For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
I wish we could test this out with only simple apparatus. Unfortunately the common people do not have access to satellites or nonstop bullet trains.
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We can and do. GPS satellites need to be regularly calibrated to Earth clock signals or they’ll start to drift their calibration by meters per day.