Questions on Galactic Coordinate Systems

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  • #1
KurtLudwig
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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

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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?
 

Answers and Replies

  • #2
davenn
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For example, why are the coordinates given in RA = 12h 51m, Dec = -27deg 42min and then in 0deg longitude?
reread you wiki link first two or three sections

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
 
  • #3
stefan r
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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.
 
  • #4
Bandersnatch
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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.
 
  • #5
KurtLudwig
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Thank you for your explanations.
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?
 
  • #6
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Thank you for your explanations.
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.
 
  • #7
KurtLudwig
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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.
 
  • #8
stefan r
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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.
Be careful though. If a star is at 80 degrees north then 12 hours later its position in the sky only changed by 20 degrees. Earth made a 180 degree rotation in those 12 hours. Polaris stays in about the same place. Orion's belt moves at 15 degrees per hour. In the summer time Altair is close enough to the equator to use it for telling time.
 
  • #9
KurtLudwig
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I have used the equations Equatorial to Galactic Coordinates, as published on Wikipedia. (All 3 equations need to be used to find galactic longitude and latitude since sine and cosine can be positive or negative.) I then used an astronomy calculator on a website clearskytonight.com. The results were exactly the same.
Thank you for your caution.
 

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