Is there a need for yet another Baedeker of the solar system? Yes, if the' author is an eminent astronomer like Fred Whipple, and the edition a handsome text profusely illustrated with line drawings and NASA photographs--including new material on Saturn. This is the fifth of the Harvard Books on Astronomy, and more accessible to the general reader than some of the earlier titles. Whipple is a patient expositor; he wants to explain about precession and the seasons, to make clear how scientists go about weighing the earth or measuring temperatures on the surface of Venus. He spends a fair amount of time discussing the Moon, pausing occasionally to commend the space agencies for providing scientists with the best pictures, with radar and radio astronomy measurements, as well as with the hard evidence of moon rocks themselves: ""We now know more about the Moon, and perhaps Mars, internally and externally, than we knew about the Earth at the turn of the century."" He takes a conservative stance on the requirements of life; and as regards Mars, he comes out on the negative, though he adds an interesting aside: the polar axis on Mars tips deeply from time to time, it seems, allowing much greater heating of the planet's surface and the possible melting of polar caps--with unknown consequences for life forms. Chapters on the outer planets and on the origins and evolution of the solar system complete the volume, along with detailed appendices for formula-minded readers and amateur telescopists. The considered approach, clear writing, and quiet ring of authority--""Astronomers now accept my theory that comets are fundamentally balls of ice and dirt. . .""--distinguish the text from others available, and may appeal to readers who prefer the straightforward over the speculative.