As always the best test of all this Gua, Hexagram and Nonagram is on the weather system...
乾 春，离 夏，坤 秋，坎 冬
and we line up the four ZhengGua (正卦) 乾 离 坤 坎 with 春 夏 秋 冬. What happen to the sequence 震 离 艮 坤 乾 兑 坎 巽 乾 兑 坎 离 艮 坤...repeats? Under this traversal, the four seasons will be mixed up.
震 离 艮 坤 乾 兑 坎 巽 乾 兑 坎 离 艮 坤 is the underlying traversal that brought 乾 震 离 艮 坤 巽 坎 兑 sequence into being. The underlying sequence also brought the weather seasons into being. The weather should follow 乾 震 离 艮 坤 巽 坎 兑, because the Sun and Earth came about after the underlying sequence, 震 离 艮 坤 乾 兑, etc...
Maybe this is the 后天八卦 and 震 离 艮 坤 乾 兑 坎 巽 乾 兑 坎 离 艮 坤...repeats is represented by the Yin Yang symbol found at the center of the BaGua.
