Well actually you can calculate alot of things. If you want I can give you a rather precise estimate of how fast they are moving based on their sail size. And well once you can do that, you can figure out anything. But applying physics to manga does not really work well.
Also on a side not, I looked up the birthday stuff and it looks as if it does not have anything to do with the plot. Since Oda just made them up in an SBS--hell he even has random people give out birhdays.