Page:The Solar System - Six Lectures - Lowell.djvu/35



proper place. Oppolzer, by assuming the major axis, showed that this may have been Biela's comet.

Since then, other comets have been observed to split up, due to the action of the planets near which they chance to pass; and Callandreau has shown that the event ought not to be so very uncommon.

Another point connected with these meteor streams must be noticed. Each of them is associated with the orbit of some particular planet. The planet in some sense shares with the Sun a control over the stream. It cannot cause the stream to circle round itself, but it can, and does, cause it to pay periodic obeisance to its might. The stream's perihelion remains at the Sun, but its aphelion becomes its periplaneta. It sweeps about the planet at the one end of its path somewhat as it sweeps round the Sun at the other.

The Andromedes are thus dependent on Jupiter, the Leonids on Uranus; while the Perseids and the Lyrids go out to meet the unknown planet which circles at a distance of about forty-five astronomical units from the Sun.

It may seem to you strange to speak thus confidently of what no mortal eye has seen, but the finger of the sign-board of phenomena points so clearly as to justify the definite article. The eye of analysis has already suspected the invisible.