Seeing is believing – but is believing seeing?

I well remember an occasion, many years ago – not so long after the end of WW2 – when I was in the dome of the Lowell Observatory at Flagstaff in Arizona. I was there for a special reason: I was one of the team devoted to mapping the Moon really accurately. The idea of lunar travel no longer seemed absurd, and visionaries such as Wernher von Braun were no longer regarded as deranged, but nobody could be sure when it would happen. On this particular night I had been using the Lowell 24-inch refractor to chart some of the lunar libration regions, but eventually the seeing fell away as the Moon lost altitude, and I was compelled to give up. Then a thought struck me. Mars had risen, so conditions ought to be at least tolerable. Why not have a look at it?

It was Lowell, remember, who drew the network of Martian canals, and believed the Red Planet to be inhabited by brilliant engineers capable of building an elaborate irrigation system, able to pump water from the melting polar caps through to the centres of civilisation in warmer latitudes.
His drawings were fascinating – and if they had been accurate, then Mars would have been inhabited, because no natural processes could possibly produce such a network. If the canals had not been seen by anyone else, most people would have assumed there was something wrong with his eyes – but Lowell was not alone. Other observers also drew canals, and some of their drawings were made with small telescopes.

Lowell died in 1916, but the canals did not die with him. Some people could not see them at all; others claimed that they were glaringly obvious, and some of the canal supporters were very experienced observers. Richard Baum, who has been a welcome contributor to our magazine, produced drawings strikingly similar to Lowell’s, even though his telescope was of very modest aperture. I had never seen canals with the 11.5-inch reflector at my own observatory, then at East Grinstead in Sussex, but the Lowell refractor was a very different proposition, and as I turned it to Mars I was frankly excited. Would I see canals?

The planet came into view; definition was good, and the view was far better than anything I had experienced before. I concentrated grimly. There were the dark markings, which I knew well enough. But canals? I had half-expected to see them in some form or other, but not a trace. Try as I might, I could not see anything remotely resembling a canal, and I never did, even with the 60-inch reflector at Palomar. I was equally unsuccessful with the ‘wave of darkening’ allegedly spreading from poles to equator as the plants were revived by the moisture-laden spring winds. To me, these phenomena were conspicuous only by their absence.

It was a puzzle. Others could see canals; I couldn’t – yet I knew quite well that Richard Baum, for example, is a much better observer than I am, as well as being a far better draughtsman. The situation was cleared up with the results from the spacecraft of the 1960s. There are no canals and no wave of darkening; the canals simply do not exist (they are not identical with features such as the Valles Marineris), and the observers were simply ‘seeing’ what they expected to see. Now that we know what Mars is really like, observers no longer draw canals!

Schoolboy errors

We can always trust the human eye, but we cannot always trust the human brain to give a correct interpretation. In 1949 I enlisted the help of 30 prep-school boys, aged between 10 and 13, all of whom were literate and intelligent. I prepared a drawing of Mars, minus canals but with a few roughly-aligned features. I set up the drawing at the far end of the school hall, gave them blank discs, and asked them to draw what they saw. The drawings were of varied quality, but reasonable. Not one showed a Lowellian canal, though a few of the boys did join up the roughly-aligned features into a broadish streak.

Later that day I went to another prep-school and repeated the experiment: same types of boys, same ages, same distance from my drawing. There was one difference: this time I showed them one of Lowell’s drawings, taken from his book. I set up my sketch, and the boys copied it. This time, more than half of the drawings showed canals. When I ‘came clean’ and explained, the boys were taken aback. Not so long ago one of them, now a famous doctor, came to see me. “You proved your point,” he said ruefully. “I would have sworn that I could really see those damned lines!”

I tried another experiment on a Sky at Night programme in the 1970s. When Venus is a crescent, very keen-sighted people can see the phase with the naked eye. So when the planet was suitably placed, I showed a photograph of it and asked viewers to draw the appearance as they saw it without optical aid. I had 422 replies, and of these 212 showed the crescent. But, ill-naturedly, I had shown the telescope view: south at the top, horns pointing to the right. Four people were puzzled. “I can see the crescent,” one of them wrote, “but the horns are pointing the wrong way. Is there something the matter with my eyesight?” I hastily reassured him: of course, these were the only four viewers who genuinely had seen the crescent. The rest were ‘seeing’ what they expected to see.

I am tempted to try another test. We have red stars, orange stars, yellow stars and blue stars, but single green stars are absent: the companions of red giants such as Antares and Alpha Herculis do look green, but this is because of contrast. In the 19th Century GF Chambers stated that the only green single naked-eye star is Beta Librae (Zubeneschamali). But is Beta Librae really green? Not to me – but I know that even with my monocle I cannot claim to have better than average eyesight. So do please go outdoors on a clear night, and check. If there is true greenness there, all well and good; if not, then many people have been guilty of self-deception.

This column first appeared in Issue 30 of Sky At Night Magazine, November 2009