The Astronomy Of Easter.

 The Date of Easter.

The date of Easter is primarily used for liturgical purposes. Up to the 8th century AD there was no uniform method for determining the date of Easter making things difficult at best.

The method favoured by the Council of Nicaea in AD 325 gradually became the accepted method. The adoption of the Gregorian calendar necessitated some modifications to this scheme but it is still basically the same.

The simple standard definition of Easter is that it is the first Sunday after the Full Moon that occurs on or after the vernal equinox. If the full moon falls on a Sunday then Easter is the next Sunday.

Unfortunately this simple definition is not strictly speaking correct. The vernal equinox used is not the true equinox but an artificial one always assumed to be on 21 March. The full moon used is not the true full moon but an artificial construct based on the Metonic cycle (below).

The reasons for this are that the method is then independent of longitude on the Earth and is thus independent of time zone. It also allows the date of Easter to be calculated in advance regardless of the actual motion of the Earth around the Sun.

The method quoted here is valid for the determination of the date of Easter in Western Christian churches; the date used by the Eastern churches can be one, four or five weeks later.

The Metonic cycle

The Metonic cycle of 19 years is one in which the phases of the Moon repeat exactly. It is thus possible to have a 19-year cycle for the dates of Full or New Moon. In the Julian calendar this 19-year cycle can be fairly easily translated into a date for Easter via the calculation of two quantities called the Golden Number and the Dominical Letter. These can readily be found from appropriate tables and another table gives the date of Easter.

In the Gregorian calendar the calculation is complicated by the definition of which century years are leap years. These leap years mess up the simple Metonic cycle by altering the number of days in different periods of 19 years. The tabular method uses the Epact instead of the Golden Number.

The Epact

The Epact is the age of the Moon, diminished by one day, on 1 January in the Gregorian ecclesiastical calendar. From the involved rules for constructing the lunar calendar a table may be drawn up to give the Epact, which can vary between 0 and 29.

The Dominical letter in the Gregorian calendar has a cycle of 28 years within each century but the century leap years again throw this into disorder. There is an overall cycle of 400 years over which they repeat.


How an ancient astronomical error affects Easter

Easter Sunday is supposed to fall on the first Sunday after the full moon that follows the spring equinox...but that isn't always the case.

Easter Sunday is supposed to fall on the first Sunday after the full moon that follows the spring equinox…but that isn’t always the case. (iStock)

Easter weekend brings a lot of things to mind — religion, family, food, bunnies and Easter eggs, to name a few. The moon usually isn’t on that list, but it actually has a big influence over the date on which the holiday weekend falls.

In fact, an ancient misunderstanding of astronomy can make us wait up to a month to celebrate Easter, depending on the year.

For hundreds of years, churches have looked to the sky to determine when they should celebrate Easter, trying to align it with the Vernal, or Spring, equinox. The holiday is a time of renewal and rebirth for the Christian church, so celebrating around the dawn of Spring made sense.

But way back when people were determining the rules for when the date would fall, humanity’s understanding of the sky was a little lacking. Robert Cockcroft, a McMaster University astronomer and physicist, explains.

“Back in 325 A.D. these rules were made, but they couldn’t accurately predict astronomical events because they didn’t understand it all, so they set up estimations,” Cockcroft said.

The church decided Easter would be celebrated on the first Sunday after a full moon that falls either on or after the Spring equinox.

The Spring equinox, Cockcroft explained, is the point in the year when the path of the sun moves along with the equator, meaning it’s making its way back to the northern hemisphere and spring is on the way.

Unfortunately, when they were making the rules nearly 1,700 years ago, they didn’t realize the date of the equinox changes slightly every year. It can occur on March 19, 20 or 21 (most often on the 20), but the church fixed the date for the equinox at March 21.

Church officials also followed an ecclesiastical calendar to determine when the full moon would fall — the 14th of a “lunar month” — which isn’t always the case, either.

“Those understandings of the words ‘full moon’ and ‘Vernal equinox’ are not the astronomical definition of those terms, which is where the complication arises,” Cockcroft said.

Out of sync

Most years the church’s definitions are pretty much in harmony with the actual astronomical events taking place. This year, for example, the equinox occured at 11:02 a.m. on March 20 and the first full moon to follow occurred early Wednesday, so Sunday’s date is accurate.

But some years, the discrepancies mean the two dates fall out of sync: six years from now, the church’s Easter date falls almost a full month after the astronomical Easter date.

“I can imagine it would be confusing during the years when they don’t agree,” Cockcroft said.

And it gets even more complicated depending on where in the world you’re celebrating Easter.

