Death on the Moon by Barbara Hambly

A novelette of 12,000 words in the Benjamin January Free Man of Color historical mystery series. All of New Orleans is agog over Professor Tixall's telescope, which can reportedly see life on the Moon. Rose Janvier is convinced that it's a hoax, and a harmless one (though profitable for Professor Tixall), until one of her servants, looking through the telescope after hours, sees one of the "bat-people" of the Moon murder another one. Rose knows that murder has been done, but how do you prove it when the events supposedly took place on another planet?