I think the calculations are extremely wasteful, since the writing being discussed would actually be visible from much of the solar system, while we only need it to be visible from Earth. The Earth subtends 1.2 millisteradians as seen from the moon, which is about 1/5000 of the whole half sphere. So if you place a mirror on the moon which throws a sunspot on the Earth, you can be 5000 times as efficient in emphasizing that patch of ground as if you simply paint it completely white (the reflectance of the moon itself is only 12%, and of course its light is diffuse and not concentrated on Earth).<p>Going with his surface area of 500,000 sq. kilometers, we would only need about 100 sq. kilometers of mirrors, spread randomly through the area to be emphasized, to produce a readable message on Earth. This is actually doable, although hard, and the message would only be visible at one particular angle (say, full moon directly overhead edit: depending on how much of the Earth you cover and how well, it may not need to be directly overhead), unless you want articulated mirrors.<p>In fact, I read, long ago, an article about a guy that performed this hack on Earth during the fairly early days of satellite photography. He drove around the desert in Southeast US, and placed small reflectors at angles calculated to reflect sunlight into a passing satellite. This resulted in huge overexposed letters in the satellite image, since each reflector exposed a very large pixel/grain of the fairly low-resolution satellite. I can't seem to find this article anywhere, although I remember it pretty clearly. If you can find it, I promise to upvote your submission!