NavList:

(While doing blockade duty,) Hornblower himself found mental exercise in working out navigational problems. Conditions were ideal for taking lunar observations, and by their aid it was possible to arrive at an accurate determination of longitude — a subject of debate since the days of the Carthaginians — at the cost of endless calculations. Hornblower was determined to perfect himself in this method, and his officers and young gentlemen bewailed the decision, for they, too, had to make lunar observations and work out the resulting sums. The longitude of the Little Girls was calculated on board the Hotspur a hundred times that summer, with nearly a hundred different results.

Hornblower and the Hotspur
C. S. Forester