# Questions on Galactic Coordinate Systems

• I
Gold Member

## Summary:

Coordinates of the galactic coordinate system are sometimes given in Right Ascension and Declination or at other times in degrees. Is the center of the milky way galaxy located on the galactic plane? And other related questions.

## Main Question or Discussion Point

Please refer to article in Wikipedia https://en.wikipedia.org/wiki/Galactic_coordinate_system

The following questions are related to the galactic coordinate system:
Is the galactic center located on the galactic plane?
Since our Sun is above the center of the galactic disk, is the galactic plane offset?
For example, why are the coordinates given in RA = 12h 51m, Dec = -27deg 42min and then in 0deg longitude?
Can the above coordinates be expressed in degrees such as 25.5deg and -27.6deg for calculations?

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davenn
Gold Member
2019 Award
For example, why are the coordinates given in RA = 12h 51m, Dec = -27deg 42min and then in 0deg longitude?

That is for an object/location that is on a line directly between the sun and the galactic centre
Anything off that line and the longitude will NOT be 0 deg

stefan r
Gold Member
Is the galactic center located on the galactic plane?
Since our Sun is above the center of the galactic disk, is the galactic plane offset?
The 0 lattitude plane is offset.

For example, why are the coordinates given in RA = 12h 51m, Dec = -27deg 42min and then in 0deg longitude?
The quote is for equatorial coordinates. If you know your location, time, and date you can point towards Coma Berenices.

Can the above coordinates be expressed in degrees such as 25.5deg and -27.6deg for calculations?
Refer to trigonometry.

Converting between hours and degrees can be confusing. Pick any star in constellations like Ursa Minor or Draco. The angular change in position is quite low even though they do a full rotation around Polaris over the course of a day.

Bandersnatch
Coordinates of the galactic coordinate system are sometimes given in Right Ascension and Declination or at other times in degrees.
(...)
For example, why are the coordinates given in RA = 12h 51m, Dec = -27deg 42min and then in 0deg longitude?
The RA/Dec ones are the location of the 0 deg longitude of the galactic coordinate system in the equatorial coordinate system. I.e. it tells you where to find the main reference points for the galactic coordinates using the standard astronomical coordinates. The galactic coordinates use longitude and latitude only.

Gold Member
Please verify or correct: One H (hour) is 30deg and one M (minute) is 0.5deg
The equatorial coordinate system is needed by astronomers to correctly point their telescopes. Date and hour of observation are needed as inputs. The equatorial coordinate system is centered on the earth's center and its equator.
The galactic coordinate system is centered on our Sun. Galactic longitude and latitude give directions to a star, but not its distance from the Sun. The distance needs to be determined by other means, such as parallax for nearby stars.
Nearby stars have relative motions to our Sun. It moves about 220km/s around the galactic center. What are typical relative motions of nearby stars?
On a galactic scale, do stars move in unison around the galactic center?
From a list of nearby stars, I have noticed quite a few Star A and B. Are binary stars that common?

Please verify or correct: One H (hour) is 30deg and one M (minute) is 0.5deg
Incorrect. 360 degrees is a full 24 hour day, so 1 hour is 15 degrees, and 1 time minute is 0.25 degrees or 15 arc minutes.
The equatorial coordinate system is needed by astronomers to correctly point their telescopes. Date and hour of observation are needed as inputs. The equatorial coordinate system is centered on the earth's center and its equator.
Equator/axis set the observations. I am actually unsure whether it is centered on Earth´ s centre or the location of telescope - when seeing Solar System objects, astronomers watching a nearby object from different parts of Earth will see it in different directions.
The galactic coordinate system is centered on our Sun. Galactic longitude and latitude give directions to a star, but not its distance from the Sun. The distance needs to be determined by other means, such as parallax for nearby stars.
That´ s correct. The distance would be the third coordinate, and much harder to measure than direction.
Nearby stars have relative motions to our Sun. It moves about 220km/s around the galactic center. What are typical relative motions of nearby stars?
Sun moves slightly faster than the local standard of rest, because Sun happens to be near periapse.
From a list of nearby stars, I have noticed quite a few Star A and B. Are binary stars that common?
Yes, that´ s typical.

Gold Member
Thank you so much for correcting me on hours to degree conversion. I was thinking of clock face hours, it is equator hours. 360 degrees / 24 hours = 15 degrees 15 degrees / 60 minutes = 0.25 degrees or 15 arc minutes.

stefan r