An ephemeris is a map of the planets. Because the planets are in constant motion, that map is going to change constantly. Ephemerides come in either midnight or noon versions, and show you where the planets are (or were for a date in the past or will be for a date in the future) at either midnight or noon GMT.
An astrologer uses this as a starting point, and then — using logarithms— calculates where the planets would be in the heavens overhead at your date, time, and place of birth to put them into your natal birth chart.
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