anet, after having received its first impulse,
is deflected from its original straight path, and bent towards that
luminary, and by the combined action of the projective and attractive
forces is made to describe an orbit which, if elliptical, has one of its
foci occupied by the Sun. So evenly balanced are those two forces, that
one is unable to gain any permanent ascendency over the other, and
consequently the planet traverses its orbit with unerring regularity,
and, if undisturbed by external influences, will continue in its path
for all time.

Milton describes the position of the planets in the sky as--

Now high, now low, then hid;

and their motions--

Progressive, retrograde, or standing still.

It is evident that Milton was familiar with the apparently irregular
paths pursued by the planets when observed from the Earth. He knew of
their stationary points, and also the backward loopings traced out by
them on the surface of the sphere.

If observed from the Sun, all the planets would be seen to follow their
true paths round that body; their motion would invariably lie in the
same direction, and any variation in their speed as they approached
perihelion or aphelion would be real. But the planets, when observed
from the Earth, which is itself in motion, appear to move irregularly.
Sometimes they remain stationary for a brief period, and, instead of
progressing onward, affect a retrograde movement. This irregularity of
motion is only apparent, and can be explained as a result of the
combined motions of the Earth and planets, which are travelling together
round the Sun with different velocities, and in orbits of unequal
magnitude.

In his allusion to the Copernican system the 'planet' 'Earth' is
described by Milton as seventh. This is not strictly accurate, as only
five planets were known--viz. Mercury, Venus, Mars, Jupiter, and Saturn;
but to make up the number Milton has included the Moon, which may be
regarded as the Earth's planet.

The three motions ascribed to the Earth are--(1) The diurnal rotation on
her axis; (2) her annual revolution round the Sun; (3) Precession of the
Equinoxes.

The rotation of the Earth on her axis may be likened to the spinning
motion of a top, and is the cause of the alternation of day and night.
This rotatory motion is sustained with such exact precision that, during
the past 2,000 years, it has been impossible to detect the minutest
difference in the time in which t