TOPICAL SCIENCE
A P R I L
APRIL 2017
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TOPICAL SCIENCE
Easter Easter is celebrated as the great spring festival in Christian countries.
The Date of Easter Easter Sunday falls on April 16th this year, 2017. However, last year, it occurred on March 27th and in Ireland, we had the main commemorations of the Centenary of the Easter Rising almost a month early. It was convenient to hold events during the Easter holiday period, when most people had a few days off work and schools were on holiday, rather than on the actual date of the rising. In 1916, the Rising in Dublin broke out on Easter Monday, which fell on April 24th in that year.
In this issue, these three questions are addressed: Why did we, in Ireland, celebrate the centenary of the 1916 Easter Rising a month early in 2016? What was the historical context for the date of Easter? What scientific methods were used to calculate the date? 1
Author: Margaret Franklin
However, it is a moveable feast and so is not held on the same date every year. Even in the same year, it is celebrated on different dates in different countries!
TOPICAL SCIENCE
APRIL 2017
Calculating the Date of Easter Astronomical Method Easter is a moveable feast whose date is determined by an astronomical method, based on a combination of solar and lunar cycles. The calculation of its date depends on rather complicated computations, involving relative movements of the Earth, Sun and Moon and has sometimes caused controversy. This arose partly because of the difficulty in carrying out the computation in an age before computers and modern scientific instruments were available; partly because of the different calendar systems adopted by different cultures.
Religious & Historical Context In the early years of the Christian church, the date of Easter was linked to the Jewish Passover, with Easter on the Sunday following the Passover. The Jews used a calendar based on the cycles of the moon and celebrated Passover on the 14th day of the month. The month began with a new moon, so that there was always a full moon at Passover, 14 days later. The Christian Easter was celebrated on the Sunday following this full moon, which then became known as the ‘Paschal moon’. In the year 325, Constantine, Emperor of the Holy Roman Empire, called a Council of bishops at Nicaea, the first ecumenical council ever held. Its main purpose was to combat heresy and achieve agreement on matters of faith, resulting in the formulation of the famous Nicene Creed. In addition, the Council decided to discontinue the use of the Jewish lunar calendar for the purpose of determining the date of Easter and instead to adopt the Julian calendar, which had been introduced by Julius Caesar in 44 B.C. and was in use throughout the Empire. 2
Author: Margaret Franklin
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TOPICAL SCIENCE
APRIL 2017
The Julian Calendar This calendar is based on the solar, rather than the lunar cycles and the length of the year is measured starting with the March equinox. This is the date on which the rays of the sun fall directly on the equator and day and night are of equal length all over the earth. It is known as the Vernal, or Spring equinox in the northern hemisphere and the Autumnal equinox in the southern hemisphere.
Another equinox occurs six months later, in September, when the positions are reversed and it is Autumn in the northern hemisphere and Spring south of the equator. However, even at the time of the Council of Nicea, it was known that the equinox does not occur on exactly the same date in March every year, due to various factors, including the slight ‘wobble’ of the earth on its axis. This discovery of the ‘precession of the equinoxes’ is usually attributed to the Greek astronomer and mathematician Hipparchus, almost 200 years before the Christian era, but it may have been known even earlier, as Aristarchus of Samos was aware of the difference between the length of the sidereal and tropical year. The sidereal year is based on the sun’s motion in relation to the stars; the tropical year uses the revolution of the earth around the sun. For convenience, it was decided to use the date of March 21st as the nominal date (even though in fact the equinox occurs more often on March 20th) and the date of Easter is reckoned as the first Sunday following the Paschal Full Moon, which is the first full moon after March 21st. In ancient times, instruments for making astronomical measurement were rather crude. The astrolabe was not invented until the 10th century and the world had to wait for Galileo before the telescope was used to observe the heavens. Nevertheless it is amazing how accurate the predictions were. In the north of England, the Venerable Bede (born ca. 672) devoted much study to the motions of the heavenly bodies and he published a text known as the ‘Computo’, which was used to calculate the date of Easter. He also gave the world the first ‘digital computer’! This was obviously not an electronic device, but it provided a method, using the fingers (digits) of both hands, for carrying out quite advanced computations. The Julian calendar continued in use for over a thousand years, but it had a major flaw. It was based on the assumption that the earth takes 365.25 days to carry out one complete revolution in its annual journey around the sun (or, in the thinking of the time, which regarded the earth as the centre of the solar system, it took 365.25 days for the sun to complete its journey!) Whichever way the relative motions of earth and sun are viewed, the fact that we don’t have an even number of days in a year creates an inconvenience for the calendar system. To compensate for this, an extra day is added to the month of February every four years, which we know as the leap years. However, this over-compensates, because the length of the year is not quite 365.25 days. So, as the centuries went by, the discrepancy between the date of the equinox and the date of Easter became greater and greater, with the equinox gradually drifting to earlier dates in March, even occurring as early as March 10th by the late 16th century. 3
Author: Margaret Franklin
TOPICAL SCIENCE
APRIL 2017
The Gregorian Calendar. Eventually, in the year 1582, Pope Gregory the Great decided to take things in hand. He decreed that ten days in the calendar would be suppressed in October of that year. And so it was done. People went to bed on the night of October 4th, 1582. When they got up the following morning, hey presto, it was October 15th! There are reports that some people grumbled at having their lives shortened by ten days! Most countries in the western world adopted this Gregorian calendar, which is the one we now use. It had the effect of bringing the astronomical full moon and the Paschal Full Moon into accord. It solved the problem for the moment, but perhaps in another millennium or so, a similar exercise would have been needed to reconcile the calendar. So it was decided not to count the centennial years as leap years, unless the first two digits are evenly divisible by four. Thus, the year 1900 was not a leap year, but the year 2000 was, while 2100 will not be a leap year. However, the Eastern and Orthodox Churches continue to use the Julian calendar for the calculation of the date of Easter, so they celebrate Easter on a different date. According to the Gregorian calendar, Easter can fall on one of 35 possible dates, between March 22nd and April 25th. This year, it falls on April 16th, but in 1916 it was on April 23rd, only 2 days before the latest date possible. The last time Easter fell on the latest possible date was in 1943 and it will not fall on that date again until 2038. It last fell on the earliest possible date, March 22nd, in 1818 and it will not do so again until 2285. So it appears that a very early Easter is more rare than a very late one! Easter fell on April 24th in 2011, just one day before the latest possible date. That year, there was a full moon on the night of March 19th, just before the equinox. The next full moon did not occur until the early hours of April 18th, which was a Monday, just narrowly missing the Sunday, giving the following Sunday, April 24th, as the date of Easter 2011. Easter is always celebrated on a Sunday and it all depends on how the days of the week fall in relation to the lunar cycle and the equinox. It turns out that April 19th is the most common date for Easter Sunday, while the cycle of Easter dates repeats after 5,700,000 years.
Happy Easter!
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Author: Margaret Franklin