Page:Philosophical Transactions - Volume 001.djvu/150

 estimates to be, 9 hours and 56 minutes.

For as Kepler did before conjecture, from the motion of the Primitive Planets about the Sun as their Center, that the Sun moved about its own Axis, but could not prove it, till by Galileo and Shiner the Spots in the Sun were discovered; so it hath been thought reasonable, from the Secundary Planets moving about Jupiter, that Jupiter is also moved about his Axis; yet, till now, it hath not been evinced, by Observation, That it doth so move; much less, in what Period of Time. And the like reason there is to judge so of Saturn, because of the Secundary Planet discovered by Monsieur Hugens de Zulichem to move about it; (though such motion be not yet evinced from Observation:) as well as that of the Earth, from its Attendant the Moon.

Whether the same may be also concluded of the other Planets, Mars, Venus, and Mercury, (about whom have not yet been observed any Secondary Planets to move,) is not so evident. Yet there may be somewhat of like probability in those. Not onely, because it is possible they may have Secundary Planets about them, though not yet discovered; (For, we know, it was long after those of Jupiter, before that about Saturn was discovered; and who knows, what after times may discover about the rest?) But because the Primary Planets being all in like manner inlightned by the Sun, and (in all likely hood) moved by it; it is likely that they be moved by the same Laws and Methods; and therefore, turn'd about their own Axis, as it is manifest that some of them are.

But, as for the Secundary Planets, as well those about Jupiter, as that about Saturn; it is most likely that they have no such Rotation upon their Axis. Not so much, because, by reason of their smalness, no such thing hath been yet observed, (or, indeed, could be, though it were true;) But because they being Analogical to our Moon, it is most likely that they are moved in like manner. Now, though it be Rh