In Majewski et al. 2003 (Paper I) we present the first all-sky view of the tidal streams of the Sgr dwarf as seen in the 2MASS database. Since tidal debris is closely confined to the Sgr orbital plane, in Paper I we introduce a longitudinal coordinate system as illustrated in Fig. 1 below. This coordinate system is an intuitive system in which to study the tidal tails of the dwarf.
The Sgr coordinate system is diagrammed above in Fig. 1 for a Galactic halo for which q = 0.9. The origin of the longitudinal coordinate
is located at the position
of the Sun (7.0 kpc from the Galactic Center), and = 0 in the direction of the Sgr dwarf core.
increases clockwise along trailing debris in the Sgr orbital plane as shown. The
Cartesian coordinate system X_Sgr, Y_Sgr, Z_Sgr is
also centered on the location of the Sun, with X_Sgr positive in the direction of Sgr, Y_Sgr positive in
the direction of = 90, and Z_Sgr
points to the pole of the Sagittarius orbital plane located at (l,b) = (273.75,-13.46). Since Sgr lies
close to the Galactic Center and is on a nearly polar
orbit, X_Sgr is roughly -X, Y_Sgr is roughly -Z, and Z_Sgr is roughly +Y, where X,Y,Z is the standard
left-handed Galactic Cartestian coordinate
system. The angular coordinate 
is the "latitudinal" angle, and is positive in the
direction of +Z_Sgr.
A variant of this system is occasionally employed in our papers, which is centered at the Galactic Center.
In this frame, the origin of all coordinates
is at the Galactic Center, and is rotated about the Sgr plane pole axis by about 14 degrees so that X_Sgr,GC and
= 0 lie in the Galactic plane.