THE A T M O S P H E R I C R E S E R V O I R Examining the Atmosphere and Atmospheric Resource Management A Solar Calendar By Mark Schneider “Centurial years are ordinary years, In fact, the dates of the Paschal full unless they are divisible by 400, in moon and the actual full moon may Our sun is the keeper of time. which case they are leap years. This vary by up to two days. Orthodox Not only does the Earth revolve causes a correction on years 1700, churches still use the Julian calen- around it, but our scheduled lives 1800, 1900, 2100, 2200, and 2300.” dar, and for this reason there are two do as well. Our modern calendar, dates on which Easter is observed the Gregorian calendar, serves as The history behind the modern annually. For example, this year the the international civil calendar and Gregorian calendar is quite interest- Western celebration of Easter was is derived by arithmetic. Each of ing. It is actually a reform of the April 12th and the Orthodox cel- our days is called a ebration occurred on solar day because LEAP SHIFTING OF THE GREGORIAN CALENDAR April 19th. Date of summer solstice over 400 years Earth makes one 23.0 rotation about its Reform from the axis as the sun’s po- 22.5 Julian to the Gregorian sition moves from a 22.0 calendar didn’t happen starting and ending overnight, and many point at the Prime 21.5 countries including Meridian. A year is JUNE England held out for en.wikipedia.org one full orbit of the 21.0 many centuries before Earth around our 20.5 making the switch. Up sun and is 365.2425 until 1752, England days in length dur- 20.0 was technically 12 1800 1900 2000 2100 2200 ing an average Gre- YEAR days behind the rest of gorian year. A solar Europe because it still year is technically about six hours Julian calendar, which is a reform followed the Julian calendar. When longer than a Gregorian year and of the Roman calendar. The reforms the British finally adopted the Gre- this necessitates a leap year every or changes between calendars were gorian calendar on Sept. 2nd, they four years, which adds one 24-hour made primarily for religious reasons. went to sleep that night and awoke day to the calendar (Feb. 29th). on Sept. 14th! The Catholic Church modified the The image here is a comparison Julian calendar in 1582 under Pope The use of calendars worldwide of the Gregorian calendar and the Gregory XIII, creating the Gregorian wouldn’t be possible without refer- June Solstice. Each dot on the chart calendar in hopes that all Christians encing our sun. Whether you use the represents the date of the solstice would celebrate Easter on the same Gregorian, Julian, or other form of between the years 1750 and 2250. day. In reality, Easter Day ended up calendar, you have a constant guide It’s very apparent that between the being decided by a lunisolar calen- in the sky. years 1900 and 2100 the date of dar. Christians use March 21st as the the solstice varies from June 20th reference date for the vernal equi- to June 23rd. The year-to-year nox (even though it varies between Atmospheric Resource Board variation is approximately equal to March 19th and 21st) and then use North Dakota State Water Commission six-hours, requiring leap years every the next Paschal full moon to deter- 900 East Boulevard, Bismarck, ND 58505 (701) 328-2788 four years for correction. mine the date for Easter each year. http://swc.nd.gov The Paschal full moon represents ND Weather Modification Association An additional rule adopted by the 14th day of the lunar month and PO Box 2599, Bismarck, ND 58502 the Gregorian calendar states that, not the true astronomical full moon. (701) 223-4232

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