{"id":2431,"date":"2017-04-01T00:01:16","date_gmt":"2017-04-01T07:01:16","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=2431"},"modified":"2017-04-08T08:21:18","modified_gmt":"2017-04-08T15:21:18","slug":"calculating-the-date-of-easter","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=2431","title":{"rendered":"Calculating the Date of Easter"},"content":{"rendered":"

Of all the annual holidays, Easter is the most difficult date to predict. It’s always a Sunday, but which one? It could land in March or April. Most people look at a calendar or (these days) use Google to find out when Easter occurs. Yet, you can write a program that tells you exactly when Easter falls.
\n
\nThe reason behind Easter’s changing date is that it’s based on a lunar calendar, not the standard solar calendar in common use. Easter occurs on the first Sunday after the first full moon after the vernal equinox.<\/p>\n

The vernal (spring) equinox is always around March 20th or 21st. That’s because the solar calendar we use is pretty accurate. Indeed, having such a predictable date for the equinox is why the Julian and then Gregorian calendar reforms occurred.<\/p>\n

The first full moon after the vernal equinox marks the Jewish passover. It can occur right after March 20\/21 or it could be up to 28 days later, in April. Easter is then celebrated on the following Sunday.<\/p>\n

To calculate the date upon which Easter falls, you must know the date of the equinox and the lunar full-moon cycle as well as the day of the week upon which the full moon falls. To gather those variables, you use a formula. This formula is so famous, it has its own name: the computus<\/em>.<\/p>\n

In Figure 1, you see pseudocode presented. It’s what’s known as the Anonymous Gregorian algorithm, which first appeared in the journal Nature<\/em> in 1876.<\/p>\n

\"\"

Figure 1. The Anonymous Gregorian computus<\/em> algorithm.<\/p><\/div>\n

The algorithm works top-down: Variables are a<\/code> through m<\/code> (I wrote l<\/code> in uppercase, L<\/code>, to be clear); Y<\/code> is the year value; mod<\/code> is the modulus operation; floor<\/code> is a mathematical function that rounds down to the next integer. At the end of the operation, values month<\/code> and day<\/code> indicate the month (3 for March, 4 for April) and day on which Easter falls for year Y<\/code>.<\/p>\n

Your task for this month’s Exercise is to use the Anonymous Gregorian computus<\/em> algorithm to calculate the dates upon which Easter falls. You must write two programs: The first displays Easter’s day for years 2001 through 2021. The second program displays the date of Easter for the current year only.<\/p>\n

I’ll present my solution<\/a> in a week.<\/p>\n","protected":false},"excerpt":{"rendered":"

Formulas, or algorithms, exist to perform many tasks. Your job is to translate them into C code. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[],"class_list":["post-2431","post","type-post","status-publish","format-standard","hentry","category-exercise"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2431","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2431"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2431\/revisions"}],"predecessor-version":[{"id":2467,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2431\/revisions\/2467"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2431"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2431"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2431"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}