Today's city-dwellers may find it hard to imagine a time when moonlight was important. Lunar phase often decided when folks would travel by night. To those who lived just two or more centuries ago, before the era of urban gas lamps, the dark of the moon meant roads frequented by predators human and animal. The Full Moon was a time of monthly import, even celebration. Few of these festivals and lunar calendars still remain. But two of them occur this week: Passover and Easter.

Although Easter's changing date is tangled in public confusion, it's decreed by the immutable motions of the moon. It starts out simply enough: Easter is the Sunday following the first Full Moon on or after the Spring Equinox. (Note: The Spring Equinox now happens exclusively on March 20, but the Church still uses March 21.) The earliest possible Easter would thus be produced if a Full Moon landed on the supposed Equinox of March 21, and additionally, if this were a Saturday. Then Easter would be the next day: March 22.

It's an unlikely state of affairs. Equally improbable is the latest Easter, which paradoxically arises if the Full Moon lands one day sooner: March 20, which, by the rules, forces us to the next Full Moon on April 18. Then, if that happens to be a Sunday, we must wait a week for the ensuing Sunday, bringing Easter to April 25. Still with us?

Easter is rather normal this year (April 12) because the first Full Moon after the Equinox happens this Thursday, April 9: neither particularly early nor late. A quick rule of thumb, then, is that if a Full Moon falls within a week after the Vernal Equinox, it'll be an early Easter. Full Moon in the week before the Equinox yields a late Easter. I hope you're writing this down.

Passover is celebrated on the very day of that first post-Equinox Full Moon - which means Thursday, the day this newspaper comes out. Throw in a few more Jewish, Islamic, Hindu and other ethnic holidays, and you've got the vestigial legacy of the moon's once far-reaching influence.

Speaking of lunar phase, a little-noticed fact of Easter is that the moon almost always assumes the same shape on that holiday: waning gibbous. Curiously, gibbous is the only phase not recognized by the average educated citizen, even though the moon can be seen in that shape for more days each month (11) than any other. Ask anyone what a crescent, half or Full Moon looks like and they'll dismiss the question as childish; ask them about gibbous and you're likely to get a blank stare. In fact, a gibbous moon looks a bit like a football. It's anything bigger than half and smaller than full. That phase repeats nearly every Easter because Easter always follows a Full Moon by a week or less, automatically eliminating any chance of an Easter full, crescent, new or First Quarter Moon.

The Passover and Easter Full Moon also distinguishes itself by displaying a dramatic effect that is the exact reverse of the Harvest Moon: The latter rises at nearly the same time for several consecutive evenings; Easter's Full Moon does the opposite. March and April Full Moons rise considerably more than an hour later each successive night, compared with the average interval of 50 minutes.

All Full Moons come up at sunset. But the days immediately following the Full Moons of March and April see moonrises so much later on successive nights that the moon soon vanishes. In contrast with the unending full moonlight of the Harvest period, by this Sunday the first half of the night is pitch-black.

This speedy lunar vanishing act a few nights after Passover and Easter - this unusually quick return to darkness - is a dependable feature of those holidays. I'm surprised that no mythic, cultural or philosophical associations have ever sprung up around it.