vik dhillon: phy105 - the celestial sphere - the galactic coordinate system
the galactic coordinate system
Our Galaxy is lens-shaped with the Sun lying approximately half-way between
the centre and the rim of the lens and near the centre of the plane
of the lens. From this vantage point, our Galaxy appears as a
band of light on the celestial sphere made
up of a myriad of stars and clouds of gas and dust. This band
of light is known as the Milky Way, part of which is
shown in Figure 18.
figure 18:
The Milky Way and the 4.2 m William Herschel Telescope on La Palma.
When astronomers study the structure of our Galaxy, they often find it
more convenient to define the positions of stars in
the galactic coordinate system rather
than the equatorial
coordinate system. The
reference plane of the galactic coordinate system
is the disc of our Galaxy (i.e. the Milky Way) and the intersection
of this plane with the celestial sphere is known as the galactic
equator, which is inclined by about 63° to the celestial equator.
Galactic latitude, b, is analogous to
declination, but
measures distance north or south of the galactic equator, attaining
+90° at the north galactic pole (NGP) and -90° at the
south galactic pole (SGP). The galactic latitude of the star
X in Figure 19 is
arc YX and is north.
Galactic longitude, l, is analogous to
right ascension
and is measured along the galactic equator in the same direction as
right ascension.
The zero-point of galactic longitude
is in the direction of the Galactic Centre (GC), in the
constellation of
Sagittarius; it is defined precisely by taking the galactic longitude of the
north celestial pole to be exactly 123°.
The galactic longitude of the star X in
Figure 19 is given by the angle
between GC and Y.
