The human naked eye has an angular resolution of approximately 280 microradians[21] (μrad) (approx 0.016° or 1 minute of arc), and the ISS targets an altitude of 400 km.[22] Using basic trigonometric relations, this means that an astronaut on the ISS with 20/20 vision could potentially detect objects that are 112 m or greater in all dimensions. However, since this would be at the absolute limit of the resolution, objects on the order of 100 m would appear as unidentifiable specks, if not rendered invisible due to other factors, such as atmospheric conditions or poor contrast. For readability from the ISS, using the same trigonometric principles and a recommended character size of about 18 arcminutes,[23] or about 5,000 μrad, each letter would need to be about 2 km tall for clear legibility in good conditions.
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