Since the position of the equator remains parallel to itself while the planet is journeying around the sun it happens that half the time the earth is elevated above the plane of the rings and the remainder of the time it lies below the plane of the rings. Twice in the period of Saturn's revolution around the sun, which occupies nearly thirty years, the earth lies directly in the plane of the rings and at this time the rings entirely disappear from view for a short time. Mid-way between the two dates of disappearance the rings are tilted at their widest angle with reference to the earth and they are then seen to the best advantage. As the date of their disappearance approaches they appear more and more like a line of light extending to either side of the planet's equator. Even in the most powerful telescope the rings entirely disappear from view for a few hours at the time the earth lies exactly in the same plane. It is at this time that the ball of the planet is best seen. Its flattening at the poles, which is nearly ten per cent. of its equatorial diameter then gives it a decidedly oval appearance. Ordinarily one of the hemispheres of Saturn is partly or entirely concealed by the rings so that the oblate form is not so noticeable. It was the change in the tilt and visibility of the rings that so perplexed Galileo when he attempted to make out the nature of these appendages of Saturn with his crude telescope of insufficient magnifying power. So great was his bewilderment when the rings finally disappeared that he cried out in despair that Saturn must have swallowed his children, according to the legend. He finally became so exasperated with the results of his observations that he gave up observing the planet. The true nature of these appendages of Saturn remained a mystery until Huygens solved the problem in 1655, some time after the death of Galileo.