If they live not at the center of the far side - and if they choose the wrong direction (because how should they know?) - they can travel more than a quarter of the moon great circle.
Ah, good point. So really the at most is `0.5*circumference - epsilon` (starting just on the far side). In the worst case, if they manage to stay on a great circle.
From the article:
"Owing to various view angles from different parts of the Earth, the most ambitious jet-setting astronomer could have seen a maximum of 59 percent of the surface of our planetary companion. "
If we take into account that fact that Earth is not a point, then epsilon is not so tiny after all, it's around 0.045 times circunference.
This would make the final result 0.455 times circunference.
