Plural of ephemeris; tables or almanacs showing the predicted positions of celestial bodies (planets, stars, moon) for specific dates and times.
From Greek ephemeris 'diary' or 'daily record' (from epi- 'upon' and hemera 'day'), via Latin. The plural follows the Greek -ides pattern.
Before computers, astronomers and navigators relied on printed 'ephemerides'—thick tables of future positions—to predict where the moon would be and navigate ships across oceans.
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