Thoughts on Masei

They traveled from Kadesh, and encamped at Hor Hahar, at the edge of the land of Edom.
Aharon the priest went up to Hor Hahar on God'scommand, and died there, in the 40th year of the children of Israel's exit from the land of Egypt, in the fifth month, on the first of the month. Aharon was 123 years old when he died on Hor Hahar. (33:37-9)

Why does the list of Israelite journeys in the desert digress to discuss Aharon's death in such detail?

I think it is because the journey's are originally introduced as being "beyad Moshe veAharon" (33:1). Now that Aharon is dead, they are no longer traveling "under his hand".

OK then - why were the journeys described as being under Aharon's hand in the first place? It makes sense that they are under Moshe's hand, but of what relevance is Aharon?

Perhaps this is related to the role of the Mishkan in their travels. When the cloud rested on the Mishkan they would encamp, when the cloud lifted they would travel, and when they traveled the Mishkan would have a defined place between their camps. Thus the Mishkan dictated their travels, and Aharon, who was in charge of the Mishkan, was (at least symbolically) in control just as much as Moshe was.