The exact nature and amount of the wobbling is a work of the
exact astronomy of the present time.
We cannot measure across oceans from island to island. Until a
recent time we have not even measured across the continent, from
New York to San Francisco, in the most precise way. Without
astronomy we should know nothing of the distance between New York
and Liverpool, except by the time which it took steamers to run
it, a measure which would be very uncertain indeed. But by the aid
of astronomical observations and the Atlantic cables the distance
is found within a few hundred yards. Without astronomy we could
scarcely make an accurate map of the United States, except at
enormous labor and expense, and even then we could not be sure of
its correctness. But the practical astronomer being able to
determine his latitude and longitude within fifty yards, the
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.
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