am always on the look out for the things Parker taught me how to understand: sunspots, chromosphere, corona, and the solar wind. Our smart telescopes, of course, show spots fairly well. They can’t show the chromosphere, but they do show cometary tails blowing in the solar wind. I conjecture smart telescopes will be able to image the corona — technically. Whether one can coax one’s scope to do that or whether I have the guts to potentially expose my scope to searing photospheric light is yet to be seen…. ;-)
Eclipse or no eclipse, I love to observe sunspots. I know my “friend” Galileo would love to use my Vespera too. It’s an especially fertile time for sunspots this year. Here’s an image I took on 19 July.
SMART SCOPEs AND SOLAR OBSERVING
Here’s what we can do and can’t. The first thing is we can actually observe the Sun in white light safely, provided we have a proper solar filter. All the manufacturers sell such filters — though IMHO they are overpriced. Some have made their own filters from solar film like that found in eclipse glasses.
What a white light filter does is to block all but about a thousandth of a percent of the Sun's light. They pass just 1/65,000 of the light coming from the Sun. These are broad band filters, so they block light all across the spectrum and even reflect out infrared light too. Any less restrictive a filter will endanger your telescope or your eye if you use it on a regular scope. And, these filters must be placed at the aperture of the scope to do the blocking right from the get go.
What I will plan on during the upcoming October partial, then April total eclipse, is to set up one of my smart scopes with its white light solar filter and take images every few minutes during the partial phases. Maybe some manufacturer will automate this process so that the apps save the images at a specified cadence? Should be easy to include in their programs, I think…. We’ll see.
I’ll let the readers of this blog post check into what their particular smart telescopes can do. I know in my case, I will probably set up my Vespera and connect it to my iPad, so the group I’m with can watch the partial phases proceed.
Although there are smart telescope filters that pass hydrogen alpha light for nighttime observing, they can’t and shouldn't be used on the Sun. The first reason is that they pass way too much light near the hydrogen alpha line (6562.8 angstroms or 656.28 nanometers as shown in the plot). Below is the transmission curve for ZWO’s Dual Band Filter. It’s great for nebulae, but if you try it on the Sun, you’ll fry your scope because the band pass is about 160 angstroms at half max. A typical H Alpha solar scope only has a band pass of 0.7 Angstroms. That’s a ratio of 230! Not only won’t you see the detailed structures, but you will certainly melt your scope's sensor chip. This isn’t counting all the light one of these filters lets in the blue green part of the spectrum.
There’s one more issue about hydrogen alpha scopes that may not be obvious. Although their hydrogen alpha interference filters can scrunch down to 0.7 Angstroms, they still pass a fair amount of infrared and ultraviolet light too. So, if you were to convert a telescope to this task, you also need to add what’s called a blocking filter. Here’s a picture of what one looks like. It substitutes for what would be an eyepiece diagonal on a normal scope.
Needless to say, these ultra narrow band filters can’t be put onto our smart scopes at this point in time. I don’t predict they will soon, either.
What about the corona during totality? I think our scopes can image it. The corona’s light is broad band, so a filter isn’t even necessary. The inner corona is supposed to be about the brightness of a crescent moon. Then it dims at a rate of about 1/(the height) out to about 2.5 solar radii. The wider angle smart scopes should be able to handle this.
Nevertheless, you know I’m going to wait to see if the manufacturers — who are responsible for the warranties on these things — will come up with a fool proof way to snap the corona.
Until then, I suggest you just stop and use your eyes to appreciate the ghostly corona, especially if you have never experienced a total solar eclipse.
GET YOURSELF TO THE CENTER LINE….
My final admonition about solar eclipse viewing is, if you can possibly manage it, make sure you’re on the center line of totality. A 90 %, partial eclipse is curious, but no big deal. Even 99% is probably not good enough. Don't pass up the chance to experience totality -- that means stopping and not fiddling with your telescope, either.... ;-)
MASS DEMONSTRATION OF THE POWER OF SCIENCE
At times my friends have heard me shrug off eclipses as being not that interesting scientifically these days — especially to heliophysicists who have their own probe now surfing the inner solar wind. Yet the coming eclipses will be huge public events — tens of millions of people will be watching.
What better message to send than the fact that science can predict such events to within seconds and meters -- especially in our science skeptical environment.
There is a golf instructor online who prefaces his little series of lessons with, “You can’t argue with physics, mate!” At least you couldn't argue about it since orrery's like the one above were put into service based on Newtonian physics in the 18th Century.
--- SPECIAL ALERT---
There will be a good virtual conference on eclipses coming up in a week. Check it out if you’re really interested:
https://astrosociety.org/get-involved/events/asp2023-a-virtual-conference/conference-overview.html
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Glossary: Solar Atmospheric Structure Terminology
Photosphere: This is the white light “surface” (~300 km thick) of the Sun. It emits a continuous spectrum of a “black body” at 5,800 K. Sunspots “live” on the photosphere.
Chromosphere: The “color sphere” is the solar atmosphere we basically see near the time of a total eclipse. It is about 2,000 km thick and just above the photosphere. It is where the solar atmosphere starts to become transparent and under the control of strong magnetic fields. But, its primary component is hydrogen, and it emits a lot of light in the red, hydrogen alpha line. You need a specialized hydrogen alpha telescope to see it — except when there’s a flash of red from it during a total eclipse.
Corona: This is the outermost region of the solar atmosphere. Some think that it’s basically connected to the solar wind and doesn’t really have an outer boundary close to the Sun. The bright inner corona is what we see that looks like a "crown" during a total solar eclipse.
Sunspots: These are cooler (~3,500K) regions of the solar photosphere where bundles of magnetic fields pierce the surface. They are relatively short lived and come and go collectively on an 11 year cycle.
Other loopy, spiky structures: These are transient chromospheric phenomena showing how magnetic fields rule the motions of the ionized solar atmosphere.
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Credits:
>>First "smart" telescope user, Galileo and one of his sketches of sunspots on June 28, 1613.
>> Parker photo courtesy of the University of Chicago.
>> Lunt blocking filter https://luntsolarsystems.com/product/lunt-blocking-filters-b600-b1200-b1800/
>> 18th Century Orrery is from the Whipple Collection:
https://collections.whipplemuseum.cam.ac.uk/objects/9010/
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