Easter Defined by Earth’s Orbit

Spring, 2014, began on March 20^th at 12:57 in the afternoon, Eastern Daylight Time.  However, the beginning of spring is not determined by a precise, to-the-minute measurement.  Several factors influence the way we determine the beginning of any of the four seasons and the important date of Easter Sunday.

The movement of Earth around the sun is the constant that is used to measure time. We depend on calendars to measure Earth’s orbit that is a bit more than 365 days.

Calendars of various types have been used for thousands of years. In western cultures, the 10- month Roman calendar was the most recognized until 45 BC when Julius Caesar introduced the 12- month calendar. It aligned closer to the changing seasons and for centuries gave the most accurate measure of Earth making one orbit around the sun each year.

Caesar, with expert mathematicians, got the measurement of a year close to accurate. Called the Julian Calendar, the twelve months were divided into 365.25 days with a leap year every four years to accept one extra day. This calendar aligned close to the changing seasons and was the accepted way of judging Earth’s movement until the 16^th century.

In 1582, Pope Gregory XIII established the Gregorian Calendar which most of the world now observes. This calendar measures a year at 365.2425 days long. Gregorian years are only 27 seconds longer than the actual time Earth orbits one year.  It allows a gain of one day in 3,200 years.

To correct for the error in the number of days that slowly accumulated since Caesar’s calendar, Pope Gregory dropped ten days from the year 1582. That was decreed to be a one- time adjustment, but 12 days were erased 170 years later from 1752 to correct the length of the year to the present accuracy.

This adjusting of the calendar makes it incredibly accurate but not exact. None of us will ever notice the one day gain Earth will have in more than 3,000 years. That slim margin of difference proves that the errors are accumulative. However, that one day gain is not noticeable.

The angle of Earth on its axis causes the season changes as our planet obits the sun. Rays of the sun hit directly onto the equator on March 20^th or 21^st. That begins the hot months of the year and days become longer. We have equal hours of day and night on this March date. This is called the spring or vernal equinox.

As Earth’s position changes on its yearly trek around the sun, the sun’s rays strike the northern hemisphere called the Tropic of Cancer. This is called the June Solstice which has the shortest day of the year. The Autumn Equinox will occur this year on Sept. 22 and the sunshine will concentrate onto the southern hemisphere called the Tropic of Capricorn.

The Tropic of Capricorn gets most of the sun’s rays in the 12th month of the year and is called the December Solstice.

Neither phase of the sun is exact. The varying times of light can be attributed to the wobble of Earth on its axis. That’s a slight shake Earth makes as it spins every 24 hours.

Pope Gregory adjusted the calendar to establish a regular method of determining which Sunday will be Easter. The important Christian celebration comes on the first Sunday after the first full moon on or just after the vernal equinox in March.

This complicated formula helps us select the date for Easter many years in advance. Here are the next five dates for Easter:  April 20, 2014; April 5, 2015; March 27, 2016; April 16, 2017; and April 1, 2018.  Just the next five Easters show a difference of 24 days. It’s the irregular movement of Earth’s orbit that complicates when the official date of Easter will be each year.

The irregularity of Easter makes for some fun celebrating in our family. On two occasions in her lifetime so far, Easter has fallen on my youngest daughter’s birthday of March 30^th (1985 & 1997). My oldest daughter, however, born on April 1, will not have the first Easter birthday of her lifetime until 2018.  

Thank you for reading this blog. Look for another topic about writing fiction next week.  See my web page at www.joevlatino.com.