Ordering the planets

There are some things that the naked eye just can’t do.

Some time ago we mentioned a calculation our astronomer was carrying out, just from curiosity.  Inspired by the Great Conjunction of Jupiter and Saturn of 2020-1, he wondered what it might look like if the former planet had actually passed in front of the latter.  One would think that actually seeing one planet pass in front of another should easily tell us which is the nearer. In a telescope, of course, one would see something moderately spectacular, the cloud bands of Jupiter occulting the rings of Saturn.  Without one, however, all the naked eye would see would be two points of light approaching each other over the space of several hours; eventually they would be too close to see separately and would appear as a single starlike object.  It would dim very slightly during the transit, too slightly and slowly to see.  Then they’d separate again.

[For planetary mutual phenomena, either “transit” or “occultation” seem to be applicable terms; we’ll use both.]

Our astronomer then made his question more general and specific at the same time: would it have been possible, in principle, for a naked-eye astronomer to have determined the order of the planets from any series of mutual occultations?  Historically they were far too few to be of any use, and there is no record of any being observed.  But he went on to consider generic central transits, first of the outer planets at opposition, then the inner planets when not too close to the Sun to see, then inner planets on outer ones.

The results were surprisingly meager.  Venus, when in the near part of its orbit, would cause a noticeable dimming of all the other planets.  The geocentric astronomer would, then, place it as the nearest planet after the Moon.  No other conclusion could be reached.

If our hypothetical astronomer were confident enough of his predictions to be sure that a transit had occurred, more information might be gleaned. (Such confidence would require an accuracy in both observation and calculation well beyond that of the historical situation.)  Since the red color of Mars persisted as it occulted Jupiter or Saturn, it would be the nearest of the three.  One might conclude that, as Jupiter was not dimmed by Saturn, it was nearer of those two.  The failure of Mercury to dim any other planet significantly (actually due its small size) would make it the most distant, a distinctly unsettling conclusion given its speed.  However, all this depends not only on much better predictions than were actually available, but also on estimates of the sizes of bodies too small to be resolved by the eye.

So even given a set of convenient phenomena, the eye alone could not produce a conclusion about the order of the planets.  The smallest telescope would settle the matter (by exploding the geocentric theory itself, but that’s another issue.)

So we’re led to further questions, bordering on science fiction.  Would a species with just a little better vision have produced a heliocentric astronomy thousands of years earlier?  Would a solar system with planets a little larger or closer together lead to an accelerated astronomy?  We’ll leave the exploration of those questions to the authors.