19. Having mentioned the Sun’s turning round his axis, and as there will
be frequent occasion to speak of the like motion of the Earth and other
Planets, it is proper here to inform the young _Tyro_ in Astronomy, that
neither the Sun nor Planets have material axes to turn upon, and support
them, as in the little imperfect Machines contrived to represent them.
For the axis of a Planet is a line conceived to be drawn through it’s
center, about which it revolves as on a real axis. The extremities of
this line, terminating in opposite points of the Planet’s surface, are
called its _Poles_. That which points towards the _northern_ part of the
Heavens is called the _North Pole_; and the other, pointing towards the
_southern_ part, is called the _South Pole_. A bowl whirled from one’s
hand into the open air turns round such a line within itself, whilst it
moves forward; and such are the lines we mean, when we speak of the Axes
of the Heavenly bodies.
[Sidenote: Their Orbits are not in the same plane with the Ecliptic.