As usual in his science books, whatever the series, Asimov tells us not only what is known about his subject but also "how we found out"--and how the different measurements are calculated. This is for youngsters not satisfied with the obvious information that a planet farther from the sun will get less light and heat; by way of offering it Asimov explains, for example, how the distance of a planet from the sun is measured by the apparent width of the sun as seen from that planet. (Later, we get the apparent size of Saturn as seen from each of its satellites.) As in his previous planet books, tables abound. (Table 10 gives "apparent width of the sun seen from the six planets" from Mercury to Saturn, and a later explanation of how distant planets could grow to be enormous involves a table of "Substances Making up the Solar System"--gases, ices, rocks, and metals. Mass, volume, density, orbital eccentricity, periods of revolution, and so on are given not only for the planets but for their satellites as well. And the further out Asimov gets, the greater emphasis he puts on the process of discovery--of which there is likely more to come. Just as irregularities in Uranus' orbit predicted Neptune before it was discovered, later observations of the same orbit indicate another, larger planet on beyond Pluto.