Converting Galactic Coordinates (copy)
Posted: 09.11.2007, 17:44
I originaly posted this in "Add-ons" but I got no reponses there, it occurs to me that I might have more luck casting a wider net and it's a suitable enough question for this thread area too, so here's crossing my fingers!
Hey guys,
In my quest to create accurately positioned fictional stars I find that my source material is based on the Galactic Coordinate System using figures like L2 and B2. Celestia, of course, uses an Equitorial (J2000) system using RA, Dec, and Distance. The need arises to convert my figures!
Selden was helpful enough to point me to the Chandra X-Ray "Precess: Coordinate Conversion and Precession Tool"...
http://cxc.harvard.edu/toolkit/precess.jsp
... which is awesome and does exactly what I'm trying to do! BUT, I find my self needing to convert a lot of figures with no desire to plug each of them manually into this lovely website. So I've begun a hunt for the equations BEHIND the Chandra site so I can use them in a program of my own or even plug them into excel or something.
I'm no math wiz but I found this in a text and I'm wondering if it's what I'm looking for...
It looks great on paper but I have no idea what it means, I'm also concerned about their use of "b" and "l" as opposed to b2 and l2 because it makes me wonder if this indicates a different type of galactic coordinate system that I'm unaware of, but maybe it's just the math-phobe in me jumping at shadows.
Do these equations make sense to anyone out there and do you think they're a step in the right direction?
Hey guys,
In my quest to create accurately positioned fictional stars I find that my source material is based on the Galactic Coordinate System using figures like L2 and B2. Celestia, of course, uses an Equitorial (J2000) system using RA, Dec, and Distance. The need arises to convert my figures!
Selden was helpful enough to point me to the Chandra X-Ray "Precess: Coordinate Conversion and Precession Tool"...
http://cxc.harvard.edu/toolkit/precess.jsp
... which is awesome and does exactly what I'm trying to do! BUT, I find my self needing to convert a lot of figures with no desire to plug each of them manually into this lovely website. So I've begun a hunt for the equations BEHIND the Chandra site so I can use them in a program of my own or even plug them into excel or something.
I'm no math wiz but I found this in a text and I'm wondering if it's what I'm looking for...
Galactic Coordinates
Sometimes, coordinates are given in a galactic reference frame, rather than a geocentric one. Given galactic coordinates (b, l), the equatorial coordinates ?? (declination) and ?± (right ascension) can be computed from the formulas
?° ?± ?? b l
cosbcos(l - 33?°) = cos??cos(?± - 282.25?°)
cosbsin(l - 33?°) = sin??sin62.6?° + cos??sin(?± - 282.25?°)cos62.6?°
sinb = sin??cos62.6?° - cos??sin(?± - 282.25?°)sin62.6?°
The reverse transformations are:
cos??cos(?± - 282.25?°) = cosbcos(l - 33?°)
cos??sin(?± - 282.25?°) = cosbsin(l - 33?°) cos62.6?° - sinbsin62.6?°
sin?? = cosbsin(l - 33?°) sin62.6?° + sinbcos62.6?°
The first system was defined in 1932 using optical observations of the Milky Way Galaxy. The new system was defined in 1958 in terms of 21 cm observations of HI (Sullivan 1984, p. 140).
It looks great on paper but I have no idea what it means, I'm also concerned about their use of "b" and "l" as opposed to b2 and l2 because it makes me wonder if this indicates a different type of galactic coordinate system that I'm unaware of, but maybe it's just the math-phobe in me jumping at shadows.
Do these equations make sense to anyone out there and do you think they're a step in the right direction?