March: So, When’s Easter?

March 2008

So, When’s Easter?

By Calvin L. Chrisman

March marks the arrival of spring in the Northern Hemisphere. Astronomically speaking, it will happen at 1:38 a.m. Eastern Daylight Time on Thursday the 20th. This is the Spring (or Vernal) Equinox when the Sun crosses the equator on its journey north. This is the earliest Spring Equinox since the year 1896.

Equinox is Latin for “equal night”. If anyone checks the statistics for sunrise and sunset on the 20th they will find that the day is actually a few minutes longer than the night. Have the astronomers missed something? No. The longer period of daylight is caused by two things. First, the Sun is a disk and covers one-half of a degree of the sky. It takes several minutes for the entire body of the Sun to sink below the horizon at sunset. The same phenomenon applies to sunrise. Second, the Earth’s atmosphere bends the rays from the rising and setting Sun making it appear to be above the horizon before (or after) it actually is. The Equinox is not actually determined by the hours of day and night, but by the Sun’s transit of the equator.

The Spring Equinox plays a part in the determination of the date of Easter. Easter is celebrated on the first Sunday after the Paschal Moon, which is the first full moon after the Spring Equinox. Confused yet? The early church certainly was. Since the Earth takes approximately 365.2422 days to orbit the Sun and the Moon takes 29.53059 days to orbit the Earth, the Equinoxes keep moving around, making it hard to predict Easter in advance. Frustrating; at times, the early church found itself celebrating Easter many weeks ahead of or behind the correct date.

Detail from meridian line at Palermo Duomo Segni
Detail from meridian line at Palermo Duomo Segni

As a result, during the seventeenth and early eighteenth centuries, the Catholic Church quietly allowed astronomers to use their large cathedrals as solar observatories. They constructed Meridian Lines on the floor with a small hole high up in the church which would project the image of the Sun on the floor. By observing the position of the image at local noon, the observers could precisely mark the Equinoxes as well as the Solstices (the Sun’s highest and lowest points in the year). This allowed more precise predictions of these events, and thus the dating of Easter. It is ironic that this use of these cathedrals implied an understanding of correct solar mechanics, i.e. the Earth orbits the Sun, at a time that Galileo was excommunicated for asserting this view. It is interesting to note that the Meridian Line of the Cathedral of Saint Sulpice in Paris figures briefly in the book and movie, The Da Vinci Code, as the hiding place of a clue to the mystery.

Clear Skies!

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