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 geoengineering 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. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Video Editor: Oli Oakley Animator: Sam Benson Video & Social: Bex Tyrrell Assistant Producer: Lucy Lipscombe Producer: Simona Rata Senior Producer: Lauren Armstrong-Carter Chief Digital Officer: Samuel Oakley Exec Producer: Neil Fearn