Solar Eclipse Shadow Card

These cards are meant to show how during a solar eclipse the resolution of the edges of shadows is different in the horizontal and vertical directions as well as show multiple images of the sun through pinhole cameras of various sizes.

On a normal day the sun appears as a disk in the sky. This means that the edges of shadows appear blurry especially as the shadow gets farther away from the object casting it. This is because part of the disk of the sun can illuminate the ground leading to a partial shadow. This is called the penumbra. Penumbras are only created when the source of light is not a point source.

Near totality both before and after, only a sliver of the sun is visible. This means that in the vertical direction, the light source still appears to be the same diameter as the sun and thus horizontal shadow edges are still blurry. However, since the sun appears to be a thin sliver in the horizontal direction, the sun acts effectively as a point source in the horizontal direction causing vertical shadows to appear much sharper. Taking an image of this cards shadow when it's properly aligned will show the difference between the edge resolution of horizontal and vertical shadows during that time period.

The gaps and hole sizes of the 3D card are 10, 8, 6, 5, 4, 3, 2, 1.5, 1, 0.5, and 0.25 mm. The pins are 5, 4, 3, 2, 1.5, 1, 0.5, and 0.25 mm. The gaps of the flat card are labeled.

PRINTING:
Print this at the best resolution you can. The smallest pins/slits/holes are only 0.25mm across for the card with a 3D reticle and 0.5mm for the flat version. It's perfectly fine if you can't get them to print perfectly or if they break off; it just means you'll have to move the card further away from the board to take a measurement image.

INSTRUCTIONS (a quick demo can be watched here: https://youtube.com/shorts/nNCVd5evXQs)

1. Place a piece of paper on a clipboard or have a large piece of something rigid, white, and flat like a posterboard on thick cardboard.
2. Prop the white board against something so it faces as close to the direction of the sun as you can.
3. Hold the card close to the board and use the reticle to align the card perpendicularly to the sun. The large openings will appear brighter the closer you are to being aligned with the sun. When the reticle is perfectly aligned with the sun and is close to the board, a small dot will appear in the exact center as light shines through the middle hole. You may have to open the middle hole with a needle if your printer isn't accurate enough.
4. Once your card is aligned perpendicularly to the sun, angle the board to be parallel to the card and rest it against an object in that new orientation. This ensures the board is also perpendicular to the sun which gives you the best shadow comparison possible.
5. When you want to take an image, hold the card about about 15-20 inches from the board.
6. The pinhole cameras will project an image of the sun. Align the edge of the card's shadow with the direction of the crescent of the sun. i.e. The line formed by the points of the crescent sun should be parallel with the edge of the card.
7. Rotate the card on it's x and y axes and use the reticle to align the card perfectly with the sun again. The center hole may not be visible while casting a shadow far away from the board.
8. You should NOT be able to resolve the difference between the two smallest vertical pins/slits, but you SHOULD be able to resolve the difference between the two smallest horizontal pins/slits if you are near totality (both before and after). If you can still resolve all pins/slits in both directions near totality, move the card farther away from the board until the smallest pins/slits in one direction become indistinguishable.
9. Take an image!

I plan to take several images leading up to and after totality to show how the edge resolution of shadows changes as the eclipse progresses.

Please make sure to share your images here after the eclipse! This allows others who had bad weather to observe the effect as well!