Early in "Januworry" I found myself nestled in a placed between Greyton and McGregor. A very beautiful valley that you can hike through between the two spots. I did not hike, no. Not to either one of those extremes, at least. I did hike down into the valley to find a river and then swim. That was rad.
Ok, the rule isn't about hiking nor pretty girls in a valley. It's about the mathematical relationship between the time it takes for stars to blur in a long exposure and the length of your lens. Ooh! As far as I understand it you divide 500 by the length of your lens to get the optimal measure of time you can expose for before the stars blur in the image.
In my case, I was using an 11-17mm lens so the math is then 500 / 17 = 29 (and a bit). You don't have to be super strict with this if you're playing around. I set the shutter to 25 seconds @ ISO200 f/3.2 and the trickiest thing here was getting the shot focused. It was hella dark! I mean like close-your-eyes-under-a-blanket-in-space dark. It's not that these came out rad but more that there were a few that had lens flares from the moon, and long shadows from how bright it was. It was more fun playing around than actually being completely meticulous!