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Srila Vyasadeva, the author of the Bhagavatam, was able to take advantage of advanced astronomy. Since he made use of a unit of distance (the yojana) defined accurately in terms of the dimensions of the earth, he must also have had access to advanced geographical knowledge. Such knowledge of astronomy and geography was not developed in recent times until the late eighteenth and early nineteenth centuries. It was not available to the most advanced of the ancient Greek astronomers, Claudius Ptolemy, in the second century A.D., and it was certainly unknown to the pre-Socratic Greek philosophers of the fifth century B.C.