Sitwell seems to be trying to change the adjective 'ephemoral' into a noun, but he is mistaken. 'Ephemorides' does not mean transitory but
A journal or account of daily transactions; a diary; a publication exhibiting the places of the heavenly bodies throughout the year, and giving other information regarding them; an astronomical almanac; a collective name for reviews, magazines, and all kinds of periodical literature — The New Webster Encyclopedic Dictionary of The English Language (1952)