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In earth as in heaven

Christian festivals and calendar are based on astronomy

detail of "The Transfiguration", by RaphaelJesus ascending to heaven: the date of Easter defines Carnivaldetail of "The Transfiguration", by Raphael

This year, two of Brazil’s main religious feasts will take place on the same day as the country’s main pagan feast. The celebrations in honor of Iemanjá, idolized by the followers of Afro-Brazilian rites, such as candomblé and umbanda, and the procession of Our Lady of the Seafarers (Nossa Senhora dos Navegantes), a secular Roman Catholic tradition, will coincide with the beginning of Carnival, February 2. Everybody thinks this overlapping is undesirable, but nothing can be done about it. In the past, the celebration of Christian holy days was designed to take place close to the time of the northern hemisphere’s spring equinox. Thus, it was through astronomy that the days of these feasts were worked out, as decided in 324 A.D. by the first Christian ecumenical meeting, the Council of Nicaea, in Turkey.

The council set Easter as “the first Sunday after the first full moon after or during the northern spring equinox, defined as March 21.” An equinox occurs when the Sun crosses the equator, lighting both the northern and the southern hemispheres equally; it is a spring (or vernal) equinox in the hemisphere that is going from winter to summer and an autumnal equinox in the one that is going from summer to winter. For a number of reasons defined in the council, Easter does not always fall on the day it should based on purely astronomical criteria. Therefore, the Nicene 21st of March is called the Ecclesiastical Spring Equinox.

From the Council in 325 until 1582, when the pope was Gregory XIII, 1,257 years passed. The problem was that every 125 years, for astronomical reasons, the real spring began one day before the date set by the Church. This discrepancy between the true equinox and the ecclesiastical equinox caused a delay of some ten days over the course of 1,257 years. Mathematicians and astronomers clamored for an adjustment of the Julian calendar then in force. Finally, in 1563, the Council of Trent, in Italy, decided to reform the calendar. Pope Gregory XIII, elected in 1572, implemented the change. However, the reasons for this were not only of a practical nature, as there was a major religious motive as well.

From Ash Wednesday to Easter the eating of meat was prohibited and to do so was regarded as heresy by the Roman Catholic Church at the time. “As Easter was defined by the Ecclesiastical Spring Equinox, which didn’t always fall on the same day as the true astronomic equinox, Catholics might eat meat when they weren’t supposed to”, explains Roberto Dias da Costa, a researcher at the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (IAG/USP). “It was to avoid future sins that the Church adjusted the calendar that was then in effect and established a safe gap between the two events.” It was ruled that Carnival Sunday would always fall on the seventh Sunday before Easter Sunday.

The reform, which became known as the Gregorian Calendar, was designed with the guidance of astronomer Luigi Lilio, his brother Antonio and the German mathematician Christoph Clavius. When it came into effect, the new calendar did away with ten days in October 1582, when the fifteenth followed the fourth, in order to make the spring equinox coincide again with the 21st of March. The Julian calendar, introduced in 45 B.C. by the Roman emperor Julius Caesar, had already instituted an extra day every four years ( leap year) for the average length of the year to become 365.25 days, closer to the true length of the solar year. The Gregorian calendar, however, sets the average length of a year at 365.2425 days, even closer to the tropical year of 365.242199 days (or 365 days, 5 hours, 48 minutes and 45.967 seconds).