There was really no way to meter that exposure, due to "reciprocity failure" and the fact that the exposed image was probably not too stable. Paper doesn't have to "hold" the exposed image more than a few minutes before it gets developed, unlike film which might sit around for weeks or months before being processed. I guess that it was in his favor that he used a very slow capture medium, choosing photographic paper rather than film.
For those unfamiliar with the concept of reciprocity failure it means that at very short or long exposures you have to give more exposure. We all know that 1/250 second at f/1 is the same exposure as 1/15 at f/4 is the same as 1 second at f/16 or 4 seconds at f/32, right? But your 4 second negative might be a bit on the thin side, and often even the 1 second exposure will be. An indicated 1 minute exposure might be a stop or more underexposed and when you get up into hours of exposure, oh WOW. You don't calculate, you experiment. Long exposures tend to increase contrast. With color film the three different layers probably react differently from one another, resulting in some unexpected color shifts as well.
It is an interesting image though, different than star trails circling the north star that's been done so many times before.