January 0 is an alternative name for December 31.
January 0 refers to the day before January 1 in an annual ephemeris. It keeps the date in the year for which the ephemeris was published, thus avoiding any reference to the previous year, even though it is the same day as December 31 of the previous year.
January 0 also occurs in the epoch for the ephemeris second, "1900 January 0 at 12 hours ephemeris time". 1900 January 0 (at Greenwich Mean Noon) was also the epoch used by Newcomb's Tables of the Sun, which became the epoch for the Dublin Julian day.
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