Page:Astronomy for Everybody.djvu/233

the keenest vision. A story has been told, by Arago, I think, of a woman who professed to be able to see them at any time and even pointed out their positions. It was found, however, that she described them as on the opposite side of the planet to that on which they were really situated. It was then found, or inferred, that she took the positions from an astronomical ephemeris, in which diagrams of them were given, but in which the pictures were made upside down in order that the satellites might be seen as in an ordinary inverting telescope. But it seems quite likely that, when the two outer satellites chance to be nearly in the same straight line, they may be visible by their combined light.

From the measures of Barnard it may be inferred that these bodies range somewhere between two and three thousand miles in diameter. Hence, they do not differ greatly from our moon in size.

Only four satellites were known until 1892; then Barnard, with the great Lick telescope, discovered a fifth, much nearer the planet than the four others. It makes a revolution in a little less than twelve hours, the shortest periodic time known except that of the inner satellite of Mars. Still, however, it is a little longer than the rotation time of the planet. The next outer one, or the innermost of the four previously known, still called the first satellite, revolves in about one day eighteen and a half hours, while the outer one requires nearly seventy days to perform its circuit.

In its visibility the fifth satellite is the most difficult known object in the solar system. Through only a few