We think we know the rule: "Easter falls on the Sunday following the first full Moon after the spring equinox." This year, the equinox is on March 20 at 5:58 p.m. EDT, and the full Moon is later that day at 9:43 p.m. The following Sunday is therefore March 24. Yet a glance at a 2019 calendar shows that Easter actually falls on April 21.
This disparity underlines the fact that the date of Easter is determined by a very old calculation called the ecclesiastical computus, which has little in common with modern mathematical tools and methods. The rule given above is in fact incorrect.
Let’s travel back 1,700 years to AD 325 when Christian bishops gathered for the Council of Nicaea. At the time, they set the rules for deciding the date of the most important Christian festival: Easter would fall “on the Sunday which follows the 14th day of the Moon which reaches this age on March 21 or immediately after that.”
It’s a mistake to give a modern twist to this very old definition. Indeed, the ecclesiastical rule implies certain simplifications, the first being to set the spring equinox on March 21. A more rigorous calculation shows that, in our day, the equinox usually falls on March 20 and can even occur on March 19. (In AD 325, the equinox fell, in fact, on March 20, so what were those bishops thinking?)
Furthermore, the full Moon calculation isn’t as sophisticated as what today’s knowledge of astronomy permits. The old calculation relies on a simplified representation of the motion of a “fictional” Moon, which still coincides rather closely with the motion of the true Moon. The lunar phase on a specific date is given as a number representing the Moon’s age (i.e., the number of days since the new Moon). Thus, the full Moon occurs on “the 14th day of the Moon” (hence, the wording found in the above quote). The precise time of the full Moon is disregarded, and the concept of time zones is absent.
Usually, the date of Easter based on the ecclesiastical rule matches the date determined by an “accurate” astronomical calculation, but there are some mismatches, like this year. The last mismatch was in 1981, and the next one is in 2038. In the 21st century, the two methods give conflicting results on 10 occasions: first in 2019 and 2038 (as stated) and then in 2045, 2049, 2057, 2069, 2076, 2089, 2095 and 2096.
The planets and the Moon
During the first days of March, you can still spot Mercury low on the western horizon, 45 minutes after sunset. Use binoculars to help find this little dot of light in the twilight glow, but also try to make it out with the naked eye. Don’t wait too long because the tiny planet dims quickly as the nights go by. After March 4 or 5, it’s too faint to be easily visible.
Also found in the evening sky at dusk is Mars. Over seven months have passed since the red planet’s opposition. It’s now four and a half times farther from Earth than it was last July, and much less bright than at that time. It now looks like an ordinary orange star, a bit fainter than the brightest stars in the winter sky. But observe it carefully for a few evenings and you’ll see that this “star” wanders through the constellations (and quite rapidly, too). In March, Mars seems to rise toward the Hyades and the Pleiades in Taurus. By late March, the red planet is only 3 degrees to the south of the bluish stars of the Pleiades cluster. The lunar crescent lies a few degrees to the left of Mars on the evening of March 11.
Also note that Mars gradually sinks in the sky at twilight. On March 1, one hour after sunset, the red planet is about 43 degrees high in the west-southwest, but by the 31st, it’s only 34 degrees above the western horizon.
The red planet sets in the west-northwest in the late evening. The other three bright planets appear hours later one after the other. Jupiter is the first to emerge after 3 a.m. EDT. At dawn, the giant planet shines about 20 degrees above the south-southeastern horizon.
Farther to its left and much less bright, Saturn rises in the southeast after 5 a.m. EDT. The ringed planet climbs only about 15 degrees high before being swallowed by the light of dawn. Even dazzling Venus now loses some of its lustre. The Morning Star appears only after dawn is well under way, very low in the east-southeast, and it rises only a few degrees at daybreak. Yet thanks to its exceptional glow, it can still be spotted quite easily.
On March 2 at dawn, admire the lunar crescent lying between Saturn and Venus. On the morning of March 27, the waning gibbous Moon is 3.5 degrees to the left of Jupiter. On March 29, the waning Moon is found only 2.5 degrees to the lower left of Saturn.
Finally, remember that the clocks spring forward early in the morning on Sunday, March 10. The lost hour of sleep may affect you for several days, but you’ll welcome an extra hour of daylight in the early evening, a sign that spring is at our doorstep.