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5 min readThe Science of Putting Sunglasses on The Sun
The sun is 1.39 million kilometers wide, sits 149.6 million kilometers from Earth, and has no ears, no nose, and no solid surface. These are the first problems you encounter when you try to put sunglasses on it. But as Michael Stevens and Hannah Fry discover in this episode of The Rest Is Science, the real question is not whether we can—it's whether the answer reveals something deeper about physics, perception, and a surprisingly real debate about geoengineering.
The Angular Diameter Solution
The trick, it turns out, is not to put sunglasses on the sun itself, but to make it look like the sun is wearing them. This is a question of angular diameter—how large an object appears in your field of view, measured in degrees and arc minutes. The sun occupies about 32 arc minutes of sky, roughly half a degree. Any object held at a distance that subtends the same angle will perfectly cover it.
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What you'll learn
- 1 (00:00) **Tease and Setup** - The hosts frame the absurd premise: how to make the sun look like it's wearing sunglasses, and hint that it leads to serious physics.
- 2 (03:37) **The Proximity Problem** - The sun's heat and lack of solid surface make physical attachment impossible.
- 3 (05:27) **Angular Diameter: The Key Concept** - How to calculate the size and distance of an object that will perfectly cover the sun.
- 4 (11:07) **The Danger of Looking at the Sun** - Why even a partial cover is unsafe and what happens to your eyes.
- 5 (13:46) **Single-Eye Solution** - A simple, safe method for one person to see the sun wearing sunglasses.
- 6 (15:01) **Binocular Vision Complication** - Why two eyes ruin the simple trick, and how to fix it.
- 7 (18:54) **Scaling Up for Multiple Viewers** - How to make the illusion work for more people by moving the glasses farther away.
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Guests on this episode
Show Notes
Why does the cartoon Sun always wear sunglasses? And how big would a real pair need to be?
Hannah Fry and Michael Stevens investigate space mirrors, Reflect Orbital and solar eclipses to find out.
Starting with a pencil held at arm’s length, they use the Sun’s angular diameter to calculate how big a pair of solar sunglasses would need to be for one eye, two eyes, everyone in London and eventually the entire planet.
Their hypothetical answer is a strange structure more than a million kilometres wide, positioned deep in space. Entirely practical. They compare its extraordinary scale with NASA’s Parker Solar Probe, the real spacecraft flying closer to the Sun than anything ever built, before using Lagrange points to work out where their creation could possibly go.
But could putting something between Earth and the Sun ever be useful?
Real proposals for space sunshades and climate geo-engineering suggest that blocking a small fraction of sunlight could help cool the planet. Moving from shade to reflection, they revisit Soviet attempts to reflect sunlight onto Earth at night and explore how Reflect Orbital’s Earendil 1 space mirror experiment might one day extend solar energy beyond daylight hours.
That leads them to the strange physics of light itself. Photons can push solar sails despite having no rest mass, revealing why E=mc² is only part of the full energy momentum equation from special relativity.
Finally, there is the optical phenomenon known as the Arago spot to contend with. It helps explain why NASA’s starshade concepts, future technology designed for hunting exoplanets, look like enormous sunflowers.
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