How do we define a second? It used to be as a fraction of the length of a day, i.e. how long it takes Earth to turn once on its axis. But that wasn’t precise enough for scientists, especially since the length of Earth’s day changes! So instead, since the 1950s the second has been defined by the number of spin transitions of electrons in a Caesium atom.


👩🏽‍💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.


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