Supernova Remnant Help!! Hi guys, Im just about to finish up my project work for my thesis which is on Cas A. I have calculate the distance between my knot and old knot from a previous experiment in arcsecs. I now need to find the velocity of my knot. Any ideas on how to calculate it?? I know i need to divide the distance by the number of years but I dont know what units. Cheers, Fran.