Aristyllus (Greek: Ἀρίστυλλος; fl. ca. 261 BC) was a Greek astronomer, presumably of the school of Timocharis (c.300 BC). He was among the earliest meridian-astronomy observers. Six of his stellar declinations are preserved at Almajest 7.3. All are exactly correct within his over-cautious rounding to 1/4 degree. See discussion (and lessons) at DIO 7.1 ‡1 p. 13 (2007).
Aristyllus was long mis-dated to c.300 BC (which made his data look among the poorest of the ancients); but when his correct date was found by least-squares (Isis 73:259-265  p. 263), it was realized that his star declinations' accuracy was unexcelled in antiquity. His data suggest that he worked in Alexandria: see DIO 4.1 ‡3 Table 3 p. 45 (2004).
A lunar crater, Aristillus, aptly near the Moon's meridian and at a lunar latitude roughly equal to the terrestrial latitude of Alexandria, is named after him.