This question originates from a YouTube video called "NASA Blue Marble 100% Debunked" by Flat Earth Asshole (pardon the language, but that is their channel name). In the video, they make a claim that NASA's Blue Marble photograph from 1972 is a fake because it is not a correct representation of Earth: the areas that should be lit up such as Europe and South America, are on the other side of the Earth in darkness. It is easy to counter the claim with a basic geometry fact: the "missing" land areas such as Europe and South America are hidden behind the horizon. You cannot get a full 180 degree view of the Earth from the distance of 45,000 km. I could also use an example from Metabunk: Debunked: "Blue Marble" Photos show a Changing Earth https://www.metabunk.org/debunked-blue-marble-photos-show-a-changing-earth.t6616/ - the three photos of the household globe gives a good explanation. But how to calculate the visible fraction of the Earth? I found a web tool that seems to do the job: With the distance of 45,000 km the tool gives a fraction of 0.43-0.44 - about 44% of the Earth. This sounds about right, but can I trust the math and calculations of this tool?