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I want the Saturn's position in terms of Declination and Right Ascension for a couple of month in the interval of 1 hour in a text file to do a simulation. Which site can provide me these data?

Or, there may be some software which can give these data in txt file. I know Stellarium 0.12.0 which show data. But does it provide the options to save the data in the text file also?

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Mathematica knows these data as functions. See e.g. Wolfram Alpha query here: wolframalpha.com/input/… –  Luboš Motl Apr 9 '13 at 12:49
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The technical name you are looking for is ephemerides. As answered below, NASA provides a wealth of data on all solar system bodies. –  Nic Apr 9 '13 at 13:55
    
Perhaps this helps. Some people are quite enthusiastic about the linked guide in it. –  Glen The Udderboat Apr 9 '13 at 14:25
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2 Answers 2

For anyone else wanting to do something similar, the JPL HORIZONS Web-Interface is a really useful and simple tool. Here is a run-down of how to use it ...

Settings

  1. Ephemeris Type would most often be "OBSERVER" to provide you with observable values like RA/DEC.
  2. Target Body: Obviously the object you want to know about. Along with the Sun, moon, and planets, you can also search for asteroids and other minor bodies.
  3. Observer Location: This is important for some statistics you will receive (e.g. Altitude/Elevation and Azimuth) but not for basic RA/DEC. Geocentric will give you results based on the Earth's location, and Heliocentric will do the same based on the Sun's position.
  4. Time Span: This is where you select the dates you are interested in and the step size. This step size is the time between each row of data you want to get. If you want to know the position of a planet for every hour between your dates, you would select a step size of 1 hour.
  5. Table Settings: Default option is usually ok, but there is where you can select what data you want to see in the table. You can select things like Alt/Az, the RA/Dec rates of movement, etc.
  6. Display/Output: Options are HTML, Plain Text, Download.

Once you are ready just click on the Generate Ephemeris button and your table of data will appear. A few things to note though.

Time

The table displays uses UTC which is Coordinated Universal Time. You will need to convert this into your local timezone. This is pretty easy and UTC is essentially the same as GMT (within 9/10ths of a second anyway). Just work out how many hours you are away from GMT and add/subtract that from the time shown in the table

Sun and Moon

Some symbols and letters are placed next to the UTC time in the table. These tell you whether the Sun or Moon are in the sky at that particular time. These are already calculated based on your location (if you provided it in the Observer Location option on the form). The notation is described below the table but here it is for your reference:

  • '*' Daylight (refracted solar upper-limb on or above apparent horizon)
  • 'C' Civil twilight/dawn
  • 'N' Nautical twilight/dawn
  • 'A' Astronomical twilight/dawn
  • 'm' Refracted upper-limb of Moon on or above apparent horizon
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That's actually a very interesting question, because I don't think that any program such as Stellarium really stores real orbit data - it rather computes them every time, i.e. solves equation of motion taking the Keplerian laws into account.

You should check out the JPL's website where they give you the key Keplerian elements needed to compute the orbit. An example of NASA's high precision computation can be found here (for Saturn).

All in all, for the time resolution you want (1 hour), you will have to use the data provided by NASA, solve the equations of motion and get the data from your soultions.

It seems the HORIZONS web-interface is capable of generating RA/Dec data with a step width of 1 hour, so my prior statement was wrong. :)

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