A long history
The stars have been getting harder to see for a long time now.
Our astronomer sometimes tells a story of a night when he was up among the telescopes at an observatory in Chile. He went outside the building to walk around, partly to keep awake and alert in the middle of the night, but mostly to enjoy the sky. As he stepped out along the path he caught a movement out of the corner of his eye. He looked, and it was his shadow, cast by Venus. It was faint but clear. When he tells it to most people, they are amazed and sometimes envious at how dark the Chilean sky must be, when something so faint should be clearly visible.
But had he told the story to any astronomer of the nineteenth century, or indeed almost anyone accustomed to the sky back then, they would have wondered what the point was. In the days before brilliantly-lit cities, towns and even roads, everywhere was dark enough to see a Venus-shadow. Indeed, it was noted that shadows cast by the brightest stars were common. There were functioning observatories on the outskirts of London.
Two factors combined to push major telescopes to distant, sparsely-populated places. One was the realization that a major investment, as telescopes were becoming, really should be located where the skies are generally clear and steady. So rather than being attached to major university sites, like those in Britain and the eastern US and northern Europe, they were constructed on mountains in deserts. The second was the remorseless spread of artificial lighting. This took effect a little later, so that (for instance) Mount Palomar could be thought an appropriate place for the world’s largest telescope in the 1940s, but by a couple of decades later was rendered a bright-sky site by the spread of San Diego. Newer telescopes were set up farther afield in the Chilean deserts, on Hawaii and in the Canary Islands.
Even here the spread of technology can be disturbing. The all-sky photographic survey that our astronomer used for one of his researches required two hour-long exposures of each field, of which there were 894 in each hemisphere (north and south). It took many years before the northern survey was completed, because of course each field had to be visible on a clear night and everything had to be working right. The southern survey started later and should have gone more easily, because the telescope was at a better site and there was the experience of the northern survey to draw on. But it was actually harder; more exposures had to be rejected because an airplane flew through them, and in the end a number of airplane-affected plates had to be accepted because there was no alternative.
The spread of lighting of course has had more effect on people who were non-astronomers, those who could not move to distant deserts for their work. In the latter 1990s one of our consultants polled the students in a Physics class he was teaching. One-third had never seen the Milky Way. A decade later the proportion would be two-thirds. We don’t ask our students now.
But there is hope for refuge from brilliantly-lit cities. Lighting can be made less obtrusive (and is often more effective that way). There are designated refuges where the sky is kept free from artificial lighting, or at least where it is under more careful control. We highly recommend the work of the International Dark Sky Association.
Our despair of last week, at the prospect of thousands of new satellites saturating the sky, comes from the fact that there is nothing we can do about it. There is nowhere to get away from them, and no way to shut them off.
