THE PLANET JUPITEK.

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But for a place in the position p, which is passing throughthe instant of noon, the two velocities are in exactly oppositedirections, as the arrows again indicate. An observer in sucha position (so far as these velocities are concerned) would there-fore at noon, or within a brief period before or after it, be almostat rest in space. And, inasmuch as Jupiter ’s velocity in itsorbit varies by as much as 1,500 miles per hour on either sideof its mean value, and that mean value only differs, as we havestated, by about 750 miles per hour from the velocity causedby the axial rotation of a point upon its equator, it follows

Fig. LXVI.—Comparison of Jupiter ’s orbital velocity with, the velocity of rotation roundits axis of a point upon its equator.

that there must be times in each period of the planet’s circuitround the Sun, when the one velocity will at noon preciselyneutralize the other.

We have therefore shown that, so far as the above velocitiesare concerned, an observer upon the equator* of Jupiter would

* That is to say, upon the equator of the surface of the globe, whichis visible to us. The compensation of the one velocity, to which wehave referred, by the other, would, of course, more or less fall short,if the real equator of the body of the planet were that of a globe situatedat a considerable distance within the limits of the sphere which we behold,according to the theory mentioned in the earlier part of this lecture. Thenearer such an equator might be to the planet’s centre, the smallerwould be the velocity communicated by the planet’s rotation to anypoint upon it.