There are no curves in Minecraft, but there are the illusions of curves created by patterns of squares. From millions of miles away, a sun made entirely of cubes would appear round.

I don't know why anyone even cares what the vanilla sun looks like anyway though. Thanks to Painterly, mine has been a very blocky circle for over a year now. No offense to the guys at Mojang, but I haven't cared about changes to the game's textures since a week or two after I started playing. If you don't like it either way, change it back.