Billiards Q. Is the Earth really as smooth as a billiard ball? A. ''On average, certainly yes,'' said John O'Keefe of the Goddard Space Flight Center in Greenbelt, Md.

Dr. O'Keefe, who was formerly a geodesist, or expert on the size and shape of the Earth, for the Army Corps of Engineers, pointed out that the Earth is more than 12,000 kilometers in diameter. Mount Everest is less than 10 kilometers high, and the profoundest depths of the ocean are around 11 kilometers. Thus, the most extreme variations in the Earth's surface ''are roughly on a scale of one part in a thousand,'' he said, ''and an Everest is very rare.'' The equivalent bump on a 2 1/4-inch billiard ball would be about two-thousandths of an inch, ''so most parts of the Earth are a lot smoother,'' he said.

''But smoothness is one thing and roundness is another,'' Dr. O'Keefe said. ''The Earth is flattened from top to bottom, so it is an ellipsoid, not a sphere.''

The deviation between an equatorial radius and a polar radius is about 20 kilometers, he said, so no one would want to play billiards with a ball shaped like the Earth.