Lustrum

Ancient RomanHistorical unit (no longer in standard use)

Five years, the interval between Roman censuses and the ritual purification that closed each one.

1 lustrum = 157,784,760 seconds (exactly).

About the lustrum

Roman censors counted citizens and their property about every five years, and the period between counts took the name lustrum. The word is tied to a Latin verb for purifying, because each census ended with a solemn purification of the people, often performed through the sacrifice of a bull, a ram and a pig in the Campus Martius. Ancient tradition credited the first one to King Servius Tullius in 566 BC.

Censors held office from 443 BC, and a later law cut their term from a full lustrum to 18 months. The interval was not always kept: Livy recorded only 21 lustra between 443 BC and 294 BC where 30 might have been expected, and some censuses took place without the ritual. The last censors were appointed in 23 BC under Augustus. In modern English the word survives for any five-year span. Using a Gregorian year of 365.2425 days, one lustrum is about 157.8 million seconds, which is the figure used here.

Sources: Wikipedia: Lustrum; Wikipedia: Roman census

Lustrum conversion chart

1 lustrum expressed in every other time unit:

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