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I was on TV these last few days (I am not sure exactly when, as it was recorded over a week ago), my purpose being to explain how the date of Easter is calculated. Easter, and the other Christian festivals which are tied into it like Mothering Sunday, Shrove Tuesday and Ash Wednesday, is the only festival I can think of which is practiced in Western civilisation where the date it is celebrated depends on the phase of the Moon.

As I have blogged about before, Chinese New Year, the Jewish New Year, Ramadan, Diwali and other religious and cultural festivals nearly always depend on what the Moon is doing, and as a consequence do not fall on the same date each year. Well, the same is true of Easter.

Of course, Easter is the most important Christian festival. In recent times, Christmas has become more celebrated, but this is mainly because Christmas has become a secular holiday. And, let’s face it, we all need something to celebrate in late December when the days are so short and dark.

Easter is more important to Christians because it represents the very essence of Christianity. That Christ, God’s only son, died and rose from the dead two days later. Even as a non-Christian, I find the story of the crucifixion a very moving one. Whether true or myth, the idea of a person (or son of God if you believe that) willingly dying in the belief that his death would help others is a very noble one. And, of course, central to the Christian faith is the belief that Christ not only died on the Cross, but that he came back to life to show that eternal life awaits those who believe in Him.


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The date of Easter

The formula for calculating Easter is very simple. Easter Sunday is the first Sunday after the first full Moon after the Vernal (spring) Equinox. This method of calculating Easter was decided in AD 325 at the 1st Council of Nicaea.

So, for example, this year, as I blogged about here, the Vernal Equinox was on the 20th of March at 11:02 Universal Time. The full Moon was on Wednesday the 27th of March and the first Sunday after this is today, Sunday the 31st of March.

Because of the way Easter is calculated, the earliest is can possibly be is the 20th of March. This would happen when the Vernal Equinox happens to also be the day of a full Moon which happens also to be a Sunday, and we are near the start of the 4-year leap year cycle (which enables the equinox to be on the 20th of March rather than the 21st of 22nd of March). In 2008 Easter Sunday was the 23rd of March, nearly as early as it possibly can be.

The latest Easter can be is the 26th of April. This will happen if there is a Full Moon just before the Vernal Equinox, and the Full Moon after it, the first one after the Vernal Equinox, happens to be on a Monday. There are 29.53 days between each full Moon, so if the full Moon falls on the 22nd of March before the Vernal equinox, and then the full Moon after that, which would be on the 20th of April happens to be a Monday, then the Sunday after that would be the 26th.

Below is a table showing the date of Easter from 2001 to 2017. As you can see, on 2011 Easter was nearly as late as it can possibly be, Easter Sunday fell on the 24th of April.


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I am certainly no expert in Christian matters, but does anyone know why it was decided to calculate Easter this way? It means, of course, that Good Friday, when Christ was meant to have been crucified, always happens near a full Moon. Usually a few days after a full Moon. Is there an indication in the Bible of the phase of the Moon when the Crucifixion happened?

+++CORRECTION+++

As Bo has pointed out below in the comments section, the Church have decided (in their infinite wisdom) that, for the purposes of calculating the date of Easter, the Vernal Equinox shall always be on the 21st of March, irrespective of when it actually is astronomically! In addition, they have also decided to alter the period between full moons! Astronomically there are 29.53 days on average between full moons. The actual time between successive full moons changes due to the speed of Earth’s orbit about the Sun not being constant and the speed of the Moon’s orbit about the Earth not being constant either.

But for the Church it is either 29 or 30 days. Obviously they are simple minded folk who cannot cope with the complications of real astronomy. As a consequence of these two simplifications, the date of Easter Sunday will always lie between the 22nd of March and the 25th of April, not the 20th of March to the 26th of April asI stated above. Sorry for any confusion!


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Today is the day of the Hindu festival Diwali, the festival of light. About 200m from my house is a Hindu temple, and it has been lit up for the last several days in celebration of this important Hindu festival. Here is a picture I took on Sunday evening.


The Hindu temple in Cardiff, lit up for Diwali


Like most religions, with the exception of Islam, the Hindu calendar is a luni-solar calendar, and so the date of Diwali changes. Last year (2011) it was on the 26th of October, and next year (2013) it is on the 3rd of November, but this year it is today, the 13th of November. How is the date of Diwali calculated?

Being a luni-solar calendar, this means it follows something both the Moon and the Sun are doing. Today is a new Moon. So, the question is, what is special about today’s new Moon? What recent important Solar event happened? The Autumn equinox of course, which was on the 22nd of September this year (2012). The firsrt new Moon after this year’s Autumn equinox was on the 15th of October, and today is the second new Moon after the Autumn equinox. So, Diwali always falls on the 2nd new Moon after the Autum equinox! It’s that simple……

I wish a very happy Diwali to all my Hindu friends and students (and readers) around the World.

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Yesterday, Sunday the 16th of September, was Rosh Hashana, the first day of the New Year in the Jewish calendar. As I have mentioned before when discussing the date of the Chinese New Year, and the date of Muslim festivals, the Jewish calendar is a lunisolar calendar. This means that it follows what the Moon and the Sun are doing.


Shofar

A shofar, symbol of the Rosh Hashana holiday


But, unlike the date of the Chinese New Year, which is always the 2nd New Moon after the Winter Solstice, the Jewish calendar works slightly differently. Although the first day of the year is the first day of the month of Tishrei, the Bible stated that Adam and Eve were created on the first day of Nisan. So, Nisan is generally taken to be the first month of the year, even though the New Year starts on the first day of the 7th month, Tishrei! The Jewish civil year starts on the first day of Tishrei, but its ecclesiastical years starts on the first day of Nisan. Because birth is associated with Spring, it is generally thought that Adam and Eve would have been created in the Spring, so the first day of Nisan needs to fall in this time of the year.


Adam and Eve

“Adam and Eve” by Lucas Cranach


The first day of Nisan would, of course, drift in a purely lunar calendar, because there are not exactly 12 Moon cycles in a year. If this were to happen, it would not fall in the Spring and its association with the creating of Adam and Eve would be lost. The Muslim calendar does follow a strictly lunar calendar, no correction is made for what the Sun is doing.

This is the reason all Muslim festivals drift forward by 11 days each year, so e.g. Ramadan moves and can be in any season. Because the Jewish calendar’s first day is tied into something which was said to have happened in Spring (the creation of Adam and Eve), the calendar needs to correct for the drift of a purely lunar calendar.

The way the drift is corrected in the Jewish calendar is slightly different to the way it is corrected in the Chinese calendar. Instead of decreeing that Nisan is on the first or second new Moon before or after some point in the Solar cycle (like the equinox, or the solstice), what the Jewish calendar does is allows the first day of Nisan to drift forwards by 11 days each year for 2 or 3 years, then it adds an extra month of 30 days, which makes it later again by 30 days. The extra month Adar I is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the 19-year Metonic cycle (when the lunar cycle and the solar cycle almost exactly match).

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