Classicly

His astronomical memory, insofar as it covered the post-nova period, was perfect, and he paid more attention to the details of star-and-planet configuration than he had ever done before. Gradually a star-map formed in his mind, a map that covered enormous distances of space. Those places he had investigated and eliminated from consideration were slowly crossed off. It was a large needle he had to find, and his own powers were considerable, but the haystack he had to search was infinite. There was no telling how many more centuries would pass before he found it.