========+=========+==============+============================ |PARABOLIC|SPEED OF SHORT|SPEED OF ACTUAL SHORT-PERIOD | SPEED |PERIOD COMETS | COMETS ABOUT JUPITER --------+---------+--------------+---------------------------- Sunrise | 2.4 | 3.5 | 3.3 Sunset | 1.0 | 1.0 | 1.0 ========+=========+==============+============================ As the actual number encountered is between 2 and 3 to 1, we see that the greater part must be travelling in the same sense as the Earth, since they come indifferently at all altitudes from the plane of her orbit. 2 DENSITIES OF THE PLANETS The densities of the principal planets, so far as we can determine them at present, the density of water being unity, are:— Mercury 3.65 Venus 5.36 Earth 5.53 Moon 3.32 Mars 3.93 ———— mean 4.36 Jupiter 1.33 Saturn 0.72 Uranus 1.22 Neptune 1.11 ———— mean 1.09 Sun 1.38 The second decimal place is not to be considered as anything but an indication. 3 VARIATION IN SPECTROSCOPIC SHIFT In the case of a body reflecting light, the shift differs from that for a body emitting it. If the planet be on the further side of the Sun, the approaching rim advances both toward the Sun and toward the Earth, thus doubling the shift. The receding rim recedes in like manner. At elongation the rims approach or recede with regard to the Earth, but not the Sun, and the shift is single as for emission. At inferior conjunction rotational approach to the Earth implies rotational recession from the Sun, and the two effects cancel. 4 ON THE PLANETS’ ORBITAL TILTS The tilts of the plane of rotation of the Sun and of the orbits of the several planets to the dynamical plane of the system tabulated are:— Sun 7° Mercury 6° 14′ Venus 2° 4′ Earth 1° 41′ Mars 1° 38′ Asteroids various Jupiter 20′ Saturn 56′ Uranus 1° 2′ Neptune 43′ where, in the determination of that plane, the latest values of the masses of the planets and the rotations of the Sun, Jupiter, and Saturn have been taken into account. These tilts suggest something, doubtless, but it is by no means clear what it is they suggest. They are just as compatible with a giving off from a slowly condensing nebula as with an origin by shock. The greater inclinations of Mercury and Venus may be due to their late birth from the central mass without the necessity of a cataclysm, the rotation of that central mass out of the general plane being caused by the consensus of the motions of the particles from which it was formed. The accordance of the larger planetary masses with the dynamical plane of the system would necessarily result from their great aggregations. So that this, too, is quite possible without shock. 5 PLANETS AND THEIR SATELLITE SYSTEMS