I was fooling around a little in Snapshot 15w51b in Creative mode, and I created a new world. As I flew upwards to see what had generated, I noticed the Sun had an odd jerky animation. It would rise up a few pixels and then fall down again. Figuring it was maybe because I was so high up, I landed on the ground and looked again, but it was still happening.

Whilst gathering information to report this, I noticed that the Sun was making some progress, but not as much as it should have been. I checked /gamerule doDaylightCycle and it said it was true, so that isn't the problem. As far as I knew, this may be the normal Sun animation, so I started up Minecraft 1.8.9 and created a new Creative world to compare. I was able to confirm that the Sun in 1.8.9 does NOT have the same jerky animation as the Sun in 15w51b.

I attempted to take several screenshots of what the sun was doing back in 15w51b, if you look at them in order you'd get a basic jerky animation of what I was seeing.