|Excel gmath library|
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The gmath add-in extends Microsoft Excel with functions for trigonometry using degrees; converting angles between decimal angles and degrees, minutes and seconds; and determining solar data such as Equation of Time (EoT) or Solar Declination (SD) for a numbered day of the year or for a specific date and time. Many of the functions use methods from Jean Meeus’ book Astronomical Algorithms. Installing gmath is a one-time exercise after which the functions are available automatically whenever you use Excel.
Download the documentation (PDF)
Download the Excel add-on file
Do not open the add-on file directly from your browser - you must download the file and then install it according to the instructions in the documentation.
As well, if you're a Windows user, depending on your version you will likely need to alter the properties of the downloaded file: before installing the add-in, right click on the downloaded file and choose 'properties'. If you see a note about the file being blocked, you'll need to unblock it. It's a Windows security thing (put the file through your anti-virus software if you feel the need to check it before unblocking). If you do not unblock the file, then Excel will install the add-in but every time you restart it after that it will report being unable to find the gmath functions.