Trigonometry.
No, seriously. The more directly a source of radiation (like light, or UV rays) hits a surface, the more of that intensity is absorbed by the surface. If you lie down in a rainstorm, you’ll catch more raindrops on your front than if you’re just leaning backwards. Sunrays work the same way. How much more? Well, if you catch all of them lying flat, you’ll catch sin(x) of them leaning, where sin is a way of measuring things that have to do with circles and triangles, and x is how far rotated you are from lying flat.
But you aren’t lying flat at certain times of day, so why do you get more sunburned? The earth is lying flat! When the sun is directly overhead, it’s like the earth (and you) are flat as compared to the direction of the sunbeams. When the sun is low in the sky, it’s more angled, and you’re catching less of them.
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