5.4 Galactic Coordinates
59
is marked by the radio source (and supermassive black hole) Sagittarius A*. The
north galactic pole is designated as having a latitude (b) of +90
◦ , the south galactic pole is designated as having a latitude of −90
◦ with longitude (l) measured in
degrees with a right-handed convention along the galactic plane, with galactic center
at l = 0
◦ and anticentre at l = 180
◦ . Hence in this system, Betelgeuse is located at
199.7872−08.9586.
Galactic coordinates are most commonly used by galactic astronomers, those
whose interests lie within the galactic plane. For extragalactic uses, the system is
without merit, and in general, the equatorial system is used in these cases, although
there also exists a supergalactic coordinate system for extragalactic use.
Because the system uses the galactic centre as its point of origin, the rate of
precession is very small, since the orbital period of an object around the galactic
centre is on the order of 200 million years. For high proper motion objects, the
proper motion being the movement of stars against the more distant background, the
rate of change is as great as it is for equatorial coordinates. However, these objects
are normally extremely close, and so are unlikely to lie on the galactic plane, where
galactic coordinates are used.
5.5 Other Minor Systems
An earlier system, now mostly used by those interested in objects within the solar
system, is the ecliptic coordinate system. The fundamental plane for ecliptic coordinates is that defined by the ecliptic, the arc traversed by the Sun and, to some extent,
the planets. Objects are located by their ecliptic latitude β and ecliptic longitude δ,
with the origin at the first point of Aries.
The supergalactic system is a coordinate system for extragalactic objects. It uses
a flat, two-dimensional structure formed by the distribution of nearby galaxies known
as the supergalactic plane as its fundamental plane and its intersection point with
the galactic plane as the origin. The system can be extended three-dimensionally
with galaxies on the plane having an SGZ value of zero. In deference to galactic
coordinates, supergalactic latitude is denoted by SGb, and supergalactic longitude
by SGb.
5.6 Spherical Geometry
As you have seen, astronomical coordinate systems are based on spheres. However,
the surface of a sphere is nonplanar, and therefore, the measurement and calculation
of distances and angles between astronomical coordinates requires the application
of spherical rather than planar geometry.
Perhaps one of the more day-to-day applications of spherical geometry is in the
routes flown by aircraft. You may have noticed on intercontinental flights that the
59
is marked by the radio source (and supermassive black hole) Sagittarius A*. The
north galactic pole is designated as having a latitude (b) of +90
◦ , the south galactic pole is designated as having a latitude of −90
◦ with longitude (l) measured in
degrees with a right-handed convention along the galactic plane, with galactic center
at l = 0
◦ and anticentre at l = 180
◦ . Hence in this system, Betelgeuse is located at
199.7872−08.9586.
Galactic coordinates are most commonly used by galactic astronomers, those
whose interests lie within the galactic plane. For extragalactic uses, the system is
without merit, and in general, the equatorial system is used in these cases, although
there also exists a supergalactic coordinate system for extragalactic use.
Because the system uses the galactic centre as its point of origin, the rate of
precession is very small, since the orbital period of an object around the galactic
centre is on the order of 200 million years. For high proper motion objects, the
proper motion being the movement of stars against the more distant background, the
rate of change is as great as it is for equatorial coordinates. However, these objects
are normally extremely close, and so are unlikely to lie on the galactic plane, where
galactic coordinates are used.
5.5 Other Minor Systems
An earlier system, now mostly used by those interested in objects within the solar
system, is the ecliptic coordinate system. The fundamental plane for ecliptic coordinates is that defined by the ecliptic, the arc traversed by the Sun and, to some extent,
the planets. Objects are located by their ecliptic latitude β and ecliptic longitude δ,
with the origin at the first point of Aries.
The supergalactic system is a coordinate system for extragalactic objects. It uses
a flat, two-dimensional structure formed by the distribution of nearby galaxies known
as the supergalactic plane as its fundamental plane and its intersection point with
the galactic plane as the origin. The system can be extended three-dimensionally
with galaxies on the plane having an SGZ value of zero. In deference to galactic
coordinates, supergalactic latitude is denoted by SGb, and supergalactic longitude
by SGb.
5.6 Spherical Geometry
As you have seen, astronomical coordinate systems are based on spheres. However,
the surface of a sphere is nonplanar, and therefore, the measurement and calculation
of distances and angles between astronomical coordinates requires the application
of spherical rather than planar geometry.
Perhaps one of the more day-to-day applications of spherical geometry is in the
routes flown by aircraft. You may have noticed on intercontinental flights that the