AstroGrav iconAstroGrav Help /Menus /Edit /Remove ObjectsVersion 3.6

Edit / Remove Objects   ('Edit / Remove Objects' icon)

Operating Systems: All
Window Types: All
Simulation Types: All

Summary

Removes the current selection of objects in the active window in such a way as to leave the orbits of other objects unchanged.

Description

When you select this command, the currently selected objects in the active window will be removed in such a way as to leave the orbits of other objects unchanged. If any of the selected objects has descendants, then these descendants will also be removed because they will no longer have anything to orbit. Each of the frontmost simulation's open windows will be updated to reflect the change.

Removing the currently selected objects is an alternative to simply deleting them using the Edit / Delete command. It's advantages are best illustrated by an example. Suppose that you had a simulation containing the Sun, Earth, and Moon, and wanted to convert this into a simulation containing just the Sun and Earth in such a way that the Earth's orbit is left unchanged. What appears as the Earth's orbit is really the orbit of the barycenter of the Earth and the Moon, so that simply deleting the Moon will result in the Earth's orbit being changed. By removing the Moon with this command instead, the Earth's orbit is left unchanged. After the removal of the Moon, the Earth's mass, position, and velocity will be equal to those of the barycenter of the Earth and the Moon prior to the removal.

To give a more complex example, when removing Mars from the Planets, All Moons simulation, the following sequence of actions takes place.

  1. Mars, Phobos, and Deimos are merged into a single object at their barycenter.
  2. Jupiter, Saturn, Uranus, Neptune, Pluto, and their moons are temporarily deleted.
  3. The barycenter of the Sun, Mercury, Venus, Earth, Moon, and Mars is recorded.
  4. The mass of Mars is transferred to the Sun, Mercury, Venus, Earth, and Moon in proportion to their masses.
  5. The semi-major axes of Mercury, Venus, Earth, and Moon are increased so as to leave their orbital periods the same as before the masses were changed.
  6. Mars is deleted.
  7. The Sun, Mercury, Venus, Earth, and Moon are translated to the barycenter that was recorded earlier.
  8. Jupiter, Saturn, Uranus, Neptune, Pluto, and their moons are replaced.

In general, for each selected object, the following sequence of actions takes place.

  1. The selected object and its descendants (if any) are merged into a single object at their barycenter.
  2. The selected object's exterior siblings are temporarily deleted.
  3. The barycenter of the selected object, its parent, and remaining siblings is recorded.
  4. The mass of the selected object is transferred to its parent and remaining siblings in proportion to their masses.
  5. The semi-major axes of the remaining siblings are increased so as to leave their orbital periods the same as before the masses were changed.
  6. The selected object is deleted.
  7. The selected object's parent and remaining siblings are translated to the barycenter that was recorded earlier.
  8. The temporarily deleted exterior siblings are replaced.

The effect of this command is independent of whether orbital elements are calculated relative to the parent object only or are calculated relative to the barycenter of all inferior objects. When calculated relative to the barycenter of all inferior objects, the orbital elements of the remaining objects are unchanged by this command, except for the semi-major axes that were increased in order to leave the orbital periods unchanged. See the Tools / Calculate Orbits... command for details about how orbital elements are calculated.

This command was used to create the Planets, All Moons, Planets, Major Moons, Planets, Moon, Asteroids, Planets, Moon, Comets, Planets, Moon, KBOs, Planets, Moon, and Planets simulations from the master Solar System simulation.

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