Why is the ninth month called September?
late 14c., from Old French considerer (13c.) "reflect on, consider, study," from Latin considerare "to look at closely, observe," perhaps literally "to observe the stars," from com- "with" (see com-) + sidus (genitive sideris) "constellation" (see sidereal).
Perhaps a metaphor from navigation, but more likely reflecting Roman obsession with divination by astrology. Tucker doubts the connection with sidus, however, because it is "quite inapplicable to desiderare," and suggests derivation instead from the PIE root of English side meaning "stretch, extend," and a sense for the full word of "survey on all sides" or "dwell long upon." Related: Considered; considering.