Galactic coordinate system

File:Artist's impression of the Milky Way (updated - annotated).jpg

Artist's depiction of the Milky Way Galaxy showing the origin and orientation of galactic longitude. The galactic longitude (Template:Math) runs from the Sun upwards in the image through the center of the galaxy. The galactic latitude (Template:Math) is perpendicular to the image (i.e. coming out of the image) and also centered on the Sun.

The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north. It uses the right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.<ref name="Blaauwetal" />

Spherical coordinatesEdit

Galactic longitudeEdit

File:Galactic coordinates.JPG

The galactic coordinates use the Sun as the origin. Galactic longitude (Template:Math) is measured with primary direction from the Sun to the center of the galaxy in the galactic plane, while the galactic latitude (Template:Math) measures the angle of the object above the galactic plane.

Longitude (symbol Template:Math) measures the angular distance of an object eastward along the galactic equator from the Galactic Center. Analogous to terrestrial longitude, galactic longitude is usually measured in degrees (°).

Galactic latitudeEdit

Latitude (symbol Template:Math) measures the angle of an object northward of the galactic equator (or midplane) as viewed from Earth. Analogous to terrestrial latitude, galactic latitude is usually measured in degrees (°).

DefinitionEdit

Template:See also

The first galactic coordinate system was used by William Herschel in 1785. A number of different coordinate systems, each differing by a few degrees, were used until 1932, when Lund Observatory assembled a set of conversion tables that defined a standard galactic coordinate system based on a galactic north pole at RA Template:Nowrap, dec +28° (in the B1900.0 epoch convention) and a 0° longitude at the point where the galactic plane and equatorial plane intersected.<ref name="Blaauwetal" />

In 1958, the International Astronomical Union (IAU) defined the galactic coordinate system in reference to radio observations of galactic neutral hydrogen through the hydrogen line, changing the definition of the Galactic longitude by 32° and the latitude by 1.5°.<ref name="Blaauwetal" /> In the equatorial coordinate system, for equinox and equator of 1950.0, the north galactic pole is defined at right ascension Template:Nowrap, declination +27.4°, in the constellation Coma Berenices, with a probable error of ±0.1°.<ref name="Reidetal" /> Longitude 0° is the great semicircle that originates from this point along the line in position angle 123° with respect to the equatorial pole. The galactic longitude increases in the same direction as right ascension. Galactic latitude is positive towards the north galactic pole, with a plane passing through the Sun and parallel to the galactic equator being 0°, whilst the poles are ±90°.<ref name="Binney" /> Based on this definition, the galactic poles and equator can be found from spherical trigonometry and can be precessed to other epochs; see the table.

J2000.0 equatorial coordinates approximating the galactic reference points<ref name="Blaauwetal"/>

Right ascension

Declination

Constellation

North Pole
+90° latitude

12^h 51.4^m

+27.13°

Coma Berenices
(near 31 Com)

South Pole
−90° latitude

0^h 51.4^m

−27.13°

Sculptor
(near NGC 288)

Center
0° longitude

17^h 45.6^m

−28.94°

Sagittarius
(in Sagittarius A)

Anticenter
180° longitude

5^h 45.6^m

+28.94°

Auriga
(near HIP 27180)

File:Galactic north pole.png Galactic north	File:Galactic south pole.png Galactic south	File:Galactic zero longitude.png Galactic center

File:2MASS LSS chart-NEW Nasa-added quadrants.jpg

Approx galactic quadrants (NGQ/SGQ, 1–4) indicated, along with differentiating Galactic Plane (containing galactic centre) and the Galactic Coordinates Plane (containing our sun / solar system)

The IAU recommended that during the transition period from the old, pre-1958 system to the new, the old longitude and latitude should be designated Template:Math and Template:Math while the new should be designated Template:Math and Template:Math.<ref name=Binney/> This convention is occasionally seen.<ref>For example in Template:Cite journal</ref>

