One step beyond the eyes

We consider an anachronism.

Some time ago we referred to calculations our astronomer was performing inspired by the Great Conjunction of 2020, in which Jupiter almost passed in front of Saturn from our viewpoint.  The idea was to work out what a naked-eye astronomer would see if one planet did pass in front of another, and if the observation could be used to tell which was closer.  You may remember that Ptolemy’s geocentric theory could predict the positions of the planets with fair accuracy, but each was treated alone.  There was no way to tell whether Mars, for instance, was closer than Saturn or farther away.  (There was a sort of consensus of opinion, but no proof either way.)

Naively, one would expect a planetary transit/occultation to settle the matter: just watch to see which one gets hidden.  But to the naked eye each planet is a point of light, with no apparent size.  In an event, one would approach another, and they would merge into one light for a time.  There would be a dip in brightness during the event, then they’d separate.  If an obviously dimmer planet passed in front of a brighter one, it would be possible to tell that the latter was further away.  But the dimmer one would have to be about as large as the brighter one (or it would only block a little of the light).  If the brighter one shone apparently undimmed, one might conclude that it was closer, but only if one trusted one’s predictions to a fine accuracy.

The main result of the calculations are that the only firm conclusion one could reach was that Venus was the closest planet.  If the predictions were accurate, and one was willing to argue from seeing no change, one could conclude that Mars was closer than Jupiter or Saturn, and that Mercury was farthest of all.  (Even though it is actually closer than Mars, Jupiter or Saturn, Mercury is so small that it dims their light imperceptibly.)  It might be possible to decide that Saturn is beyond Jupiter.  So mutual phenomena could settle the order of the universe, but maybe not.

All this would change with even a small telescope.  Galileo’s first efforts showed the planets to be disks of a certain size.  It would be clear, with such an instrument, which planet was coming in front and which going behind.  The order of the outer planets would be clear.  It would be revealed that Venus passes sometimes in front of Mercury and sometimes behind.  Of course the observations that Galileo actually made, of the phases of Venus and the moons of Jupiter, showed that the question of the ordering of the planets in distance from the Earth was the wrong one to ask: the Solar System is centered on the Sun, not Earth.

So what would have happened if the ancient observers had been able to go one step beyond the eye?  It would be too much to say they’d immediately adopt a heliocentric universe, and that the science of the seventeenth century would have appeared over a thousand years earlier.  There was a whole structure of thought that had to change first.  But we’re struck by how much difference a small step could make.