The last of the preliminary points to be taken up is the form of the planet. Consideration of it makes in some sort a bridge from the planet’s past to its present. For its deviation from a perfect sphere tells us something of its history.
Between the shapes of the large planets, Jupiter, Saturn, Uranus, and probably Neptune, and those of the small ones, Mercury, Venus, the Earth, and Mars, there is a striking dissimilarity, the former being markedly oblate spheroids, the latter almost perfect spheres.
Into the cause of this, very interesting as it is, we have not here space to go. The effect, however, is so noticeable that while the most casual glance at the disk of Jupiter will reveal its ellipticity, the most careful scrutiny would fail to show Mars other than perfectly round.
Nevertheless, the planet is slightly flattened at the poles. Measures have repeatedly been made to determine the extent of this flattening, with surprisingly discordant results, most of the values being much too large.
Observations at Flagstaff during this last opposition have not only shown that most of the values were too large, but have revealed the cause of their discrepancy. There turns out to be a factor in the case, hitherto unsuspected, whose presence proves to be precisely such as would cause the observed variations in measurements. It not only accounts for the fact of