Why does Mercury get thrown out of the Solar System when I turn up the time step?
As you turn up the time step you lower the accuracy of the simulation. If the accuracy is too low bodies will get thrown out of the system.
The numbers: Mercury takes about 88 days to make a single orbit around the sun. A time step of 22 days would only be calculating a new position for Mercury 4 times in that period. This isn't enough accuracy to maintain a stable orbit. The Earth is further out and takes 365 days to orbit the sun. This same time step of 22 days results in about 16 position calculations for the Earth which is enough to maintain an orbit.
Why don't you include any moons in the system 'Our Solar System'?
Since some moons make full orbits of their respective planets in less than 1 day, the maximum speed (or time step) that you can run the system is much lower than without them. (see the question above for more info on time steps)
Load the system 'Our Solar System - all known moons' to see all the 160+ moons in our Solar System.
I made a black hole but everything just got flung into space. Why didn't they get sucked in?
Your time step is too high for a body with such a large mass.
Press the A key or the numpad Enter key to turn on auto time step before you create the black hole.
Auto time step adjusts the time step to maintain an reasonably accurate simulation.
Is there any difference between a black hole and any other body in Universe Sandbox?
No. For the purposes of the simulation a 'preset' black hole in Universe Sandbox is body equal in mass to 100 of our Suns. Real black holes can be larger or smaller than this.
What's the math behind all of this?
It's Newtonian physics using a basic n-body (particle-particle) algorithm.
Are their other programs like Universe Sandbox?
Gravity Simulator - Tony Dunn
An n-body simulator similar in concept to Universe Sandbox and capable of scale simulations of our Solar System.
Using the rotating frame feature, you can easily study the complex orbital relationship between the various moons of Saturn, or the relationship between Earth's orbit and the horseshoe orbit of Crutched, an asteroid often referred to as "Earth's 2nd moon".
It also includes some advanced features for recording data that can be graphed in a spreadsheet app.
Gravity Simulator is for Windows systems.
AstroGrav
A gravity simulator available for both Mac and Windows as a 15 day trial.
Uses Newton's Law of Gravitation to run scale simulations of our Solar System.
A space simulator that lets you explore our universe in three dimensions (but it's not interactive and doesn't simulate gravity).
All movement in Celestia is seamless and you can zoom from the Earth to the edge of our Galaxy.
Celestia runs on Windows, Linux, and Mac OS X.
Digital Universe
A free program from Hayden Planetarium for Windows, Mac, Linux, and IRIX.
Includes data on thousands of stars and galaxies.
This is not a gravity simulator, only a data viewer.
Gravit
Gravit is a gravity simulator which runs under Linux, Windows and Mac OS X. It's released under the GNU General Public License.
Uses Newtonian physics using the Barnes-Hut N-body algorithm but isn't setup to simulate real systems.
How accurate is Universe Sandbox?
Solar System sized simulations at a small time step (~2 minutes) are very accurate.
Galaxy sized simulations are only representative and not very accurate (as Universe Sandbox doesn't take dark matter into account which doesn't have much of an observable effect on our solar system but noticeably affects galaxies. Nor does Universe Sandbox consider Einstein's theory of general relativity as the basis of any of its calculations.).
Where did you get your data?
Data used in Universe Sandbox was collected from a variety of sources:
JPL's Horizon System - Planet, moon, and asteroid starting positions & velocities
Star Positions & Velocities - David Nash
Star Color Values- Mitchell Charity
Atlas of the Universe - Richard Powell
Wikipedia - arbiter of truth :)
What's the version number mean?
Version numbers for Universe Sandbox look like this: 1.0.424.0. The first two numbers are the major and minor version numbers. The third (424 in this example) is an approximate count of the number of days spent working on it. The fourth is used only when there's minor fixes.
How'd you come up with this?
I've been wanting to make something like this for a long time. I've always been fascinated by how such a simple equation can result in such beauty. Here are some screenshots of the predecessors to Universe Sandbox that I worked on in
1993 & 1994, 1997, and 2000.
If your question isn't answered, just ask.
