septum n.

loose ("open") star cluster (M44) in Cancer, 1650s, from Latin praesaepe the Roman name for the grouping, literally "enclosure, stall, manger, hive," from prae "before" (see pre-) + saepire "to fence" (see septum).

It is similar to the Hyades but more distant, about 600 light-years away (as opposed to about 150 for the Hyades), consists of about 1,000 stars, mostly older, the brightest of them around magnitude 6.5 and thus not discernible to the naked eye even on the clearest nights, but their collective light makes a visible fuzz of nebular glow that the ancients likened to a cloud (the original nebula); Galileo was the first to resolve it into stars (1609).

The modern name for it in U.S. and Britain, Beehive, seems no older than 1840. Greek names included Nephelion "Little Cloud" and Akhlys "Little Mist." "In astrology, like all clusters, it threatened mischief and blindness" [Richard Hinckley Allen, "Star Names and Their Meanings," 1899].

"Manger" to the Romans perhaps by influence of two nearby stars, Gamma and Delta Cancri, dim and unspectacular but both for some reason figuring largely in ancient astrology and weather forecasting, and known as "the Asses" (Latin Aselli), supposedly those of Silenus.

"enclosed area," 1540s, from Latin septum (see septum). As "division of a nation or tribe," 1510s, it is apparently a different word, "prob. a var. of sect" [OED].

"transverse section of a cruciform church," 1530s, from Medieval Latin transeptum, from Latin trans "across, beyond" (see trans-) + saeptum "fence, partition, enclosure" (see septum). Rare before 1700. Related: Transeptal.