Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
vespĕrasco, āvi, 3, v. inch. n. [vesper], to become evening, grow towards evening: vesperascente jam die, Tac. A. 16, 34: vesperascente caelo, Nep. Pelop. 2, 5.
Impers.: vesperascit, Ter. Heaut. 2, 3, 7: ubi jam vesperaverat, Gell. 17, 8, 1.