Western Christianity follows the Gregorian calendar — that’s the civic calendar we follow — to determine the date. Eastern Christianity follows the Julian calendar, which is 13 days out of sync.

Therefore, Easter can fall on any Sunday between April 4 and May 8 for Eastern Christians; this year it’s May 5.

Scientific errors

Cockcroft explained it’s not just churches that can get caught up in old traditions that don’t mesh with new understandings; it happens all the time in science.

“This is how science works as well. We look for a pattern and categorize it, then realize later on those categories are incorrect, but we’re kind of stuck with them,” he said, pointing to the categorization of stars as an example.

“Rather than label stars hottest to coolest from A to Z, we label the hottest O, followed by B, then A, F. It doesn’t make any sense, but it’s because it’s arranged differently than when we first made the rules.”

On the bright side, Easter may change from year to year and hemisphere to hemisphere, but at least the cream eggs stay the same. Source: CBC


The Astronomy Of Easter According To The Scriptures

The image shown above was the recent Full Moon called the Paschal Full Moon. In Christianity, the first astronomical full moon after the vernal (spring) equinox is usually designated as the Paschal Full Moon or the Paschal Term. Traditionally, Easter is observed on the Sunday after the Paschal Full Moon. Credit: Raven Yu

 Most Christians know that Jesus was crucified in Jerusalem at the time of the Passover, and in this way became the perfect Passover sacrifice, “the Lamb of God which taketh away the sin of the world” (John 1:29). Since the early centuries of the church, Christians have honored the death and resurrection of Jesus in the celebration of Easter. And while the observance of Easter has changed over the centuries, it is based on the Hebrew Passover.

Hebrew Roots

We read in the Book of Exodus how the LORD used Moses lead the Israelites out of Egypt. In Exodus 12, the LORD sent the final plague, smiting the firstborn of all Egypt. The Israelites were saved by sacrificing a lamb and covering their doorframes with its blood, so that the LORD would “pass over” the house and not smite the firstborn of Israel. And the LORD commanded that the Israelites remember the Passover in a seven-day feast….

“And this day shall be unto you a memorial; and ye shall keep it a feast to the LORD throughout your generations; ye shall keep it a feast by an ordinance for ever.” – Exodus 12:14. 

The LORD instructed Israel as to when the Passover should be kept: “In the first month, on the 14th day of the month at even, ye shall eat unleavened bread, until the one and 20th day of the month at even.” – Exodus 12:18.

In the lunar calendar, the New Moon is the first day of the new month. The Full Moon is at mid-month, the 14th day of the month. So Israel was instructed to celebrate the Passover on the Full Moon of the first month. The LORD even tells us the name of the month….

“Observe the month of Abib, and keep the passover unto the LORD thy God: for in the month of Abib the LORD thy God brought thee forth out of Egypt by night.” -Deuteronomy 16:1

The word “Abib” means “sprouting” or “budding” and is the first month of Spring, falling among our months of March and April. Abib is the only month of the year named by God in the books of Moses.  

The Bible doesn’t clearly indicate the calendar method used to keep track of Passover and other Hebrew holidays. However, for centuries the Jewish calendar has been based on the well-known, 19-year cycle of the Sun and Moon. God in His providence established that 235 cycles of the Moon’s phases is nearly equal to 19 solar years. This means that every 19 years, the Moon’s phases will recur on the same dates of the solar year.

This 19-year “luni-solar” cycle was the basis for the calendar used in Babylon. In the west, its discovery is attributed to the Greek philosopher Meton, who may have learned it from the Babylonians. The 19-year cycle was generally well known and understood in antiquity, and was apparently used by the Persian rulers of Babylon after the restoration of Israel. 

It appears that the modern Jewish calendar was directly influenced by the Babylonian calendar. In addition to using the 19-year cycle, the modern Jewish calendar uses months with names nearly identical to the Babylonian names. In the books of Nehemia, Ezra and Esther (which record events after the Babylonian exile) you can find the month names “Sivan,” “Elui,” and “Adar,” nearly the same as their Babylonian counterparts. 

Most notable is the month “Nisan,” the first month of Spring, corresponding to the Mosaic month of “Abib.” In the modern Jewish calendar, Passover occurs on the fourteenth day of Nisan. The Jewish historian Flavius Josephus, writing in about 90 A.D., gives us an astronomical reference for the month of Nisan….

“In the month of Xanthicus, which is by us called “Nisan,” and is the beginning of our year, on the 14th day of the lunar month, when the Sun is in Aries (for in this month it was that we were delivered from bondage under the Egyptians, the law ordained that we should every year slay the sacrific which I before told you we slew when we came out of Egypt, and which was called the Passover….)” Source: Crosswalk.Com

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