calculation is easy, especially with the astronomical tables of those
days, and there were very few sailors who were capable of, or patient
enough to make them, nor was the result, as a rule, very accurate. For
one thing, the motions of the moon, which are extremely complicated, were
not enough known to allow her calculated position in the heavens to be
very accurate, and a very small error in this position considerably
affects the time, and therefore the longitude.

Luckily for Cook, the Nautical Almanac had just been started, and
contained tables of the moon which had not previously been available, and
which much lightened the calculations.

The great invention of the chronometer, that is, a watch that can be
trusted to keep a steady rate for long periods, was at this time
completed by Harrison; but very few had been manufactured, and
astronomers and sailors were slow to believe in the efficacy of this
method of carrying time about with a ship. Thus Cook had no chronometer
supplied to him.

Green had accompanied Mr. Maskelyne, afterwards Astronomer Royal, to
Barbados in 1763 in H.M.S. Princess Louisa, in order to test Harrison's
timekeeper, and also a complicated chair, from which it was supposed
observations of Jupiter's satellites could be observed on board ship; and
as this trial afforded the final triumph of the new method, one would
have thought that on a voyage of circumnavigation he would have made
every effort to get one of these watches.

Be this as it may, the Endeavour had no chronometer, and lunars were the
mainstay of the expedition.