Not only is the orbit of a planet round the sun elliptic in form, but the path of the moon round the earth, or the path of any satellite, as for example a satellite of Mars or Jupiter or Saturn, is also that of an ellipse, the planet round which it revolves occupying one of the Foci. It has also been found that certain comets have orbits which cannot be distinguished from that of an elongated ellipse, the sun occupying one of the Foci. Now let us apply the Law of Gravitation to Kepler's First Law, and note carefully its application. [Illustration: Fig: 1.] Let _A_, _B_, _C_, _D_ be an ellipse representing the orbit of the earth, and let _S_ represent the sun situated at one of the Foci. We will suppose that the earth is projected into space at the point _A_, then according to the First Law of Motion, it would proceed in a straight line in the direction of _A_ _E_, if there were no other force acting upon the earth. But it is acted upon by the attraction of the sun, that is the Centripetal Force which is exerted along the straight line _S_ _A_ (Art. 20), which continues to act upon it according to the principle already explained in Arts. 21 and 22. Now, according to the Second Law of Motion and the Parallelogram of Forces, instead of the earth going off at a tangent in the direction of _A_ _E_, it will take a mean path in the direction of _A_ _B_, its path being curved instead of being a straight line. If the sun were stationary in space, then the mean distance, that is, the length of the imaginary straight line joining the sun _S_ _A_ to the earth, would remain unaltered. The Radius Vector _S_ _A_, or 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.