Page:Popular Science Monthly Volume 75.djvu/536

532 the hands of long-continued ones. We can calculate about how much that flattening would be. That of the earth is that of Mars, and both planets rotate in approximately twenty-four hours. That of Venus for a like spin would lie between the two figure, because in mass and density she falls between the earth and Mars. Let us say for it, which would be close to the truth.

Now Venus on occasion offers peculiar opportunity to measure any such flattening if it existed. For at times she passes in transit across the sun's face. At that moment she presents an absolute absence of phase and in consequence any correction due to asymmetrical illumination is self-eliminated. Furthermore, she then shows the largest of all planetary disks, one of 60 seconds in diameter. A flattening of would amount therefore in her case to 0″.22. Such a quantity could not possibly miss of detection. For that of Mars, which is only half as much and is not so well displayed, has nevertheless been measured. Yet no divergence from perfect sphericity has ever been found in the globe of Venus, though diameters at all azimuths have been carefully taken when she is seen silhouetted in transit against the sun.

I may have seemed to dwell at unnecessary length upon the time that Venus takes to turn. But there is cause. The rotation time of Venus, the determination, that is, of the planet's day, is one of the fundamental astronomical acquisitions of recent years. It is not a question of academic accuracy merely, of a little more or a little less in actual duration, but one which carries in its train a completely new outlook on Venus and sheds a valuable sidelight upon the history of our whole planetary system. For upon it turns our whole knowledge of the planet's physical condition. More than this, it adds something which must be reckoned with in the framing of any cosmogony.

To this we shall now proceed and if the deductions and the phenomena which corroborate them appear almost romantically strange it is in the facts themselves that the romance exists.

In the first place such isochronism gives us a glimpse into the planet's past. That the day should coincide with the year means that it has been brought to this condition. For that it can always have been so is mechanically highly improbable. On the other hand, there is a cause continually tending to bring about such a result; tidal friction. Under the immense forces at work the planetary masses behave as if they were plastic. In consequence tides of the whole substance are set up in them if they rotate, and these tides act as a brake upon the rotation until they finally retard it to coincidence with the orbital revolution.

That Venus now turns the same face in perpetuity to the sun, lets us look down a long vista in her career, and gives us a very instant idea of a phase in a planet's history: that long slow change by which a day is lengthened to infinity. That Venus should have suffered such action is in keeping with theory, though it could not have been predicted in the absence of fact?. For Venus falls exactly on debatable ground, on