The equatorial system of coordinates (Right Ascension
and declination) is the one most often used.
But the galactic
system is sometimes more useful,
e.g. for seeing how
objects are distributed with respect to the galactic plane.
In
this system, the fundamental great circle is the galactic
equator,
which is the intersection of the galactic plane with
celestial sphere,
with corresponding galactic poles.
We
define the North Galactic Pole as that pole in same hemisphere
as the North Celestial Pole.
The positions of the poles were
determined
by the International Astronomical Union (IAU) in 1959.
To fix the galactic coordinates of object X,
draw
a great circle between the two galactic poles, passing through X.
The galactic latitude (b) of object X is the
angular distance on this circle from galactic equator to X,
from
- 90° at South Galactic Pole to +90° at North Galactic Pole.
The zero-point for longitude is the centre of
galaxy;
again, the position was fixed by the IAU.
The
galactic longitude (l) of object X is the angular distance
around the galactic equator from the centre of the galaxy to the
great circle through X,
measured eastwards 0-360°.
Although later research may come up with better
values
for the positions of the galactic poles and the centre of
the galaxy,
the IAU values will still be used to determine this
coordinate system.
To convert between galactic and equatorial
coordinates,
draw the spherical triangle with points at P (North
Celestial Pole), G (North Galactic Pole) and X,
and apply the
sine and cosine rules.
Exercise:
The North Galactic Pole is at
Right Ascension 12h49m, declination +27°24'.
What is the tilt
of the galactic plane to the celestial equator?
Click here for the answer.
Previous section:
Conversion between horizontal and
equatorial systems
Next section: Ecliptic
coordinates
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