Trapezium? Trapezoid? Theta-1 Orionis.

[Zermelo]

I've been giving the Trap some attention on the clear nights we've been having, and belatedly been wondering about its name.

The four stars ABCD don't really form a trapezium, because no two sides are parallel. I've always thought this to be yet another example of the astronomical licence that allows, for example, a group of very mediocre stars to be identified as a crab. But recently I recalled from my teaching days that this is one of those points of linguistic confusion that divides UK and US speakers of English. In the UK, a trapezoid is a quadrilateral with "nothing special", i.e. no two sides parallel, whereas a trapezium has a pair of parallel sides (opinions still divide on whether that also includes those quadrilaterals with two pairs of parallel sides). This convention is also applied in other European countries, but in the US the names are reversed. The Europeans follow the names introduced by Proclus in the fifth century AD, and the confusion was bizarrely introduced via a mathematical dictionary error by Charles Hutton in 1795. This was not corrected for nearly a century, but afterwards the US continued to use Hutton's transposed definitions.

That made me wonder whether theta-1 was christened sloppily by a UK English speaker, or perhaps accurately by an American. Apparently, though the cluster was discovered by Galileo, it was not given its name until 1931 by Robert Trumpler, who also devised a classification scheme for open clusters. Trumpler was born in Switzerland but was half American, and he emigrated to the US in 1915. So it seems quite possible that the name he gave to the cluster later on was accurately following the linguistic conventions of his new home.