Here in the United States, it’s the 4th of July. This normally means fireworks, though this year many are canceled due to the pandemic. But if you have them in your area and you are wondering how high they are, you can use this handy Altitude-Distance Finder. These were handed out to all Ground Observer Corps observers and came in a few varieties, calibrated to type of aircraft. They’re about the size of a credit card, so you could carry it in your wallet or purse and always be ready when you heard the sounds of unknown aircraft.