Answer: Solar eclipses and shadows?

The eclipse of April 8, 2024... 

... was seen by millions.  And if you were lucky enough to see it, you almost certainly know the answers to this week's Challenge.    

The shadows we see every day are round things.  The Challenge was why, and now we have some clues.    

1. Why ARE the dapples of sunlight round in shape?  Why isn't the shape of the sunlight exactly like the shape of the hole in the tree canopy?  Related: Why are only the small patches of light round while the bigger ones are other shapes?  Super related: Will the round sun patches change shape during the eclipse? 

I started with a statement of the Challenge: 

     [ why are dapples of sunlight round ] 

which led me to a blog post about dappled light effects (Gurney Journey) about the effects of dappling light in artwork, which led me to Edward Tufte's article about visualization, which led me to M. G. J. Minnaert's brilliant book Light and Color in the Outdoors.  In there, we find this page: 

The text goes on to say that at a large enough distance, the gaps in the tree foliage act as pinhole cameras, creating images of the disk of the sun on the ground.  THAT's why most of the dapples we see are round--they're little suns projected onto the surface of the earth.  (Keep reading to learn why some of the dapples are other shapes.)  

To pursue the second question about the changes in shapes of the dapples during the eclipse, SRS Regular Reader Ramón searched for this:   

     [tree canopy eclipse light shape]

which led him to this article about the Mystery of the Crescent Shadows, with a nice explanation of why eclipses will produce crescent-shaped shadows.  

Regular Reader remmij contributed this wonderful YT video about why solar eclipses make those crescent (with some nice animation in the middle). https://www.youtube.com/watch?v=wGeKWOD468k 

This change becomes clear when you see photos of the dapples taken during the eclipse: 

25% covered. Pacman-like.

Very near totality

Note that the "pinhole" (or leafy equivalent) has to be far enough away from the surface to have a focusing effect.  If the non-circular "pinhole" is near enough to the focus plane, it will project the shape of the hole, and not have any focusing effect.  Example:  

Triangle aperture in my "pinhole" projection.
Distance from hole to surface, about 4" (10cm)

But when you move the triangular "pinhole" farther away from the surface, the dapple becomes circular. 

 

The projection throw is about 2 meters.

As it turns out, I was in Austin, TX with some friends to see the eclipse.  The weather was cloudy, but everything worked out, more-or-less, with clouds sometimes parting enough for us to see the eclipse happening.  I did't have any fancy photo gear, but I got  a couple of okay shots with my iPhone 17--here's my best from the day.

Totality, with some cloud cover (and Texas telephone wires)

2. What other solar light phenomena should I be looking for?  Are there any other extraordinary shadow and light things I should be looking for?  

My query was: 

     [ other phenomena associated with eclipse ] 

which easily led to a bunch of other effects, some of which I expected (the temperature drops), and several of which I did NOT expect.  One of the ones I didn't expect was the 360-degree sunset, brighter towards the north and south, which are outside the path of totality. Apparently, darkening happens first to the west and as the eclipse progresses, the darkening moves to the east as the Moon's shadow rushes over you.  

Regular Readers provided some additional ideas:  Almaden Mike pointed out that "There are optical phenomena such as Bailey's Beads, the Diamond Ring and shadow bands, which are described here [on Space.com]."

Shadow bands!  What are those?  I haven't heard about them before either.  

RR Mark said that shadow bands are "are kind of a cherry on the top of the whole eclipse dessert experience" explaining that "When the first tiny bit of sunlight appears at the end of the eclipse, all the photons from the sun are aligned. This provides something of a free 'Schlieren Optics' setup, that makes the pressure of the wind visible. So the bands are letting you see the 'wavelength', of the speed of wind in the air. Very cool to see the invisible, made visible."  

Remmij pointed to an article at Shore & Islands, with a nice pic of the shadow bands.  

My quick search for [ shadow bands ] led to this really nice article on Space.com  (Shadow bands are a solar eclipse mystery), including a great video by Destin ("Smarter Every Day") about how to photograph the bands.    (Update: Here's a excited video of shadow bands from the April 8th eclipse. Worth the 1 minute watch time.)  

What a marvelous physical phenomenon!  Moar eclipses, please!   

SearchResearch Lessons

There was SO MUCH written about the eclipse that this wasn't a difficult Challenge.  (It's good to have them every so often as well.)  But I did want to point out: 

1.  Notice the things you don't know about. Good searching involves reading and understanding... and picking up on the things that are new to you.  In this case, there was SO MUCH written about the eclipse, that there was a hyper-abundance of content.  As I scanned the material, every so often I'd notice something that I didn't know about (like, Shadow Bands, or the 360-degree sunset).  

Keep searching!