This is an easy-to-use coordinate transformation, precession, and position angle calculator provided for your convenience. It is flexible enough to convert accurately between Besselian and Julian equinoxes, taking the epoch of observation into account when needed. It assumes that Besselian dates refer to the FK4 system, that Julian dates refer to the FK5 system, and makes the appropriate transformations. Though the International Celestial Reference Frame (ICRF) is not explicitly supported, the FK5 optical system is consistent with ICRF to within the known errors of the FK5 system (see e.g. Ma et al. AJ 116, 516, 1998).
The coordinate calculator also returns the foreground Galactic extinction at your input position using the Schlafly & Finkbeiner 2011 (ApJ 737, 103, 2011) recalibration of the Schlegel, Finkbeiner & Davis 1998 (ApJ 500, 525, 1998; SFD98) extinction map. The original SFD98 extinction values are also returned for comparison purposes. See this FAQ page for more information and cautions on using the extinction values returned by the calculator.
Select the input and output coordinate systems that you want to use. They may be any of the following:
The input and output equinoxes may be any date between AD 1500.0 and AD 2500.0. If you do not prepend a "B" (Besselian dates) or "J" (Julian dates) to the equinox, the calculator will use Besselian dates for equinoxes before 1990.0, and Julian dates for equinoxes after 1990.0.
If you know the epoch of the observation (between AD 1500.0 and AD 2500.0), enter that in decimal years. The default epoch, 1950.0, is adequate for all but high-precision conversion between the FK4 and FK5 systems.
Enter the input coordinates in decimal degrees or in sexagesimal units. Input is checked by a software interpreter, and re-formatted for display. The best way to gauge these interpreters is to test them; they will post an error message if the input is not interpretable. Examples of legal coordinate input are given below.
The position angle (default is 0 degrees) is optional. Enter it in decimal degrees if you need to precess it.
Click on "Calculate" to run the calculator. It will replace the input page with a new page showing your input parameters as well as the output. If the calculator does not recognize some aspect of your input, it will display an error message. In this case, use your browser's "Back" button to return to the input page to correct the input.
Different systems may be used for input and output; for example, equatorial B1950.0 coordinates on the FK4 system may be precessed and transformed to ecliptic J2000.0 coordinates on the FK5 system.
Position angles are in degrees, measured from north through east.
The precession and coordinate conversion routines were written by J. Bennett who has also provided extensive documentation on them.
All of the following examples are legal input. In cases of ambiguous input, the output may not be what you intend. If the interpreter returns a warning message instead of doing the calculation you requested, click on your browser's "Back" button to return to the calculator input page and reformat the coordinates in your request. If you use decimal hours or decimal degrees in the RA field, we suggest noting this explicitly as in "14.3256h" or "214.8840d". The examples below show the default behavior of the interpreter.