WordNet records no synonym and no antonym for ephemeris. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an annual publication containing astronomical tables that give the positions of the celestial bodies throughout the year
“today computers calculate the ephemerides”
Synonyms, antonyms and the broader and narrower relations for ephemeris come from Princeton University’s WordNet. Full sources and licences.