The nongravitational part of the moon's orbital acceleration is negative and is due entirely to tidal friction, so far as we know. Ancient astronomical data show with high confidence that the amount of tidal friction 10 centuries ago was about twice what it is now. It is controversial whether mechanisms that have been studied can account for the present amount of tidal friction. They cannot account for the amount found for an epoch 10 centuries ago. Several considerations, including the large changes within historic times, suggest the speculation that much friction originates in the Antarctic ice shelves. The acceleration of the earth's spin comes from tidal friction and from nonfrictional sources. The average acceleration over the past 10 or 20 centuries is negative. However, the corresponding average of the nonfrictional component is positive and larger than the frictional component is now, and it is possible that the present secular acceleration is positive. Thus it is not safe to assume that the spin acceleration has been governed by tidal friction within historic times. Paleontological data indicate that the values of tidal friction estimated for historic times, though variable, are typical. They suggest that the historic values of the nonfrictional component of the spin acceleration are somewhat larger than the average over geologic time. This in turn suggests that much of the nonfrictional component comes from internal processes that conserve angular momentum.
