What Determines When Easter Comes?

The date of Easter is based on the positon of the moon to Earth.

Unlike other holidays such as Christmas, Easter is a moveable feast, one which takes place at different time every year. The date of Easter varies as greatly as a month, between March 22 and April 25. This shift in date is caused by the different rotations of the moon around the Earth and the Earth around the sun, making it rather complicated to calculate when Easter will occur in any given year.

Easter

Easter comes at the end of the 40-day period of fasting, prayer and penance known as Lent. Despite its moveable nature, Easter always falls on a Sunday, with the preceding Friday known as Good Friday, and the entire preceding week known as Holy Week. Easter is also followed by a 50-day period called the Easter Season, which ends with Pentecost Sunday. Easter itself is determined as the first Sunday after the full moon following the northern hemisphere's vernal equinox.

Vernal Equinox

The equinox is a point in the year where the sun is directly overhead of any point on the Earth's equator. This occurs twice a year, in spring and autumn (at least in the northern hemisphere), with the earlier equinox called the vernal and the later equinox called the autumnal. Although the actual equinox typically occurs on March 20, the vernal equinox is generally occurs on March 21, giving the occasion the additional nickname of the March equinox. The equinox then begins the period in which Easter may occur, with the exception of years when the full moon falls on the same day as the vernal equinox. In this case, Easter falls on the second full moon following the equinox.

Moon Cycles

With the exception of an eclipse, the sun always illuminates half of the moon. How much of this side is seen on Earth depends on the angle where Earth sits between the sun and the moon. It takes approximately 27.5 days for the moon to complete a cycle from new moon, beginning when the sun and moon are on the same side of the Earth. This rotation causes the date of Easter to depend on the positions of both the sun and the moon in relation to Earth.

Different Calendars

The date of Easter can be further complicated when considering the difference between Eastern and Western forms of Christianity. Whereas Western Christianity uses the Gregory calendar, the standard calendar used worldwide, Eastern Christianity bases its date for Easter off the Julian calendar. The calendars differ by approximately 11 minutes, meaning that as time has progressed, the differences in dates have widened. The period of Easter therefore runs between April 4 and May 8. Of course, since the equinox occurs regardless of the calendar, this difference is purely based on date and not on the position of the sun. However, whether Eastern or Western, Easter is still the first Sunday after the first full moon following the vernal equinox.

References

• StarrySkies.com: April's Full Moon Has Important Distinction
• First United Methodist Church of Heath: All About Lent and Easter
• Oremus: Calculating the Date of Easter
• TimeandDate.com: The March Equinox Explained
• AstronomyOnline.org: The Moon - Importance of the Moon
• Calendars Through the Ages: The Christian Calendar