Comments on: When is Noon? https://blogs.mathworks.com/community/2018/01/24/when-is-noon/?s_tid=feedtopost News from the intersection of MATLAB, Community, and the web. Fri, 29 Jun 2018 22:22:40 +0000 hourly 1 https://wordpress.org/?v=6.2.2 By: Ned Gulley https://blogs.mathworks.com/community/2018/01/24/when-is-noon/#comment-72282 Mon, 05 Feb 2018 22:39:28 +0000 https://blogs.mathworks.com/community/?p=5146#comment-72282 I love that time-wrongness map. China really jumps out for being one gigantic time zone. And you can see the world is mostly skewed late (red) rather than early (green). Thanks for sharing!

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By: Grant Cook III https://blogs.mathworks.com/community/2018/01/24/when-is-noon/#comment-72280 Mon, 05 Feb 2018 22:07:50 +0000 https://blogs.mathworks.com/community/?p=5146#comment-72280 Great article. I was reminded of this other article that illustrates deviations in solar noon across the world:
http://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/

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By: Ned Gulley https://blogs.mathworks.com/community/2018/01/24/when-is-noon/#comment-72274 Thu, 25 Jan 2018 23:15:56 +0000 https://blogs.mathworks.com/community/?p=5146#comment-72274 Thanks for the comments Eric! In astronomy there are always opportunities for more precision. Like your Gregorian calendar story, I’ve always been amused about how astronomers and historians differ about dates before 1 CE. For historians, there is no year zero. 1 BCE gives way to 1 CE. Astronomers, on the other hand, insist on using math with a zero between -1 and 1! So you have to apply a correction factor to make the two square with one another.

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By: Eric Shields https://blogs.mathworks.com/community/2018/01/24/when-is-noon/#comment-72272 Thu, 25 Jan 2018 16:14:30 +0000 https://blogs.mathworks.com/community/?p=5146#comment-72272 Great post! I see on the FEX that this algorithm is accurate to +/- 1 degree. Those interested in calculating solar zenith angles more accurately can look here: http://rredc.nrel.gov/solar/codesandalgorithms/spa. The authors demonstrate an accuracy of +/- 0.0003 degrees. They provide C-code that can be converted to Matlab or a MEX file.

Note that in their PDF, the value for H in Table A5.1 of Section A5 has a typo. It should be 11.105902 degrees rather than 11.105900. This is corrected in their code and is necessary to obtain their answers in the examples.

Interestingly, I also found that the algorithm in their paper for calculating Julian dates disagrees with Matlab’s juliandate() function for dates before October 15, 1582. The discrepancy has to do with the 10 day error in the Julian calendar in 1582 when the Gregorian correction was made, I believe. See this PNG on Wikipedia.

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