Em 28/09/2010 Kevin Karney enviou para a SML (Sundial Mailing List) a mensagem, que reproduzo abaixo, descrevendo uma nova forma de desenhar a equação do tempo, que foge da tradicional curva com forma de "8". Vale a pena considerar este tipo de enfoque quando se desejar um aspecto mais artístico.
You may be interested in the attached chart of the Equation of Time for 2011. It is of interest for a number of reasons...
- It is an 'intrinsic' plot - not the familiar cartesian, polar or analemmatic form. An intrinsic plot has equal increments of the independent variable along the curve of the plot rather than along the x-axis. This allows the curve to be folded - making a very compact form.
- It is much easier to read than a normal plot, because the day points are all equally spaced along the curve.
- It looks good !
The use of an intrinsic plot for the equation of time was first used in the Bury St Edmunds dial in the 1860s. See F. Sawyer: ‘The Bury St Edmunds Curve’, NASS Compendium 12(3), p.29 (Sept 2005) or John Davies: 'More on the Equation of Time on Sundials', BSS Bull 17(ii), p.75 (June 2005).
This particular 'Flame' form is one of many possible compact folded forms that can easily be generated with a spreadsheet.
For the number-obsessed, the chart is very precise. Though you cannot of course read it on the chart, the values of EoT are exact to within a fraction of a second, since I have used the 2011 polynomial algorithms issued to surveyors and navigators by HM Nautical Almanac Office. The chart is for noon UT. If anyone would like a copy - correct for their own time zone and longitude - please let me know. Or if you are familiar with Excel, you can download the spreadsheet used to draw the curve from www.precisedirections.co.uk/Sundials/Index.html. The file is called "Almanac 2010_2011.xls". This has all the polynomial coefficients for the Sun and navigational stars for both 2010 and 2011. You can see a number of other intrinsic plots of the EoT at the above website. Click on the 'Grange-over-Sands Conference' link.
For the really observant, you will notice a tiny additional gap between 31 Dec and 1st Jan: that is there because there are 365 1/4 days in a year not 365...
The zodiac signs on the chart mark the day starting the astronomical constellations when the Sun's longitude are passes 30, 60, 90 degrees etc. This may not conform with the astrological dates.