Yesterday was quite a good night to watch the sky using my 8 inch dobsonian (until moonrise at least). I was lucky enough to catch some satellites flying into my sight as I was searching for some deep sky objects. One of those satellites was a little different though.
I followed it a bit and noticed a period oscillation in its brightness. When I first saw it, it was at its brightnesses peak point, going fainter from there until it eventually vanished into darkness. I thought it might have set over the horizon, without any more sunlight for it to reflect, but a few seconds later it became visible again. I've seen three full cycles of it going from invisible, to visible, to invisible again, before I lost it.
The object went overhead from east-northeast to west-southwest and according to my sky tracking app it appeared to be a Cosmos 1400 rocket stage. Approximate position was Frankfurt/Germany, at about 11pm-12am local time. My question is, how come the satellite's brightness is oscillating so periodically?
My initial thought was that the satellite (rocket stage) is rotating and only reflective on one of its "sides".
Am I right with my initial assumption? If not, how else could it be explained?