The
argument on which the view in question rests may be made clear in
the following way.
Let us chose for our observations that hour of the night at which
the Milky Way skirts our horizon. This is nearly the case in the
evenings of May and June, though the coincidence with the horizon
can never be exact except to observers stationed near the tropics.
Using the figure of the grindstone, we at its centre will then
have its circumference around our horizon, while the axis will be
nearly vertical. The points in which the latter intersects the
celestial sphere are called the galactic poles. There will be two
of these poles, the one at the hour in question near the zenith,
the other in our nadir, and therefore invisible to us, though seen
by our antipodes. Our horizon corresponds, as it were, to the
central circle of the Milky Way, which now surrounds us on all
sides in a horizontal direction, while the galactic poles are 90
degrees distant from every part of it, as every point of the
horizon is 90 degrees from the zenith.
Let us next count the number of stars visible in a powerful
telescope in the region of the heavens around the galactic pole,
now our zenith, and find the average number per square degree.
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