Request for Aid: SSC Scripting Guide
Posted: 19.09.2008, 20:33
Not wanting to appear too incompetent, I spent some time reading up on various aspects of celestial mechanics (primarily those noted in "Selden's List of Resources for Celestia") but swiftly found myself overwhelmed with concepts and mathematics I am thoroughly unfamiliar with. I then stopped by this forum and read the first topic on "orbits" I found, which led me to Ulrich Dickmann? and Bob Hegwood?'s SSC-Scripting Guide for Celestia. After the first few pages I believed myself to have chanced across the ultimate "Celestia for Dummies" guide...then I reached the portion appertaining to orbits. That's where I am stuck and, consequently, where I am requesting some help.
To elaborate, the following is taken directly from said guide:
This is the lead in to the "EllipticalOrbit" section, then we get down to the point where each factor is separately explained and the means of defining it made clear. Here's the section on Period:
What?
How did we go from "the time it takes for an object to orbit its host" to the code of "Period 0.6152". What does that mean? 0.6152?
Here's what I know: my system has two planets, the first of which is tidally locked with one side facing the sun, a day of 2167.79 hours and a year of 90.32 days. The second planet has 17.74 hours in the day and a year of 227.84 days. Still, how would I go about turning that into something like 0.6152? Is that even the correct data from which to derive it?
Any aid in this matter would be greatly appreciated (and with the other aspects of defining an orbit in Celestia), however, do keep in mind that I am a complete amateur in regards to both astronomy and this program. Thank you...
Postscript:
One other thing. Could the administrator of this forum allow searches for the word "orbit" alone to go through? I attempted to search for posts concerning such at first (hoping to avoid drafting this topic whose contents, most likely, have been harped upon before), however, when searching regularly and with an advanced search (browsing topic titles alone) it was dismissed for being "too common" or something.
To elaborate, the following is taken directly from said guide:
EllipticalOrbit
{
Period 0.6152
SemiMajorAxis 0.7233
Eccentricity 0.0068
Inclination 3.3947
AscendingNode 76.681
LongOfPericenter 131.533
MeanLongitude 181.979 (also: MeanAnomaly)
Epoch 2451545.0
}
In this example, the author has defined the elliptical orbit of Venus...
This is the lead in to the "EllipticalOrbit" section, then we get down to the point where each factor is separately explained and the means of defining it made clear. Here's the section on Period:
Period = The time it takes for an object to orbit its host.
Planetary periods are defined in terms of years, while satellite orbits are
defined in days. The Period element MUST be defined. Period is simply the
reciprocal of Mean Motion and so a satellite with 2 revolutions per day, for
example, has a period of 12 hours.
Kepler's third law of orbital motion gives us a precise relationship between the speed of the
satellite and its distance from the earth (or the speed of a planet and its distance from the
sun). Satellites (or any other objects) that are close to the earth (or any other objects) orbit
very quickly. Satellites far away orbit slowly. This means that we could accomplish the same
thing by specifying either the speed at which the satellite is moving, or its distance from the
earth!
Satellites in circular orbits travel at a constant speed. Simple. We just specify that speed, and
we're done. Satellites in non-circular (i.e., eccentricity > 0) orbits move faster when they are
closer to the earth, and slower when they are farther away.
The common practice is to average the speed. You could call this number "average speed",
but astronomers call it the "Mean Motion". Mean Motion is usually given in units of
revolutions per day.
In this context, a revolution or period is defined as the time from one perigee to the next.
Typically, artificial satellites have Mean Motions in the range of 1 rev/day to about 16 rev/day.
What?
How did we go from "the time it takes for an object to orbit its host" to the code of "Period 0.6152". What does that mean? 0.6152?
Here's what I know: my system has two planets, the first of which is tidally locked with one side facing the sun, a day of 2167.79 hours and a year of 90.32 days. The second planet has 17.74 hours in the day and a year of 227.84 days. Still, how would I go about turning that into something like 0.6152? Is that even the correct data from which to derive it?
Any aid in this matter would be greatly appreciated (and with the other aspects of defining an orbit in Celestia), however, do keep in mind that I am a complete amateur in regards to both astronomy and this program. Thank you...
Postscript:
One other thing. Could the administrator of this forum allow searches for the word "orbit" alone to go through? I attempted to search for posts concerning such at first (hoping to avoid drafting this topic whose contents, most likely, have been harped upon before), however, when searching regularly and with an advanced search (browsing topic titles alone) it was dismissed for being "too common" or something.
sympa