  Each of the two pulsars in the double pulsar PSR J0737-3039A/B system
exhibits not only the pulses emanating from itself, but also displays
modulations near the pulse period of the other. Freire et al. (2009, MNRAS,
396, 1764) have put forward a technique using the modulation of B by A to
determine the sense of rotation of pulsar A relative to its orbital motion,
among other quantities. In this paper, we present another technique with the
same purpose. While the Freire et al. approach analyzes pulse arrival times,
ours instead uses periods or frequencies (their inverses), which can be
experimentally determined via power spectral analysis similar to that used in
pulsar searches. Our technique is based on the apparent change in spin period
of a body when it is measured from an orbiting platform (the other pulsar), and
is shown to be entirely analogous to the difference between the sidereal and
solar spin period of the Earth (i.e., the sidereal and solar day). Two benefits
of this approach are its conceptual and computational simplicity. The direct
detection of spin with this technique will observationally validate the
rotating lighthouse model of pulsar emission, while the detection of the
relative directions of spin and orbital angular momenta has important
evolutionary implications. Our technique can be used on other binary systems
exhibiting mutually induced phenomena.
