Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9) Previous Document: C.10 Why isn't the earliest Sunrise (and latest Sunset) on the longest day of the year? Next Document: C.12 What is the time delivered by a GPS receiver? See reader questions & answers on this topic!  Help others by sharing your knowledge John Horton Conway (the Princeton mathematician who is responsible for "the Game of Life") wrote a book with Guy and Berlekamp, _Winning Ways_, that describes in Volume 2 a number of useful calendrical rules. One of these is an easy "in your head" algorithm for calculating the phase of the Moon, good to a day or better depending on whether you use his refinements or not. In the 20th century, calculate the remainder upon dividing the last two digits of the year by 19; if greater than 9, subtract 19 from this to get a number between 9 and 9. Multiply the result by 11 and reduce modulo 30 to obtain a number between 29 and +29. Add the day of the month and the number of the month (except for Jan and Feb use 3 and 4 for the month number instead of 1 and 2). Subtract 4. Reduce modulo 30 to get a number between 0 and 29. This is the age of the Moon. Example: What was the phase of the Moon on DDay (June 6, 1944)? Answer: 44/19=2 remainder 6. 6*11=66, reduce modulo 30 to get 6. Add 6+6 to this and subtract 4: 6+6+64=14; the Moon was (nearly) full. I understand that the planners of Dday did care about the phase of the Moon, either because of illumination or because of tides. I think that Don Olsen recently discussed this in _Sky and Telescope_ (within the past several years). In the 21st century use 8.3 days instead of 4 for the last number. Conway also gives refinements for the leap year cycle and also for the slight variations in the lengths of months; what I have given should be good to +/ a day or so. User Contributions:Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9) Previous Document: C.10 Why isn't the earliest Sunrise (and latest Sunset) on the longest day of the year? Next Document: C.12 What is the time delivered by a GPS receiver? Part0  Part1  Part2  Part3  Part4  Part5  Part6  Part7  Part8  Single Page [ Usenet FAQs  Web FAQs  Documents  RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM

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