‘Fl (3) 15s’. So what, precisely, does that mean to the practical navigator ? Obviously, it’s a light flashing in a group of three at fifteen second intervals. The thing is, when do you start counting? The answer is, begin as soon as you see the first flash and carry on numbering up the seconds until the same flash comes around again. Whether a light flashes or occults, the time period is the whole cycle, not just the period of darkness. It pays to train yourself to count seconds accurately. Try the old stand-by ‘a thousand and one, a thousand and two etc. It works like a charm up to twenty seconds.