18 December 2015

IAU Calls for Consistency in Use of Polarization Angle

Richard Fienberg

Richard Fienberg, AAS Press Officer

This announcement is posted on behalf of the International Astronomical Union:

Scientists working on the polarization of the cosmic microwave background (CMB) use a convention for the polarization angle (PA) that is opposite to the International Astronomical Union (IAU) approved standard. This may cause confusion and misunderstanding.


The convention astronomers follow for the PA (polarization angle) goes back to the 19th century and has been in use for observations going from radio to gamma rays: the PA increases counterclockwise when looking at the source. This convention is consistent with the one used for position angle and has been enforced by the IAU with a resolution by Commissions 25 and 40 at the IAU XV General Assembly in Sydney in 1973 (see Transactions of the IAU, Vol. XVB, pg. 166).

Recently, scientists investigating the polarization of the CMB (discovered in 2002) have unfortunately adopted the opposite convention (PA increasing clockwise when looking at the source). This corresponds to a change of sign of the U Stokes parameter and is causing confusion and misunderstanding, in particular in the case of polarization data coming from experiments and satellites that are used by the CMB community and by other astronomers.


The IAU recommends that all astronomers, including those working on the CMB, follow the IAU resolution for the polarization angle in all their publications.

Piero Benvenuti¹, Pietro Ubertini² & Saul J. Adelman³

¹IAU General Secretary
²President, IAU Division B
³President, IAU Commission B6