Friday, March 21, 2008

A date with Easter

Ever since the Council of Nicea "fixed" the date for Easter, it has almost always occurred at different dates from one year to the next. By the council's definition, Easter occurs on the first Sunday after the first full moon after the spring equinox (which we call the beginning of spring).

As a result, the Easter date can vary by more than a month. This year, it occurs very early (March 23, 2008). The reason is that equinox took place on March 20 at 5:49 Universal Time (UT) - that is, March 19 at 10:49 Pacific Daylight Time (remember, we just switched to PDT on March 8) - and the first full moon after that occurs today (Friday, March 21, which also is "Good Friday" this time) at 18:40 UT (11:40 PDT, about an hour from now, as I write this). The next Sunday occurs the day after tomorrow - so that one is the Easter Sunday.

If the full moon had occurred even just one second before equinox, we would have to wait a full lunar cycle to the Sunday after the following full moon (29.5 days from one full moon to the next, approximately, called a Synodic month) for Easter to happen. If that full moon were to happen on a Monday, we'd have to wait an additional six days to get to the next Sunday - Easter. You do the math for each particular year of your interest.

I'm not addressing the religious aspects of Easter here. It's obvious to me that the Easter date is totally artificial and man-made. But it's interesting to me, because it has to involve astronomical observations and calculations (as most of you who read this blog know, one of my hobbies is Astronomy). The whole thing is an outgrowth of man's preoccupation with the concept of "time" (another anthropocentric idea).

If you're interested in the more detailed aspects of the lunar orbit and the concepts behind our definition of time, an excellent source of information is the "Observer's Handbook", published by the Royal Astronomical Society of Canada. For quick and detailed references, this handbook is used worldwide by amateur and professional astronomers alike. I took the times and dates information above from that handbook.


  1. Thanks for the explanation, I did wonder why Easter was so ridiculously early this year (our "Easter vacation" will actually start on April, 19 th, which sounds more like an "Easter date" to me).

    Now why would this whole equinox thing be, and why doesn't the Christian Easter fit with the Jewish Easter ? Doesn't make sense to me, as Jesus was supposedly crucified on the first day of the Jewish Pessah, which is Easter, so... why ?

  2. The difference comes from adoption of the Gregorian Calendar ( in 1582. It dropped 10 days (once only) from the then current Julian Calendar. The Julian Calendar was about 10 days "out-of-sync" at the time. Important festivals (such as Easter) along with the vernal equinox (the beginning of spring)had progressively been "creeping" forward with relation to the calendar date. This reform brought date and seasons back to a state of synchronization. In order to maintain this synchronization, leap year rules were instituted.

    The "Gregorian year", defined as 365.2425... days, is slightly shorter than an actual year (which is defined as the Earth reaching the exact point that it occupied one complete revolution earlier in its orbit around the sun). The actual time for a complete revolution of the Earth around the sun is 365.25636... days, as "seen" from the sun. This difference gives rise to the leap year rules, which drop some years from being leap years.

  3. Something I didn't address in my previous comment: The Hebrew Calendar is a "lunisolar" calendar. Its months have a different length from those in the Gregorian Calendar. Laurence's question in her comment asks about the Easter date differences between the Jewish and civil (Gregorian) calendars.

    Here's what Wikipedia says about this: "On our Gregorian calendar, the months are merely arbitrary divisions of the year. On a lunisolar calendar, every month begins at the new moon, when the moon is invisible, or on the first night when it is barely visible in the west after sunset as a very thin crescent. It takes 29.53 days for the moon to complete its cycle of phases, so lunar months approximately alternate between 29 and 30 days so that they average 29.53 days in length. On a lunisolar calendar, simply knowing the day was the 15th of the month tells you that it was within a day of the full moon."

    With reference to the length of a year, Wikipedia shows the following: "A lunisolar calendar also needs to have a year length equal to the seasonal cycle of 365.24 days, but 12 lunar months only total 354 days, which is about 11 days short of a year! It cannot use "leap days" to make the year come out right, because they are being used for the month. It was because this problem seemed hard and because people decided they didn't care where the moon was that led them to abandon the lunisolar calendar and switch to the Julian Calendar.
    But the problem is not all that hard. One simply inserts an entire "leap month" when necessary to make the year come out right on the average. If 12 lunar months are about 11 days short of a year, then 36 would be about 33 days short, so about every three years a lunisolar calendar inserts ("intercalates") an entire 13th month. The Hebrew Calendar inserts 7 extra months every 19 years in what is called the Metonic Cycle, after a Greek astronomer. That gives an average year of 365.247 days which is better than the Julian Calendar but not as accurate as the Gregorian.

    What is implied in a lunisolar calendar is that it is more important to know where the moon is than to have every season begin on exactly the same solar day every year. In reality, there is a lot of variation in just when seasons actually begin every year, so in practice a one month variation in the beginning of the year is not a problem for most farming efforts. In our modern world, however, there are other endeavors for which we would like more precision in the length of a year (such as financial dealings), so it is preferred to use the Gregorian solar calendar for commerce.

    Other differences between the Hebrew calendar and the Gregorian are that the Hebrew day begins at 6:00 p.m. (formerly at the appearance of the third star after sunset), and the year begins in the fall, usually in September (on Rosh Hashanah). The Chinese year begins in January /February."