[X] My Assistant

[Rob Pinnegar]

Several years ago, when I was still at U of Western Ontario, I was browsing through some of the old books in the astronomy section of the main campus science library. One of these dated from the late 1800s and contained a couple of hypotheses, current at the time, that are worth passing along. (I don't think I've posted these before... apologies if I'm repeating myself.)

The first of these had to do with the number of moons around each planet. After Asaph Hall discovered Phobos and Deimos in 1877, no planetary satellites were found for another fifteen years. This book must have been published during that time, because the author pointed out that Earth had one moon, Mars two, Jupiter four and Saturn eight. This had apparently led people to speculate that Uranus had sixteen moons, and Neptune thirty-two, most of them waiting to be discovered. (It seems that Bode's law died very, very hard.) My guess is that this probabaly dates the book to just before 1892, when Amalthea was discovered, as it would have taken people a while to start thinking that all the moons out to Saturn had been found. The death blow came six years later, when Pickering picked up Phoebe.

The second hypothesis was the really neat one. In the late 1800s, Jupiter and Saturn's axial tilts were well known because Jupiter's bands were clearly visible, and of course Saturn's rings provided even better evidence of its tilt. However, since Uranus and Neptune are so bland and so far away, estimates of their axial tilts had to be based on satellite orbits. As a result, although Uranus' axial tilt was pretty well determined, the estimate at the time for Neptune was terribly wrong because it was thought to be same as the plane of Triton's orbit. Since Triton's orbit is circular, everyone (quite reasonably) thought Triton was a regular satellite. In fact, there weren't any irregular satellites known then -- Iapetus and Luna would've had the largest known orbital inclinations. And Proteus hadn't been discovered. So there was no way of telling that this assumption was wrong.

This gave estimates of the axial tilts of Jupiter, Saturn, Uranus and Neptune as 3, 27, 98 and 160 degrees. At this point, someone had the bright idea that there was an unknown process operating in the outer Solar System that was causing the outer planets to flip over from the outside in. I thought this was terrific. Naturally, when better telescopes were invented, this was found to be wrong... but it was still a great idea.

Although it's unscientific to say so, in a way it's always a pity to see a brilliant idea blown out of the water by fact. Nonetheless, even if we can't keep wrong ideas around for sentimental reasons, at least we can appreciate them for the merits they had, given the information that was available at the time.