Radio source Sagittarius A*, which is the best physical marker of the true Galactic Center, is located at Template:Nowrap, Template:Nowrap (J2000).<ref name="Reidetal"/> Rounded to the same number of digits as the table, Template:Nowrap, −29.01° (J2000), there is an offset of about 0.07° from the defined coordinate center, well within the 1958 error estimate of ±0.1°. Due to the Sun's position, which currently lies Template:Val north of the midplane, and the heliocentric definition adopted by the IAU, the galactic coordinates of Sgr A* are latitude Template:Nowrap south, longitude Template:Nowrap. Since as defined the galactic coordinate system does not rotate with time, Sgr A* is actually decreasing in longitude at the rate of galactic rotation at the sun, Template:Mvar, approximately 5.7 milliarcseconds per year (see Oort constants).

Conversion between equatorial and galactic coordinatesEdit

An object's location expressed in the equatorial coordinate system can be transformed into the galactic coordinate system. In these equations, Template:Math is right ascension, Template:Math is declination. NGP refers to the coordinate values of the north galactic pole and NCP to those of the north celestial pole.<ref>Template:Cite book</ref>

<math>

\begin{align} \sin(b) &= \sin(\delta_\text{NGP})\sin(\delta)+\cos(\delta_\text{NGP})\cos(\delta)\cos(\alpha-\alpha_\text{NGP}) \\ \cos(b) \sin(l_\text{NCP}-l) &= \cos(\delta)\sin(\alpha-\alpha_\text{NGP}) \\ \cos(b) \cos(l_\text{NCP}-l) &= \cos(\delta_\text{NGP}) \sin(\delta) - \sin(\delta_\text{NGP}) \cos(\delta) \cos(\alpha - \alpha_\text{NGP}) \end{align} </math>

The reverse (galactic to equatorial) can also be accomplished with the following conversion formulas.

<math>

\begin{align} \sin (\delta) &= \sin (\delta_\text{NGP}) \sin(b) + \cos (\delta_\text{NGP}) \cos(b) \cos(l_\text{NCP} - l) \\ \cos (\delta) \sin(\alpha - \alpha_\text{NGP}) &= \cos(b) \sin(l_\text{NCP} - l) \\ \cos (\delta) \cos(\alpha - \alpha_\text{NGP}) &= \cos (\delta_\text{NGP}) \sin(b) - \sin(\delta_\text{NGP}) \cos(b) \cos(l_\text{NCP} - l) \end{align} </math>

Where:

<math>\alpha_\text{NGP}=12^h 51.4^m \qquad \delta_\text{NGP}=27.13^{\circ} \qquad l_\text{NCP}=122.93314^{\circ}</math>

Rectangular coordinatesEdit

In some applications use is made of rectangular coordinates based on galactic longitude and latitude and distance. In some work regarding the distant past or future the galactic coordinate system is taken as rotating so that the Template:Math-axis always goes to the centre of the galaxy.<ref>For example Template:Cite journal</ref>

There are two major rectangular variations of galactic coordinates, commonly used for computing space velocities of galactic objects. In these systems the Template:Math-axes are designated Template:Math, but the definitions vary by author. In one system, the Template:Math axis is directed toward the Galactic Center (Template:Math = 0°), and it is a right-handed system (positive towards the east and towards the north galactic pole); in the other, the Template:Math axis is directed toward the galactic anticenter (Template:Math = 180°), and it is a left-handed system (positive towards the east and towards the north galactic pole).<ref name="Johnson" />

File:Milky Way infrared.jpg

The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those that require high densities of stars at low galactic latitudes, and those that require a low density of stars at high galactic latitudes. For this image the Mollweide projection has been applied, typical in maps using galactic coordinates.

In the constellationsEdit

The galactic equator runs through the following constellations:<ref name="SEDS" /> Template:Div col

Template:Div col end

ReferencesEdit

Template:Reflist

External linksEdit

Template:Milky Way Template:Galaxy Template:Celestial coordinate systems Template:Portal bar