NavList:

Roger,

Thanks for the article recommendation, I will try to find it. I was not aware that Jupiter satellite mutual attraction perturbs their motions to the extent which make prediction difficult. From what I read I made another conclusion: that the main difficulty was stabilizing sufficiently powerful telescope on a ship (For example, this is addressed in Derek Howse, Nevil Maskelyne, Seaman's Astronomer, and in Euler's Letters to a German Princess. And even in fction, Umberto Eco, The Island of the day before).