He had been studying the red comet, believed to be a herald of great wars, conflagrations and massacres. Aarhenius had hoped that he would be able to calculate when the comet would appear again, rationalising that it would be possible to better prepare for the war it would herald than the present one.
After escaping from Tir ná Lia with Kelpie and Ihuarraquax, Ciri started travelling between different worlds, trying to return in the Northern Kingdoms. At one point, she appeared on the property of Aarhenius and inquired where and in what time period she had found herself. Aarhenius had been so overwhelmed he was unable to formulate a coherent sentence, and Ciri vanished once more, unaware that she had in fact reached her destination.
Ironically, the astronomer was a believer in other worlds and had been waiting for the opportunity to prove that travel between them was possible, and yet when said opportunity manifested itself in the form of Ciri he had been completely oblivious.