       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:= AstronomicalData["Sun", {"LastRiseTime", {2010, 10, 1, 12, 0,
0}, {39., -77.}}, TimeZone -> 0]

Out= {2010, 10, 1, 11, 4, 27.962}

and the time at which the attitude is zero

In:= AstronomicalData["Sun", {"Altitude", {2010, 10, 1, 11, 8,
45.474}, {39.`, -77.`}}, TimeZone -> 0]

Out= 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
> --

```

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