positions of the principal points in all great cities of the
country are known, and can be laid down on maps.

The world has always had to depend on astronomy for all its
knowledge concerning times and seasons. The changes of the moon
gave us the first month, and the year completes its round as the
earth travels in its orbit. The results of astronomical
observation are for us condensed into almanacs, which are now in
such universal use that we never think of their astronomical
origin. But in ancient times people had no almanacs, and they
learned the time of year, or the number of days in the year, by
observing the time when Sirius or some other bright star rose or
set with the sun, or disappeared from view in the sun's rays. At
Alexandria, in Egypt, the length of the year was determined yet
more exactly by observing when the sun rose exactly in the east
and set exactly in the west, a date which fixed the equinox for
them as for us. More than seventeen hundred years ago, Ptolemy,
the great author of The Almagest, had fixed the length of the year
to within a very few minutes. He knew it was a little less than
365 1/2 days. The dates of events in ancient history depend very
largely on the chronological cycles of astronomy. Eclipses of the
sun and moon sometimes fixed the date of great events, and we
learn the relation of ancient calendars to our own through the
motions of the earth and moon, and can thus measure out the years
for the events in ancient history on the same scale that we
measure out our own.

At the present day, the work of the practical astronomer is made
use of in our daily life throughout the whole country in yet
another way. Our fore-fathers had to regulate their clocks by a