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      Telescope Types

      comics.movim.eu / XKCD • 17 December 2025

    I'm trying to buy a gravitational lens for my camera, but I can't tell if the manufacturers are listing comoving focal length or proper focal length.
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      Jumping Frog Radius

      comics.movim.eu / XKCD • 15 December 2025

    Earth's r_jf is approximately 1.5 light-days, leading to general relativity's successful prediction that all the frogs in the Solar System should be found collected on the surface of the Earth.