John Henry said to his captain:
A man ain't nothing but a man.
But before I let that steam drill beat me down,
I'll die with my hammer in my hand.
Most important of all, imaging leaves an objective, auditable trail. Visual observations, by contrast, are notoriously unreliable. The history of astronomy is littered with sightings that proved to be false dating back to the invention of the telescope and before. Most people have heard of the canals that Percival Lowell fantasized seeing on Mars. But did you know that Galileo saw cities on the Moon? That particular error has been swept under the rug by people eager to present science as an inexorable, inevitable, one-way march to the truth.
Yet amateur astronomers continue to make valuable contributions to astronomy using nothing but their eyes and wits. How is this possible? And how much longer can it go on?The discoveries that loom largest in the popular imagination are comets. That's no doubt partly due to the fact that the brightest ones are spectacular to look at, and many fainter ones make spectacular photos. But perhaps even more important is that fact that comets are named after their discoverers. We're a society that idolizes the individual despite the fact that never before in history have individual contributions counted for so little.
But only a small fraction of comets are discovered by amateurs these days, and of those, more are found by imaging than visual observing. I bet that most of the amateur-discovered comets would be found by the pros not long after, and that the time lag wouldn't matter much.
Supernova discoveries don't get nearly as much press, but they probably have more scientific value. Again, these would eventually be found by the pros. But supernovas change a lot faster than comets (usually!), and the early stages of a supernova's outburst are very interesting and important. Even if the amateurs only speed discovery by a few hours, that can have considerable value.
Supernovas are extreme examples of variable stars, and amateurs have played a central role in variable-star observing for a long time. But these days, the lion's share of the good work is done with CCD cameras. A skilled amateur can measure a star's brightness with an error of 0.1% using a CCD camera, compared to 5% or 10% for the best visual estimates.
But Arne Henden, head of the American Association of Variable Star Observers, assures me that visual observers still play a key role in monitoring cataclysmic variables stars that erupt unpredictably, including supernovas, novas, and other less glamorous categories. That's because people skilled at observing these stars are scattered all around Earth, eager to be mobilized with a moment's notice. That makes it possible to monitor the stars continuously; it's always nighttime somewhere on Earth.
To my mind, the area where visual observing is most important is meteor science. It just so happens that humans can monitor a huge field of view in very dim light and spot anything that moves. This ability, which presumably evolved for avoiding predators, happens to be ideally suited to detecting meteors as well.
And as with variable-star observers, there's a worldwide network of people who like nothing better than lying outside in the freezing cold, at a time when all self-respecting people are asleep, keeping careful track of minuscule blips of moving light. So most of what we know about meteors comes from visual observation.
I have to conclude that visual observers are most valuable when they're acting in concert, not as heroic individuals. And just as John Henry achieved glory by doing a purely mechanical job, visual observers are at their best when they're emulating machines: objective and dispassionate. Really, their main advantage is cost. If you had to pay meteor observers by the hour, it would cost a fortune; machines would do the same job cheaper. But amateurs do it for love, not money.