Shadow bands: the ripples that race across the ground just before and after totality

Shadow bands are thin, wavy ripples of light and shadow that slide across the ground or a wall in the moments just before totality and just after it. They are caused by turbulence in the air, which scrambles the light from the last thread of Sun. They are faint, they don’t appear at every eclipse, and photographing them is very hard. If you want to see them on 2 August 2027, you will have to look at the ground, at something white, in the seconds before totality begins (C2, second contact) where you are: in Tarifa, according to the IGN (Spain’s National Geographic Institute), C2 comes at 10:45:00 Spanish time (CEST, UTC+2).

What they are

NASA describes them as long, dark bands separated by lighter spaces that move quickly and can be seen on the walls of buildings or on the ground just before and just after totality, although they can be very faint and hard to photograph. The glossary of the American Astronomical Society (AAS) speaks of very faint, shimmering ripples of light and dark that run across the ground, walls or even clouds. English Wikipedia adds their shape: thin, wavy, parallel lines, best seen on smooth surfaces of a single colour.

Why they appear

In the last seconds before totality, the Sun’s disc has shrunk to a crescent so thin that its light arrives almost as if it came through a slit, and that light crosses an atmosphere that is never still. NASA talks of cells of turbulent air in the upper layers that distort the sharp-edged light coming from the Sun; the AAS puts it down to atmospheric scintillation of that thread of Sun, and Wikipedia to refraction in turbulent air.

According to Wikipedia, the narrower the crescent, the more ordered (collimated) its light is when it reaches the ground, which is why the bands only appear at those moments. During totality there is no direct sunlight, so there are no shadow bands either. When the first thread of Sun reappears, at C3, they can come back.

When to look on 2 August

As the AAS says, they appear just before second contact, C2, which opens totality, and just after third contact, C3, which closes it. None of the sources gives a number of seconds, so the sensible thing is to be watching the ground for a good minute before your C2. These are the IGN’s times for some municipalities in the path:

The IGN’s times run about 10 seconds ahead of our engine’s. The page for each of the 105 municipalities with totality gives both.

A simple plan:

• Before the countdown starts, spread a white sheet on the ground, in full sun and away from the shade of trees or awnings. Wikipedia shows a test done in exactly this way, with a sheet on the ground, and the AAS says the bands show up best against a white background.
• In the last moments before C2, watch the sheet. You don’t need the glasses for that, but if you look up at the Sun before C2 it must be with them on: until the last flash, that thread of Sun burns.
• During totality, glasses off and eyes up, because that is the moment for the corona. When to take them off and when to put them back on is explained in this article: with the later C2 and the earlier C3.
• After C3, glasses back on. And then, yes, look at the sheet again.

Why hardly anyone photographs them

NASA warns about it: they can be very faint and hard to photograph. They are dim and they move quickly. Besides, according to the AAS, they are not recorded at every total eclipse, so not seeing them is perfectly normal.

If you want to try, think video rather than photos, because what gives the bands away is their movement: the phone propped up and pointed at the sheet, recording from a couple of minutes before C2. That way you don’t have to deal with it when totality arrives. How to leave it recording without burning the camera is in film the eclipse with your phone.

And during the partial phase: crescents under the trees

Shadow bands last only a moment. The crescents, on the other hand, can be watched for more than an hour, before and after totality. The AAS explains that the tiny gaps between a tree’s leaves act as pinhole projectors and fill the ground with images of the crescent Sun. A pasta colander does the same; NASA suggests letting its shadow fall on a white card some distance away. The AAS stresses that projecting is not looking at the Sun through a hole: you look at the image on the ground or on the paper, with your back to the Sun.

What works and what doesn’t for watching safely is in homemade eclipse viewer (in Spanish). And what surrounds C2, such as the diamond ring, is covered in our learn section.

Sources: 2024 Total Eclipse: What to Expect — NASA Science (what they are, where and when they are seen, air turbulence, and that they are hard to photograph); A Solar Eclipse Glossary — AAS (ripples of light and shadow on the ground, walls or clouds, before second contact and after third, not at every eclipse, best against a white background); Shadow bands — Wikipedia (shape of the bands, refraction in turbulent air when only a slit of Sun is left, the test with a white sheet on the ground); Pinhole & Optical Projection — AAS (crescents under the trees, the colander, projecting is not looking through a hole); Eclipse Photography, NASA/TP-1999-209484 (the colander over a white card); C2, C3 and lengths of totality in Spain: IGN — eclipses.ign.es (in Spanish); eclipseando data (skyfield + DE440s engine).