Planet Data - The National Air and Space Museum
What to know the Polar Radius of Neptune? The National Air and Space Museum at the Smithsonian offers a great chart with lots of data about all of the planets and Pluto. Feel free to check it out!
Deepspace Group - University of Cal, Santa Barbara
The Deepspace Group is the official Experimental Cosmology Group of the University of California in Santa Barbara. This provides an array of data about experimental astrophysics and many active projects in astronomy.
Other Sites about Astronomical Data and Terms:
What is Orbital Velocity?
Confused by Orbital Velocity? Windows to the Universe, defines orbital velocity at this helpful site!