The Earth-Moon problem is a special case of the empire problem for countries with 
disjoint regions, with one region of each country lying
on the Earth and one on the Moon (Ringel 1959; Frederickson 2002, p. 32). The
additional constraint means that fewer than 12 colors might suffice. And in fact,
Ringel (1959) gave an example requiring only 8 colors. Gardner (1980) reported an
example requiring 9 colors, but it is not known if configurations exist requiring
10, 11, or 12 colors (Frederickson 2002, p. 32).
Every pair of countries shares a border on at least one of the two maps in the illustration, so this example requires eight different colors.
