r the
straight line referred to, would then be perpendicular to the tangent,
and the velocity of the earth round the sun would be uniform, its path
being that of a circle.

The Radius Vector _S_ _A_, however, is not always perpendicular to the
tangent _F_ _E_, and therefore the velocity of the earth is not always
uniform in its motion in its orbit, as sometimes it travels at a lesser
or greater speed than its average speed, which is about 18 miles per
second.

It has to be remembered that the sun itself is in motion, having a
velocity through space of about 4-1/2 miles per second, so that, while
the earth is travelling from _A_ to _B_ the sun is also travelling in
the direction of _S_ _B_. Thus the orbital velocity of the earth, and the
orbital velocity of the sun, together with the Centripetal Force or
universal Gravitation Attraction, are all acting in the same direction
when the earth is travelling from _A_ to _B_, that is, in the direction
of the orbit situated at _B_. This point of the orbit is known as the
perihelion, and at that point the velocity of the earth is at its
greatest, because the earth is then nearest the sun.

According to Newton, the planet when at _B_ would still have a tendency
to fly off into space owing to its Centrifugal Force, but it is held in
check by the Centripetal Force, so that instead of it flying off into
space, it is whirled round and starts off on its journey away from the
sun in the direction of _B_, _C_. The sun, however, is still continuing
its journey in the direction of _S_, _H_, so that not only is the
increased orbital velocity of the earth, which it obtained at its
perihelion, urging the earth away from the sun, but the sun itself in
its advance through space is leaving the earth behind it. The combined
effect of the two motions, the advancing motion of the sun, and the
receding motion of the earth, due to its increased orbital velocity,
drives the earth towards the aphelion, where its distance from the sun
is greatest, and its orbital velocity is the least.

By the time the planet has arrived at point _C_, its motion through
space has gradually decreased, and the Centripetal Force begins to
re-assert itself, with the result that the earth is slowly made to
proceed towards the point _D_ of the ellipse, at which point its motion
is the slowest in orbital velocity, only travelling about 16 miles per
second, while the distance of the earth from the sun is the greatest and
h