Astronomical Coordinate Calculator version 0.31
User Input: Enter One Coordinate Set
J2000 RA Dec
B1950 RA Dec
User Year: RA Dec
Current Year: RA Dec
Galactic: Longitude Latitude
Old (pre 1958) Galactic: Longitude Latitude
Calculated Output
User Year:
Current Year:
Modern Galactic l b:
Old Galactic l' b':


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 conversion-tools, 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 Sharpless-2 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 result-values are all round-tripped via J2000, so if you enter B1950 or Galactic Coordinates, the "calculated output" has been round-tripped and you can compare the output value to your input value and check precision.

To aid copy-pasting 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 direction-cosine matrics were calculated from the rotation values. You should not expect all the transforms to be good to an arc-sec. The direction-cosine matrix I got from "Spherical Astronomy" has only six-digit values. And the Old Galactic Coordinate system I am using has an imprecise (guessed) longitude-shift relative to modern Galactic.


The transformations between J2000.0 and B1950.0 use direction-cosine matrices I found in the literature. The transforms involving the user's special year use the algorithms in the Explanatory Supplement to create a direction-cosine 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 user-supplied dates (note that one row is called "Current Year" but the user can over-ride that, so there can be two user-supplied 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 arc-sec per year, date-errors of a day or two are inconsequential.


The "early galactic" output is presented first with a matrix-transformed 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 credit-line on the page(s) that contain it.

If you like the calculator but would like to suggest improvements, feel free to write me at