If you're interested in this type of computation and have some basic knowledge of Python, there is a package called pyephem that you can install that allows you to perform all sorts of calculations on the planets and other astronomical bodies. It uses the same ephemerides data set that is used by professional astronomers so it is quite accurate.

PyKEP (https://esa.github.io/pykep/) is also quite good.

Some of the links from http://www.stjarnhimlen.se/comp/ppcomp.html#22 are dead. If you want human-readable physical and orbital data for:

* planets, go to http://ssd.jpl.nasa.gov/?planet_phys_par and http://ssd.jpl.nasa.gov/?planet_pos (it points you to a table http://ssd.jpl.nasa.gov/txt/p_elem_t1.txt where elements need to be corrected for a given date, see http://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf)

* moons, go to http://ssd.jpl.nasa.gov/?sat_phys_par and http://ssd.jpl.nasa.gov/?sat_elem

* dwarf planets, asteroids, etc, go to http://ssd.jpl.nasa.gov/sbdb.cgi

Also, some more data here http://ssd.jpl.nasa.gov/?constants. If you want more precise orbital elements at a given epoch (they vary through the year), use http://ssd.jpl.nasa.gov/?horizons (using the Telnet interface, first choose "Ephemerides" (e) and then "Elements" (e).

It is a great fun to compute Planetary positions. You should really read a book named "Astronomical Algorithms" by Jean Meeus. That is the ultimate source for those computations, for hobby use. (There is a really nice chapter in there, about accuracy, and interpolation of values). You'll find various implementations of the algorithms on the net.

Apropos - does anyone have suggestions for good Android apps for things like this? I'm looking for something pretty for my kids, that lets you look at some different things, like how the moon goes around the earth, night and day on the earth, seasons, and maybe some of the other planets too.

It's a web site but it has a number of simulations of various astronomy topics and is very well done. It's labeled as being introductory but, depending upon the ages of your children, it might still be too advanced.

http://astro.unl.edu/naap/

not sure about mobile, but I would think Kerbal Space Program has most of the physics done, as well as an active mod community. I've always thought an accurate solar system mod would be pretty cool

It exists. Check this out: http://forum.kerbalspaceprogram.com/threads/55145-0-90-Real-...

It's also a lot more difficult than the stock solar system.

Kerbal Space Program has a good physics model for fun but not for study: it's very basic. Their drag and aerodynamic model for example is not accurate (if I remember correctly they assume drag to be constant)

Jet engine modeling and life support are not accurate either

Does it show how to compute the error margins?