Celestia Forums

Vincent wrote:Rinoa79,

First, lines 1, 2 and 4 of your script are useless.
Then, your script gets the observer's speed at one single time t0.
What you want here is a loop which continuously gets the
current observer's speed and then displays it:

Code: Select all

while true do
  obs = celestia:getobserver()
  speed = obs:getspeed()
  celestia:print("Speed = " ..speed)
  wait(0)
end

Note that getspeed return the speed value in microlightyears/s.
You'll have to make the required conversion if you want to display it using another unit.

You can terminate the script by typing the 'ESC' key.

rinoa79 wrote:I have selected my object and then I have started your code but the speed is always zero.

Actually, the getspeed method doesn't really return the speed of the observer,
but rather the speed of a virtual spacecraft in which would be placed the observer.
You can change the speed of this virtual spacecraft using the 'A' and 'Z' keys.

However, if what you want is to calculate the speed of the observer relative to an object,
you shouldn't use the getspeed method, but rather something like:

Code: Select all

while true do
   obs = celestia:getobserver()
   sel = celestia:getselection()
   dist1 = obs:getposition():distanceto(sel:getposition())
   t1 = celestia:gettime()
   wait(0.1)
   dist2 = obs:getposition():distanceto(sel:getposition())
   t2 = celestia:gettime()
   speed = (dist2 - dist1) / ((t2 - t1) * 86400)
   celestia:print(speed)
end

This will give you an average speed calculated between t1 and t2 such as dt = t2 - t1 ~ 0.1s

Vincent wrote:

rinoa79 wrote:I have selected my object and then I have started your code but the speed is always zero.

Actually, the getspeed method doesn't really return the speed of the observer,
but rather the speed of a virtual spacecraft in which would be placed the observer.
You can change the speed of this virtual spacecraft using the 'A' and 'Z' keys.

However, if what you want is to calculate the speed of the observer relative to an object,
you shouldn't use the getspeed method, but rather something like:

Code: Select all

while true do
obs = celestia:getobserver()
sel = celestia:getselection()
dist1 = obs:getposition():distanceto(sel:getposition())
t1 = celestia:gettime()
wait(0.1)
dist2 = obs:getposition():distanceto(sel:getposition())
t2 = celestia:gettime()
speed = (dist2 - dist1) / ((t2 - t1) * 86400)
celestia:print(speed)
end

This will give you an average speed calculated between t1 and t2 such as dt = t2 - t1 ~ 0.1s

Vincent,

Your script returns the radial speed and not the total speed. To have the total speed (which includes also the transversal speed) your code should look like that:

Code: Select all

while true do
obs = celestia:getobserver()
sel = celestia:getselection()
dist1 = obs:getposition()-sel:getposition()
t1 = celestia:gettime()
wait(0.1)
dist2 = obs:getposition()-sel:getposition()
t2 = celestia:gettime()
speed = ((dist2 - dist1):length()) / ((t2 - t1) * 86400)
celestia:print(speed)
end

Paul

rinoa79 wrote:Excuse me,
I didn't understand a piece of your code:

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speed = ((dist2 - dist1):length()) / ((t2 - t1) * 86400)
I have looked for this method but I didn't find it.
What is its utility?
Is it the formula of the average tangential speed or the sum of both tangential and radial speed?

Mary

Mary,

In my code "dist2" and "dist1" are 3D vectors, that describes relative positions of the observer with respect to the selected object ("dist2" at time "t2" and "dist1" at time "t1"). "dist2 - dist1" is then a vector, which desribes the displacement of the observer in time period "t2 - t1". "(dist2 - dist1):length()" is the length of this vector, so it approximates the distance traveled by the observer in time period "t2 - t1". To calculate the total speed of the observer with respect to the selected object it is enough to divide the distance "(dist2 - dist1):length()" by time "t2-t1".

Another thing:

[tex]total speed = \sqrt{(transversal speed)^2+(radial speed)^2}[/tex]

I'm not sure, what do you mean by "tangential speed".

Paul

rinoa79 wrote:If I wanted to calculate only tangential speed, can you tell me what should I do?

Mary.

SU(3)xSU(2)xU(1) wrote:Mary,

You can use the following script:

Code: Select all

while true do
obs = celestia:getobserver()
sel = celestia:getselection()
pos1 = obs:getposition()-sel:getposition()
t1 = celestia:gettime()
wait(0.1)
pos2 = obs:getposition()-sel:getposition()
t2 = celestia:gettime()
dist = (pos2:length()+pos1:length()) / 2
tspeed = dist * math.acos((pos2 * pos1)/pos2:length()/pos1:length()) / ((t2 - t1) * 86400)

celestia:print(tspeed)
end

Paul

rinoa79 wrote:Excuse Paul,
I have controlled again your code. I think that a line is wrong. Can you tell me if I'm made a mistake?
For me the right line is:

Code: Select all
tspeed = dist * math.acos((pos2 * pos1)/pos2:length()*pos1:length()) / ((t2 - t1) * 86400)
Mary

SU(3)xSU(2)xU(1) wrote:
Mary, your code is wrong, because "pos1:length()" is placed in the numerator and not in the denominator. I can give you a correct, alternative version of this line, which is equivalent to my original one:

Code: Select all

tspeed = dist * math.acos((pos2 * pos1)/(pos2:length()*pos1:length())) / ((t2 - t1) * 86400)


Paul

Hi Paul,
I forgot parenthesis. In your previous version there was a "slash" instead "asterisk".

Now all it's clear!
Thank you, Mary.