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PLANETS AND THEIR SATELLITES

eter, it must considerably exceed the satellites of Mars in size.

The satellites have been most useful to the astronomer in enabling him to learn the exact mass of Mars. How this is done will be explained in a subsequent chapter, where the methods of weighing the planets are set forth.

The satellites also offer many curious and difficult problems in gravitation. Their orbits seem to have a slight eccentricity, and the position of the planes in which they revolve changes in consequence of the bulging of the planet at its equator, produced by its rotation. The calculation of these changes and their comparison with observations have opened up a field of research in which Professor Hermann Struve, now of the University of Koenigsberg, Germany, has taken a leading part.

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