Sun, moon, pope, scientist helped set Easter

Cassini's sun calendar, installed in 1651 in San Petronio Cathedral in Bologna, Italy, was a work of genius that tracked the Gregorian calendar.

(CNN) -- The holiday wanders each year from March to April. Exactly how is the Easter date set? The short answer is it falls on the first Sunday after the first full moon that follows the first day of spring in the Northern Hemisphere. The long answer requires a trip back through time more than 1,700 years ago.

In 325 A.D., Christian church leaders established the astronomical requirements that much of the Western world uses today to determine when to observe the holy day, which falls anytime between March 22 and April 25.

At the time, astronomers and clergy had a lot of trouble predicting when Easter would take place in the years ahead, in large part because of the absence of an adequate annual calendar.

The reason? Mathematicians at the time were working on the premise that the sun orbited the Earth, which gave them considerable headaches in plotting the future.

After many centuries of patchwork solutions, the Catholic Church in 1582 adopted the much improved Gregorian Calendar, named for then-Pope Gregory XIII, which eventually became the norm in Europe and most of the world.

Giovanni Cassini, 1625 -1712

In 1651, the scientist Giovanni Cassini installed a pinhole camera in the roof of the San Petronia Cathedral in Bologna, Italy, that helped monitor the effectiveness of the new calendar.

The observatory, which concentrated sunlight onto the floor, allowed Cassini to calculate future vernal equinoxes with considerable accuracy.

His observations helped solve the perplexing question of when future Easters would take place. Moreover, his measurements of variations in the sunbeam on the meticulously arranged tiles lead to one of the first accurate calculations of the distance between the Earth and sun.

Interestingly, Cassini's findings supported the controversial "theory" that the Earth revolved around the sun, contrary to the official Catholic cosmology that the Earth was the center of the solar system.