To the serious observer of a total solar eclipse every instant is
extremely precious. Many distinct observations have to be crowded into a
time all too limited, and this in an eclipse-party necessitates constant
rehearsals in order that not a moment may be wasted when the longed-for
totality arrives. Such preparation is very necessary; for the rarity and
uncommon nature of a total eclipse of the sun, coupled with its
exceeding short duration, tends to flurry the mind, and to render it
slow to seize upon salient points of detail. And, even after every
precaution has been taken, weather possibilities remain to be reckoned
with, so that success is rather a lottery.

Above all things, therefore, a total solar eclipse is an occurrence for
the proper utilisation of which personal experience is of absolute
necessity. It was manifestly out of the question that such experience
could be gained by any individual in early times, as the imperfection
of astronomical theory and geographical knowledge rendered the
predicting of the exact position of the track of totality well-nigh
impossible. Thus chance alone would have enabled one in those days to
witness a total phase, and the probabilities, of course, were much
against a second such experience in the span of a life-time. And even in
more modern times, when the celestial motions had come to be better
understood, the difficulties of foreign travel still were in the way;
for it is, indeed, a notable fact that during many years following the
invention of the telescope the tracks were placed for the most part in
far-off regions of the earth, and Europe was visited by singularly few
total solar eclipses. Thus it came to pass that the building up of a
body of organised knowledge upon this subject was greatly delayed.

Nothing perhaps better shows the soundness of modern astronomical theory
than the almost exact agreement of the time predicted for an eclipse
with its actual occurrence. Similarly, by calculating backwards,
astronomers have discovered the times and seasons at which many ancient
eclipses took place, and valuable opportunities have thus arisen for
checking certain disputed dates in history.

It should not be omitted here that the ancients were actually able, _in
a rough way_, to predict eclipses. The Chaldean astronomers had indeed
noticed very early a curious circumstance, _i.e._ that eclipses tend to
repeat themselves after a lapse of slightly more than eighteen years.