Most of us would not, I suspect, look at the 17-year reign of Chilean dictator Augusto Pinochet, and then the branch of astronomy concerned with the study of stellar remnants and the origin of heavy elements, and say to ourselves, "You know, those two things seem to me like two sides of the same coin". And I further suspect that most of us would regard anyone who did say that with considerable doubt, particularly in regard to their sanity. And that is why Patricio Guzmán, a 70-year-old documentary who has directed a great many films of which I have seen none before now, had such a steep uphill climb with his Nostalgia for the Light, which isn't quite as idiotically reductive as I've just made it sound, but still sets off to compare two subjects as alike, superficially, as apples and xylophones. The amazing thing is that he does this so well that the film has barely even introduced this comparison before you've completely forgotten that it doesn't make any intuitive sense, until in the one poorly judged moment of the entire 90-minute film, Guzmán hands the mike, as it were, to a man who states in needlessly straightforward language what the rest of the movie has implied with such delicacy and subtlety, and thereby snaps, if only for a brief moment, the precise balance of the film's unconventional but hugely persuasive argument.

The film is about two things literally, and two other things figuratively. Literally, it is about the astronomers who've set up shop in the Atacama Desert of Chile thanks to its unmatchable geological and ecological conditions (it is the driest place on the face of the Earth), and about the women who lost loved ones to Pinochet's regime, and know nothing else beyond the fact that those victims were buried in the vastness of the desert where they were theoretically never meant to be found.

Figuratively, then, it is about the desert as a place and a concept and a force above and beyond the human beings who can do little more than exploit it along the edges; and mostly, it is about the attempt to contextualise the present and create a working plan for the future, by carefully studying the past. Both the astronomers and the mourning women are trying to make sense of something that has already happened, decades ago in one case, billions of years ago in the other (Guzmán, a self-professed astronomy buff, presents nifty facts like "when we look at the sun, what we're seeing is already eight minutes old" with the gosh-wow enthusiasm of somebody who learned a neat fact a long time ago and has never seen any reason to stop being impressed by it); and if this sounds like a slightly arbitrary and forced hook for an entire feature-length argument, it is one of the chief glories of Nostalgia for the Light that you don't think about it in those terms until long after it's over.

Simply in terms of content, this is all tremendously fascinating stuff, individually and for the fascinating collision of the two threads. The authoritarian rule of Pinochet, like most of the history of South America, hasn't really penetrated the cocoon of self-involvement that defines the North American-European corridor, and if only for the unique perspective the film offers on that period of time, and the psychological impact it has had on the survivors, this would be an invaluable document. Guzmán's interview style is tremendously effective - asking his subjects questions that aren't leading, so much as they are curious (we hear the director's voice frequently), and the camera simply waits and watches them as they run through their answers. Not in the interrogative way that you feel with an Errol Morris documentary, for example, where it seems to be the filmmaker's greatest hope to catch the subject up and make them sweat for a bit; but out of a knowledge that what these women have to say is personal and important, and it is more important for the filmmaker to follow where they go rather than lead them to where he wants the movie to be. The same is true of his interviews with the scientists; every time the film switches from the women in their fruitless search the the astronomers in their comfortably sterile labs, one has the immediate pang of wondering, "how much can this really matter in the face of human suffering?", and every time, Guzmán finds the answer: because it's interesting, and because it tells us who we are.

As I said, the content here is endlessly worthwhile: a man recalling with admirable steadiness the time he measures out the precise dimension of the prison camp he was thrown in, a scientist enthusiastically pointing out that the calcium in our bones could be millions of years old (a nugget which is pointedly contrasted with the solemn image of a library of unidentified skeletal remains), and the manner in which the film travels from one idea to the next is awfully clever even when Guzmán strains a little to make his point. What pushes Nostalgia for the Light to the next level is the considerable elegance of its construction as a film. First, in the way it is anchored by the poetic musings of the director's narration, which humanises abstract concepts and draws abstract readings from human problems, and gives the whole film the feeling an essay rather than just a talking heads piece. Then there are the images, which run the gamut from that same heart-stopping sight of anonymous blue boxes, each of which holds the last remains of a person, to photography of the grandest sights in space, to the slow, stately movement of the ancient telescope that, despite its uselessness in modern science, is taken here as the physical manifestation of the thirst to find the truth of the past. It's a bit dizzying to find a documentary that does so much of its storytelling through visuals, and the contrast between visuals (out of many potent moments, the one that hit me the most was when the rocks of the desert floor were shot in such a way as to mirror the images we'd seen already of galaxies and nebulae), but this is what makes Nostalgia for the Light not just a phenomenal human story - which was guaranteed from the moment Guzmán chose his subject - but a gripping work of cinema as well.