By Catherine Pepinster
According to Psalm 104 verse 19, God “made the moon to mark the seasons” — a line that suggests that the people around the southern shores of the Mediterranean and the desert as long ago as the ninth century before the common era (BCE) made a connection between the moon and changing climate of different times of year.
The phases of the moon became not only important to ancient peoples, marking growing times and the harvest, while its light helped navigation at night, often through the desert; it also became integral to the practice of their faiths.
It is still true today. Last week, the Muslim month of fasting, Ramadan, began with the appearance of a new crescent moon.
Today is Ash Wednesday, the start of the Christian season of Lent which lasts 40 days until Easter Sunday, a moveable feast also determined by the moon.
And then there’s Jewish Passover, also linked to the lunar calendar and moveable.
Once, the movement of the moon was determined for these ancient religious events by the naked eye. But determining when they take place has, through the centuries, involved popes, the English historian the Venerable Bede, the Council of Nicaea, moon-sighting committees, and nowadays, complicated algorithms.
Passover
The most ancient use of the moon for marking religious occasions of the Abrahamic faiths began with the Jewish Passover, which has been celebrated since at least the fifth century BCE to mark the exodus of the Israelites from slavery in Egypt. According to the Book of Exodus, part of the Hebrew Scriptures, God ordered Moses to tell the Israelites to mark their doors with the blood of a lamb, and eat the slaughtered lamb, so that when the firstborn of Egypt were killed by the Angel of Death, he would pass over the Jewish homes (hence the name of Passover). Ever since, Jewish families have gathered at Passover to celebrate what is called a Seder meal, and eat lamb.
Passover begins on the 15th day of Nisan, the month of barley ripening and the first month of spring. It is calculated to begin on the night of a full moon after the vernal, or spring, equinox in the northern hemisphere — the day when the sun appears to rise due east and to set due west, making night and day of equal length.
The spring equinox happens on 19, 20 or 21 March — this year on 20 March. But because the appearance of the full moon varies, so Passover varies. This year, Passover will begin at sundown on Saturday, 12 April, and ends on the evening of Sunday, 20 April.
Easter
It is no coincidence that Easter Sunday this year is also 20 April. The next full moon after the equinox is the “Pink Moon” which happens this year on 13 April, and the this is also known as the Paschal Moon, which reflects its connection to Easter and Passover. Easter Sunday is always observed on the following Sunday after that post-equinox full moon.
It has always been a convention to ensure that Easter coincides with the date of Passover, given that the Christian gospels relate that Jesus gathered with his disciples to mark Passover before he was arrested and crucified, and then rose from the dead.
The early Christian church regarded that Jesus’s sacrifice fulfilled the symbolism of the Passover meal, and that he was the sacrificial lamb. Paul in his first Letter to the Corinthians, wrote: “For our Passover lamb has been sacrificed, namely, Christ.”
The Christian churches used to depend on the annual announcement of the date of Easter by the pope but this tradition eventually faded away.
Up to the eighth century, there was no uniform method for determining the date of Easter, but a method favoured by the Council of Nicaea in 325 gradually became the accepted method. By 725, the English historian Bede was writing about a computus, or mathematical calculation, for the date of Easter.
By 1582, Pope Gregory XIII introduced adjustments to the measurement of time, called the Gregorian calendar after him. This took over from the previous Julian calendar which had over-estimated the average length of the solar year, and had introduced a large number of leap years, causing the March equinox to be marked well before 21 March.
Making the amendment was vital for calculating the date of Easter, while the other great feast of Christianity, Christmas, depends on the solar calendar and so stays fixed on 25 December. Meanwhile the Julian calendar is still used by Orthodox churches, but this year orthodox Easter falls on the same Sunday as the churches using the Gregorian calendar. (See explainer on Orthodox dates for Easter and Christmas, here)
So far, so good — if a bit complicated. But according to the Greenwich Royal Observatory: “Unfortunately this simple definition is not strictly speaking correct. The spring or 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.”
The Metonic cycle? According to the observatory: “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.
“In today’s 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.”
Ramadan
While a full moon is crucial for Judaism and Christianity, it is the crescent moon that matters most for Islam. In the Quran (2:189), God says: “They ask you [O Prophet] about the phases of the moon. Say ‘they are the means for people to determine time and pilgrimage’.”
The Islamic calendar is a lunar one and so its months match a lunar cycle of 29.5 days, so months are either 29 or 30 days (but never 31).
During the month of Sha’ban, the month before Ramadan, the ninth month of the calendar, people search the skies for the crescent moon, called the hilal, and when they see it, that marks the start of Ramadan.
As the weather and the spherical shape of the earth can impede people seeing the hilal, some countries, such as Saudi Arabia have a moon-sighting committee that can make an official declaration about the crescent moon.
Although Muslims in Britain have made use of moon-sighting committees from other countries, there is now a UK-based New Crescent Society that organises moon sightings. Its next project, with organised sightings across Britain, is tracking the moon to mark the end of Ramadan.
Muslims then celebrate Eid, the breaking of the Ramadan fast, which takes place at the sighting of the first new moon after the month of Ramadan. This year that new moon will be seen on 30-31 March. As well as celebratory meals, Eid is marked by end-of-Ramadan donations for the poor.
In some years, depending on the lunar calendar, Muslims mark Ramadan twice. It last happened in 1997 and will occur again in 2030.
Passover, Lent and Easter are always annual events, and lunar predictions by astronomers show that the next dates for Easter are:
2025 20 April
2026 5 April
2027 28 March
2028 16 April
2029 1 April
But if you want to have a go yourself at calculating the date, here’s a link to the algorithm recommended by Greenwich Royal Observatory. Click here
