Celestia Forums

Vincent wrote:Mary,

On top of the issue raised by Selden, your script does not display the right Celestia values for R, Ra and Dec.
Though I don't have the time to investigate what is wrong with your script, you can use the following script that
displays R, RA, Dec for the selected object. RA/Dec are displayed respectively in hour/min/sec and in deg/min/arcsec.
You should be able to easily edit the script to make them displayed in decimal degrees.

Code: Select all

-- Title: Display current RA/Dec for selected object

KM_PER_LY = 9460730472580.8
KM_PER_AU = 149597870.7
PI = math.pi
degToRad = PI / 180;
J2000Obliquity = 23.4392911 * degToRad

LOOK  = celestia:newvector(0, 0, -1)
earth = celestia:find("Sol/Earth")

-- Convert coordinates from cartesian to polar:
xyz2rtp = function (x, y, z)
   local r = math.sqrt(x * x + y * y + z * z)
   local phi = math.atan2(y, x)
   local theta = math.atan2(math.sqrt(x * x + y * y), z)

   return r, theta, phi
end

-- Return current distance of object from Earth:
getR = function (obj)
   return earth:getposition():distanceto(obj:getposition())
end

-- Return current RA, Dec for object:
getRADec = function (obj)
   local base_rot = celestia:newrotation(celestia:newvector(1, 0, 0), -J2000Obliquity)
   local rot = earth:getposition():orientationto(obj:getposition(), LOOK) * base_rot
   local look = rot:transform(LOOK):normalize()
   local r, theta, phi = xyz2rtp(look.x, look.z, look.y)
   local phi = math.mod(720 - math.deg(phi), 360)
   local theta = math.deg(theta)
   if theta > 0 then
      theta = 90 - theta
   else
      theta = (-90 - theta)
   end
   return phi, theta
end

km2Unit =
   function(km)
      local sign, value, units
      if km < 0 then sign = -1 else sign = 1 end
      km = math.abs(km)
      if km > 1e12 then
         value = km / KM_PER_LY
         units = "ly"
      elseif km >= 1e8 then
         value = km/KM_PER_AU
         units = "AU"
      else
         value = km
         units = "km"
      end;
      return string.format("%.2f", sign * value).." "..units
   end

deg2dms = function(deg)
   local   a = math.abs(deg)
  local   d = math.floor(a)
   local   r = (a - d) * 60
   local   m = math.floor(r)
   local   s = (r - m) * 60
   if deg < 0 then d = -d end
   return string.format("%0.0fd %02.0f' %2.0f''",d,m,s)
end

deg2hms =   function(deg)
   local   a = math.abs(deg / 15)
  local   d = math.floor(a)
   local   r = (a - d) * 60
   local   m = math.floor(r)
   local   s = (r - m) * 60
   return string.format("%0.0fh %02.0fm %2.0fs", d, m, s)
end

while true do
   obj = celestia:getselection()
   objR = getR(obj)
   objRA, objDec = getRADec(obj)
   if objR >= 0 then
      -- Display geocentric coordinates for object:
      objRStr = km2Unit(objR)
      objRAStr = deg2hms(objRA)
      objDecStr = deg2dms(objDec)
      celestia:print("Geocentric coordinates for "..obj:name()..":\nR: "..objRStr.."\nRA: "..objRAStr.."\nDec: "..objDecStr, 1, -1, -1, 1, 7)
   end
   wait(0)
end

selden wrote:Mary,

What values do you get?

Vincent wrote:Mary,

Actually, my script only gives accurate RA/Dec values for distant objects (stars, DSO's) and
when the observer is in the neighbourhood of Earth. If you want to get RA/Dec for spacecraft,
please replace the getRADec function with the following one that uses an observer centered
coordinate system. Then, the displayed values should match the Orbitron values, and also the values
obtained from the projection onto the Celestial grid.

Code: Select all

-- Return current RA, Dec for object:
getRADec = function (obj)
   obs = celestia:getobserver()
   local base_rot = celestia:newrotation(celestia:newvector(1, 0, 0), -J2000Obliquity)
   local rot = obs:getposition():orientationto(obj:getposition(), LOOK) * base_rot
   local look = rot:transform(LOOK):normalize()
   local r, theta, phi = xyz2rtp(look.x, look.z, look.y)
   local phi = math.mod(720 - math.deg(phi), 360)
   local theta = math.deg(theta)
   if theta > 0 then
      theta = 90 - theta
   else
      theta = (-90 - theta)
   end
   return phi, theta
end