astrolibR (version 0.1)

posang: Compute position angle of source 2 relative to source 1

Description

Computes position angle of source 2 relative to source 1

Usage

posang(u,ra1,dc1,ra2,dc2)

Arguments

u

binary indicator describing units of inputs and output: 0 = radians; 1= RAx in decimal hours, DCx in decimal degrees, ANGLE in degrees

ra1

Right Ascension of point 1

dc1

declination of point 1

ra2

Right Ascension of point 2

dc2

declination of point 2

Value

angle

angle of the great circle containing (ra2, dc2) from the meridian containing (ra1, dc1), in the sense north through east rotating about (ra1, dc1). See u above for units.

Details

Computes the rigorous position angle of source 2 (with given RA, Dec) using source 1 (with given RA, Dec) as the center based on the "four-parts formula" from spherical trigonometry (Smart 1977, p.12)

If (ra1,dc1) and (ra2,dc2) are vectors, then angle is a vector giving the position angle between each element of (ra2,dc2) and (ra1,dc1). Similarly, if (ra1,dc1) are vectors and (ra2,dc2) are scalars, then angle is a vector giving the position angle of each element of (ra1,dc1) and (ra2,dc2). If both (ra1,dc1) and (ra2,dc2) are vectors, then angle is a vector giving the position angle between each element of (ra1,dc1) and the corresponding element of (ra2,dc2). If then vectors are not the same length, then excess elements of the longer one will be ignored.

Note that posang is not commutative: the position angle between A and B is theta, then the position angle between B and A is 180+theta

References

Smart, W. M., 1977, "Textbook on Spherical Astronomy", Cambridge Univ. Press (originally published 1931) http://adsabs.harvard.edu/abs/1977tsa..book.....S

Examples

#  For the star 56 Per, the Hipparcos catalog gives a position of 
#  RA = 66.15593384, Dec = 33.94988843 for component A, and 
#  RA = 66.15646079, Dec =  33.96100069 for component B.   
#  What is the position angle of B relative to A?
#  Result:  21.4 degrees

ra1 = 66.15593384/15. ;  dc1 = 33.95988843
ra2 = 66.15646079/15. ;  dc2 = 33.96100069
posang(1,ra1,dc1,ra2,dc2)