Enter an astronomical location using one of the coordinate systems and use "Calculate" and that location in all of the coordinate systems is returned. The coordinate systems are the equatorial systems J2000 and B1950 plus the user's choice of year and "current date", and galactic coordinates "new" and "old". I created this small tool because existing conversiontools, in particular tools dealing in galactic coordinates, are narrow in function. And old Galactic Cordinates in particular get little support, though they are used in the Sharpless2 catalog (ApJ 1959 4 p257). (I only found one tool that deals in old Galactic Coordinates, and its computations appear to be incorrect.)
I try to be permissive on the input grammar. You can enter RA as hours minutes seconds, or hours and minutes with a decimal, etc. The same for Dec: "45.51" = "45 30.6" = "45 30 36". You can include " h m s " and " ° ' " " or not, to aid pasting. If you have a RA in degrees, you can enter it in a "RA" field with an initial plus sign: "12h 0m 0s" = "+180". (Galactic coordinates are always degrees and decimal fractions.)
The resultvalues are all roundtripped via J2000, so if you enter B1950 or Galactic Coordinates, the "calculated output" has been roundtripped and you can compare the output value to your input value and check precision. To aid copypasting the results, I supply J2000 and B1950 outputs first in h m s and ° ' ", and then in alternative formats.
The transformations used come from a variety of sources: "Explanatory Supplement", ApJ, "Fundamental Astronomy" by Karttunen et al, "Spherical Astronomy" by Green. Some of the directioncosine matrics were calculated from the rotation values. You should not expect all the transforms to be good to an arcsec. The directioncosine matrix I got from "Spherical Astronomy" has only sixdigit values. And the Old Galactic Coordinate system I am using has an imprecise (guessed) longitudeshift relative to modern Galactic.
The transformations between J2000.0 and B1950.0 use directioncosine matrices I found in the literature. The transforms involving the user's special year use the algorithms in the Explanatory Supplement to create a directioncosine matrix. The two methods are not the same. So the user can enter a "user year" of 1950.0 to check on a round trip through J2000 that uses different transformation equations coming and going.
For usersupplied dates (note that one row is called "Current Year" but the user can override that, so there can be two usersupplied dates), I convert to an ephemeris time without any of the fine details about the precise meaning of "Besselian Year" etc; indeed I assume that months have thirty days. Since precession is only about twentieth of an arcsec per year, dateerrors of a day or two are inconsequential.
The "early galactic" output is presented first with a matrixtransformed value, then again using trigonometric equations in Green that transform new Galactic to Old galactic and will be good at low latitudes but poor near the poles.
I thank Patrick Wallace for pointing out that I should print RA in tenths of seconds since RA 'seconds' are bigger. If you copy the Javascript, I would appreciate a creditline on the page(s) that contain it. If you like the calculator but would like to suggest improvements, feel free to write me at astroayers@gmail.com
