how?

Thanks!!! It was at night, I put the camera on a tripod. The clouds were moving fairly quickly. I focused manually and used a remote trigger in bulb mode. Here's the info: Nikon D70 with Nikkor 24-85 f/3.5-4.5G lense set at f/5.6 & 36mm. ISO 400. Exposure was for 23 sec. The light showing on the tree limbs was from a street light behind and to the right of me.