skysep: Compute arc separation of two RA/Dec values

Package: nproto

Synopsis

Given two RA/Dec value pairs the spherical separation is computed. This task can be used in scripts and has both text and parameter output.

Usage

skysep ra1 dec1 ra2 dec2

Parameters

ra1, dec1, ra2, dec2: The RA and Dec of two points on the sky for which a separation is to be computed. The RA may be in hours or degrees and the Dec is in degrees. The values may be in decimal or sexagesimal format.

raunit = "hr": Units for right ascension. The value "hr" selects hours and "deg" selects degrees.

verbose = no: Print a verbose output to the standard output?

sep: This output parameter will contain the separation in arc seconds after the task is run. It may then be referenced as the variable skysep.sep.

Description

This simple script task computes the separation between two celestial coordinates given as RA and Dec. The RA units may be hours or degrees, as selected by a parameter, and the Dec units must be degrees. The result may be printed to the standard output (in restricted precision) and is also record in a task parameter for later use.

Examples

1. The verbose output appears as follows:

cl> skysep 12:12:12 32:32:32 12:12:24 32:32:52 verb+
153.05 arcsec = (12:12:12.00, 32:32:32.0) - (12:12:24.00, 32:32:52.0)
cl> = skysep.sep
153.04686934468

2. To use in a script:

cache skysep   # Cache to avoid problems with updating par files

# To use scan to get the value.
skysep (r1, d1, r2, d2, raunit="deg", verbose+) | scan (sep)
printf ("The separation is %f\n", sep)

# To use the saved value.
skysep (r1, d1, r2, d2, raunit="deg", verbose-)
printf ("The separation is %.5f\n", skysep.sep)