The association of eggs and rabbits with Easter is not immediately apparent. Easter is a Christian holiday, and it is a celebration of fertility and rebirth, the modern continuation of ageless rites of spring that we have borrowed and adapted from ancient pagan rituals.

The name comes from Eostre, the pagan goddess of spring, whose name derives from the direction of the sunrise, the rebirth of the day. In Germanic mythology, Eostre saved the bluebird, a symbol of spring, from perishing in late winter’s frost by appearing at sunrise to warm the dying bird with her clothing of light.

The Easter egg as a fertility symbol goes back at least to ancient Mesopotamia, circa 1500 B.C., and its identification with Astarte, a goddess of fertility.

The Easter bunny is a carry-over from the festival of Eostre, an annual celebration of the rebirth of nature in which people feasted on hares (not rabbits) to promote fertility, for which hares (and rabbits) are famous.

Unlike other holidays, Easter does not fall on the same day of the month from one year to the next; it can occur anywhere from March 22 to April 25. The date is not arbitrary: It links the calendar with the March equinox and the full moon.

The connection of Easter with Christianity dates back to A.D. 325 A.D. when the emperor Constantine convened the Council of Nicea to promote unity among different factions of this new church. It was at this first Ecumenical Council that church officials forged the fundamentals of modern Christianity, including specifying the date of observance of the Resurrection.

The decision was that Easter would fall on the first Sunday after the first full moon after the March equinox. This was problematic because the lunar month is not a whole number of days, the year is not a whole number of lunar months and it was not easy to pinpoint the equinox.

At the Council’s request, astronomers calculated the approximate dates of the full moons. These became the ecclesiastical full moon tables, which are lists of virtual full moon dates far into the future, regardless of when the actual full moon might occur.

Although the actual date of the equinox moves between March 19 and 22, the Council established March 21 as the date of reckoning regardless of the actual equinox, and set the EFM as the date of the full moon regardless of the actual full moon.

The EFM tables remained in use for more than 1,000 years until 1582 when the Gregorian calendar replaced the Julian calendar. The old calendar had drifted out of sync with the seasons by 10 days.

With the adjusted dates in an accurate new calendar, the old EFM tables were obsolete, so Pope Gregory XIII had new EFM tables calculated. By that time there were much better data on the actual length of a month, and the introduction of numbers in the ninth century had allowed refined methods and better accuracy of calculations.

Even with today’s astronomical precision, the actual date for Easter is the first Sunday after the first EFM after March 21, regardless of the actual full moon or the actual equinox.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.