Thursday, April 09, 2009

Full moon

Yesterday evening I was at a choir rehearsal. One of our sopranos came in late, apologizing that she had stopped to watch the full moon rise through a thin layer of clouds.

I've heard people remark in years past how it always seems like there is a full moon pretty close to Easter; they marked it up to an amazing coincidence. In fact, it is no coincidence at all.

The Christian holiday of Easter is always held on the first Sunday following the first full moon of spring, where spring is defined to start on March 21. (For all of us except the occasional rogue curmudgeonly astronomer, spring is assumed to start on the Vernal Equinox, which is often not March 21, but usually this distinction doesn't matter.) The full moon was 5 minutes ago (by my watch), so Easter falls on Sunday. And, if the weather is nice, people enjoying the spring weather in the week before Easter will probably notice a big, nearly-full moon rising just after sunset.

Why is Easter set by the full moon and not by a specific date, like the Christian Christmas holiday? It's because the events celebrated by Easter occurred in the days following the Jewish holiday of Passover, so the Catholic Church set the date of Easter to mimic the timing of the Passover.

The Jewish Passover celebration is celebrated starting on the 15th day of the month of Nisan. The Jewish calendar is a quasi-lunar calendar, meaning that the months start at the start of the new moon. Since there aren't exactly 12 cycles of the moon in a calendar year, the Jewish calendar contains either 12 or 13 months, depending on the year, so that the months always roughly align with the same seasons. Since the month begins at the new moon, and the full moon is about 14 3/4 days after the new moon, the Passover feast always begins right at the full moon.

There's yet another complication. Eastern Christian churches, like the Greek Orthodox and Russian Orthodox churches, base their calculation of Easter on the Julian calendar, while western churches use the Gregorian calendar (what most of us consider "the calendar"; read here about the differences between the two.) These two calendars are 13 days apart (with the Gregorian calendar "ahead" of the Julian calendar), so roughly half of the time the first full moon of spring in the Gregorian calendar is one lunar cycle earlier than the first full moon of spring in the Julian calendar. This means that, roughly half of the time, the Orthodox Easter happens one full moon after the western Easter holiday. Confusing? Definitely. And then there's this year, when the Orthodox Easter is April 19, while the western Easter is one week earlier, on April 12. I don't know how that happened; I'm sure that is explained online somewhere.

The sum total of all of this means that even though the Christian and Jewish holidays are closely related, they often don't fall on the same date. Even more, even different branches of religion disagree on the date of the same holiday. And all of it comes down to the fact that the length of the Earth's rotation (day), the duration of one lunar cycle (a lunar month), and time it takes the Earth to complete one orbit around the sun (year) are not even multiples of each other. (Heck, there are even different types of years, but let's not go there). Aren't calendars fun?


  1. Julian Easter is a lunation later than Gregorian Easter in 5 years out of every 19: Years 3, 8, 11, 14, and 19 of the cycle. (The year 2009 is year 15). So this situation happens somewhat less than "half the time".

  2. Thanks for the clarification.

  3. To Jewish people, it's just known as Passover rather than the Jewish Passover. The term Jewish Passover is used by people of other faiths who prefer to distinguish their version of celebrating Passover from the Passover of the Jewish people, as, for instance, there are Christian Passover versions of the Passover celebrated by Jewish people.

    At my Pesach / Passover website, you can plan for future Christian Passover or Passover celebrations by viewing a list of future Passover dates : Passover Dates.

    Sincerely, Eli Ha-Levi, BA, M.L.I.S. (professional librarian)
    Website: Montreal, Quebec, Canada