Planetary Orbits
Planetary Orbits
| Group Name | solsys |
| Reference | Jet Propulsion Lab's HORIZONS |
| Prepared by | Brian Abbott, Ryan Wyatt (AMNH/Hayden) |
| Labels | No |
| Files | solsys-mw.speck |
| Dependencies | halo.sgi |
| Census | 9 orbits |
Because the Solar System is among the most familiar and widely understood aspects of astronomy, we have included the paths of the planetary orbits in the Milky Way Atlas. This is a bit unnatural, since the scale of the Solar System is so small when compared with the nearest star to the Sun. But with a few modifications, you can see how the Solar System fits in the overall scale of the Milky Way Atlas.
This is not a “working” Solar System. There are no planets that move along these trajectories. Only the orbits themselves are present for context within the Atlas.
The orbits' color coding is loosely based on the planets' colors. Mercury is gray, Venus is yellow, Earth is blue, Mars is orange, and so on for the outer planets. The data set also includes what we call the mini-sun. This is a smaller Sun exactly like the Sun in the stars group but far less luminous.
Recall that the average Earth-Sun distance is defined to be 1 astronomical unit (AU) and equals 149.6 million kilometers (93 million miles). Yes, that small distance between the Sun and the blue orbit is 93 million miles. We can also express this distance in light-travel time as 8.3 light-minutes. Light-travel time is the time it takes light to traverse some distance in space. It takes the Sun's light a little more than 8 minutes to reach Earth. It takes more than 4 hours to reach Neptune. For more on light-travel time, see “Light-Travel Time and Distance.”
Seeing the Solar System Group
The scale of the Solar System is so small compared with the parsec-scaled Milky Way Atlas (Earth is 0.00000485 parsecs from the Sun) that you must alter the display characteristics of Partiview to see the solsys (and probes) data group. Once you bring up the Milky Way Atlas, follow these steps to view the orbits:
- Turn on the solsys data group.
- Change Partiview's clipping planes using the command clip 1e-5 1e5. You should see the orbits appear. If you see nothing, see the “Troubleshooting” paragraph below.
- Turn off the star polygons. Make sure stars is the active data group and press the Poly Toggle Button. This is necessary because the Sun's polygon will be too bright on the scale of the Solar System, so we must turn it off.
- Fly backward away from the Sun using the right mouse button and moving the mouse to the left.
- Now you can orbit the Solar System and view it from outside. Once you leave the Solar System and begin flying away from the Sun, remember to switch your clipping planes back to their preset values using the command clip 0.1 1e6 and return the stars' polygons.
What Are Clipping Planes?
In any 3-D graphics software, the program needs to know what to draw and where to draw it. When you look into Partiview's Graphics Window, imagine you're looking down onto a pyramid from its apex. The sides of the pyramid are represented by the four sides of the window, which is set by the field of view. A small field of view will make your pyramid narrow, resulting in a narrow sightline and less data in view; a large field of view will expand the range for drawing, resulting in a pyramid with a wide base.
The location of the base and apex of the pyramid are set using clipping planes. There is a near clipping plane, the apex, in front of which nothing is drawn, and a far clipping plane, the base, behind which nothing is drawn. The Milky Way Atlas is initialized with the command clip 0.1 1000000, meaning objects will be drawn only when they are between 0.1 and 1 million parsecs from your position. Anything closer than 0.1 pc and farther than 1 million pc will not be drawn. The Solar System is very small on the parsec scale, so we must bring the near clipping plane closer to us to see it. Hence the 1e-5 (0.00001) pc setting.
Troubleshooting
Depending on your computer and graphics support, you may have problems with these clipping-plane values. On most systems, you will see flashing and blinking of data with such small values and large differences between planes. There are some graphics cards that draw nothing when you enter a near clip of less than 1. Some cannot handle a large range between the near and far planes. If you are having trouble, experiment with the values to see if you can make something work. Because Partiview has never been rigorously tested on various graphics cards, we cannot provide any hard, fast rules for its performance.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott