How the Moon ties into Easter and Passover

(Dion Ogust | Almanac Weekly)

(Dion Ogust | Almanac Weekly)

Unlike today’s city folk, for whom moonlight is charming but useless, in the pre-electric centuries the Moon was vital. Although moonlight is 450,000 times less bright than sunlight, it adequately lights up paths and roads and provided safety for nighttime activities for countless millennia. It may be no coincidence that the average human menstrual cycle, now known to be very close to 29 days, seems so in-sync with the Moon’s synodic (phase cycle) of 29 ½ days. Quite possibly, courtship and thus fertility usefulness marched in tune with the times of the month when it was safe to venture out on dates without being munched upon by predators, whose nocturnal eyesight was generally better than ours.

We’ve recently learned that the Moon’s existence provides a physical gravitational “brake” on the amount that our planet’s axis can tilt. Our tilt permanently stays within a few degrees of its present value. Without the Moon, our poles would sometimes point at the Sun, boiling off the oceans and making life impossible.

While few of the festivals and calendars that the Moon once inspired remain, one of them, the most celebrated, happens on Sunday, March 27: Easter. Since its date is decreed by celestial things like the Moon and the Equinox, and since it’s one of the few Western holidays set by the heavens, it deserves to be understood.

It starts out simple enough. Easter is the Sunday following the first Full Moon that’s on or after the Spring Equinox, which the Church considers to be March 21. The earliest possible Easter would thus be produced if a Full Moon landed on the Equinox itself, and, additionally, if this were a Saturday. Then Easter would be the very next day, March 22.

It’s unlikely. Equally improbable is the latest Easter, which paradoxically arises if the Full Moon lands one day sooner: a day before the Equinox, which, by the rules, forces us to the next Full Moon on April 17. Then, if that happens to be a Sunday, we must wait a whole week for the following Sunday, bringing Easter to April 25. We won’t see that earliest-possible Easter anytime this century or even next century; but that latest-possible Easter of April 25 will come up just 22 years from now.

The fact that Easter is quite early this year becomes clear when we notice that the first Full Moon after the Equinox happens on Wednesday, March 23, just two days after the Equinox. So Easter merely had to wait until the first Sunday, next weekend.

So a quick rule of thumb is that if a Full Moon falls within a week after the Equinox, it’ll be an early Easter. Passover is often celebrated on that same Full Moon, but this year the rules push it to the next one, a month from now.

Throw in a few more ethnic holidays, and you’ve got the vestigial legacy of the Moon’s once far-reaching influence.


Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at

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