e. The
ancients were in error in supposing that it stood entirely apart--the
curve of curves. As a matter of fact it is merely _a special kind of
ellipse_. To put it paradoxically, it is an ellipse which has no
ellipticity, an oval without any ovalness!

Notwithstanding all this, astronomy had to wait yet a long time for a
definite proof of the revolution of the earth around the sun. The
leading argument advanced by Aristotle, against the reality of any
movement of the earth, still held good up to about seventy years ago.
That philosopher had pointed out that the earth could not move about in
space to any great extent, or the stars would be found to alter their
apparent places in the sky, a thing which had never been observed to
happen. Centuries ran on, and instruments became more and more perfect,
yet no displacements of stars were noted. In accepting the Copernican
theory men were therefore obliged to suppose these objects as
immeasurably distant. At length, however, between the years 1835 and
1840, it was discovered by the Prussian astronomer, Bessel, that a star
known as 61 Cygni--that is to say, the star marked in celestial atlases
as No. 61 in the constellation of the Swan--appeared, during the course
of a year, to perform a tiny circle in the heavens, such as would result
from a movement on our own part around the sun. Since then about
forty-three stars have been found to show minute displacements of a
similar kind, which cannot be accounted for upon any other supposition
than that of a continuous revolution of the earth around the sun. The
triumph of the Copernican system is now at last supreme.

If the axis of the earth stood "straight up," so to speak, while the
earth revolved in its orbit, the sun would plainly keep always on a
level with the equator. This is equivalent to stating that, in such
circumstances, a person at the equator would see it rise each morning
exactly in the east, pass through the _zenith_, that is, the point
directly overhead of him, at midday, and set in the evening due in the
west. As this would go on unchangingly at the equator every day
throughout the year, it should be clear that, at any particular place
upon the earth, the sun would in these conditions always be seen to move
in an unvarying manner across the sky at a certain altitude depending
upon the latitude of the place. Thus the more north one went upon the
earth's surface, the more southerly in the sky would the sun's path l