An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

22 THE EARTH.
of the sun, and the sun sets. During the next twelve hours,
the sun continues on the invisible side of the sphere, until the
horizon returns to the position from which it started, and a
new day begins.
59. Let us next contemplate the similar phenomena at the
Jpoles. Here the horizon, coinciding as it does with the equator, would cut the sun through its center, and the sun would
appear to revolve along the surface of the sea, one half above
and the other half below the horizon. This supposes the sun
in its annual revolution to be at one of the equinoxes. When
the sun is north of the equator, it revolves continually round
in a path which, during a single revolution, appears parallel
to the horizon, and it is constantly day; and when the sun is
south of the equator, it is, for the same reason, continual night.
60. We have endeavored to conceive of the manner in
which the apparent diurnal movements of the sun are really,
produced at two stations, namely, in the right sphere, and in
the parallel sphere. These two cases being clearly understood,
there will be little difficulty in applying a similar explanation
to an oblique sphere.
ARTIFICIAL GLOBES.
61. Artificial globes are of two kinds, terrestrial and celes.
tial. The first exhibits a miniature representation of the
earth; the second, of the visible heavens; and both show the
various circles by which the two spheres are respectively traversed. Since all globes are similar solid figures, a small
globe, imagined to be situated at the center of the earth or of
the celestial vault, may represent all the visible objects and
artificial divisions of either sphere, and with great accuracy
and just proportions, though on a scale greatly reduced. The
study of artificial globes, therefore, cannot be too strongly
recommended to the student of astronomy.*
It were desirable, indeed, that every student of the science should have the
celestial globe at least, constantly before him. One of a small size, as eight or
nine inches, will answer the purpose, although globes of these dimensions cannot
usually be relied on for nice measurements.