In and out of phase on the Arran ferry

I was watching the radar aerials on the top of the ferry at Ardrossan whilst we were waiting to go on. There were two horizontal rotating aerials. The top one was abput twice the length of the lower one. The lower one was spinning about twice as fast. This meant that sometimes they had their full length facing me. At this point we'd say that they were in phase - doing the same thing at the same time in their rotation. They'd spin a bit longer and whilst the top one was showing it full length, the bottom one would be end on. Now we'd say that they were 90 degrees out of phase. Next, one would show its full front length but the other would show its full back length. They were 180 degrees out of phase. And so on. Fans of maths will realise that 180 degrees out of phase is the same as pi radians out of phase.