things are learnt either by experience or by inquiry or by scientific investigation.
It is of course far best for a general to be himself acquainted with the roads, the spot he is bound for and the nature of the ground, as well as with the people by whose agency and in concert with whom he is going to act.
But the next best thing is to make careful inquiries and not to rely on chance informants. The pledges of good faith given by those who act as guides in such a case must be in the hands of those who follow their guidance.
Skill, therefore, in these and similar matters can perhaps be acquired by a general just through military experience, partly by practice, and partly by inquiry; but what depends on scientific principles requires a theoretical knowledge more especially of astronomy and geometry, which, while no very deep study of them is required for this purpose at least, are exceedingly important and capable of rendering the greatest services in projects such as we are speaking of.
The most necessary part of astronomy is that dealing with the variations day and night. If day and night were always of equal length, the matter would give us no trouble and the knowledge of it would be common property;
since, however, days and nights differ not only from each other, but also from themselves it is evidently necessary to be acquainted with the increase and decrease of both.
For how can one rightly calculate the distance traversed in a day's march or in a night's march without knowing the different lengths of day and night
Indeed it is impossible for anything to come off at the proper time without such knowledge; it is sure to be either too late or too soon.
And in such matters alone it is a worse fault to be in advance than behind hand.
For he who arrives later than the hour decided upon is disappointed merely in his hope — since he becomes aware of the fact while still at a distance and can get away in security —
but he who arrives too soon, approaching the enemy and being discovered by him, not only fails in his attempt, but runs the risk of total destruction.
Walbank Commentary