Re: Astronomical Data Accuracy

*To*: mathgroup at smc.vnet.net*Subject*: [mg113005] Re: Astronomical Data Accuracy*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Sun, 10 Oct 2010 06:43:35 -0400 (EDT)*References*: <i8h7kb$ceu$1@smc.vnet.net>

The refraction of the atmosphere alone accounts for a difference of 0.5 deg (http://www.asterism.org/tutorials/tut28-1.htm). I understood from an email exchange I had with Jeff Bryant (WRI) two years ago that Mathematica does not take refraction into account. And is the sun's altitude that of its centre or it's boundary? Makes a difference for it sunrise/sunset times. It's not in the documentation, but if you look at the sunrise time In[4]:= AstronomicalData["Sun", {"LastRiseTime", {2010, 10, 1, 12, 0, 0}, {39., -77.}}, TimeZone -> 0] Out[4]= {2010, 10, 1, 11, 4, 27.962} and the time at which the attitude is zero In[23]:= AstronomicalData["Sun", {"Altitude", {2010, 10, 1, 11, 8, 45.474}, {39.`, -77.`}}, TimeZone -> 0] Out[23]= 0.*10^-6 We may assume that the difference of 4 min accounts for the finite size of the sun. Cheers -- Sjoerd On Oct 6, 9:14 am, "1.156" <r... at piovere.com> wrote: > Tim, I find that AstronomicalData is not much use for celestial > navigation work. Note that AstronomicalData[] does not list "Aries", or > anything involving Vernal Equinox, nor does it list any of the 57 > navigational stars. But I have made only very limited trials with it. > > Rob > > ************* > Hi, > Does anyone know what the accuracy is for the AstronomicalData > calculations for "Altitude" for the sun, moon, planets? I'm writing a > Mathematica program to solve celestial navigation problems. I used the > following code: > > AstronomicalData["Sun", {"Altitude", {2010, 10, 1, 12, 0, 0}, {39., > -77.}}, TimeZone -> 0] > > This gives a result of 9.83748 deg. > > However, when I ran the same computation using The United States Naval > Observatory (USNO) online data service (www.usno.navy.mil/USNO) the > altitude was given as 9.93889 deg. This is a difference of 0.1 deg. > I'm sure the Mathematica AstronomicalData functions were never intended > to replicate the accuracy and precision of the USNO computation. > However, 0.1 deg seems a bit much, especially when doing > navigation/astronomy calculations. Does Mathematica use low accuracy > formulae, or did I do something wrong? Any help would be appreciated. > Thanks, > Tim > --