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Earth vs. the Center of the Galaxy

Seeing Through Different Eyes | What is an image? | I'm not lost . . . I've got a map!
So what if the Earth is round?! | A new wrinkle . . . the third dimension. | Earth vs. the Center of the Galaxy


When was the last time you stood in the center of the galaxy? I'm going to guess NEVER!!!! We are on Earth so don't you think a coordinate system with the Earth as the origin would be easier for astronomers to use? This "Earth centered" mapping  is called the Equatorial Coordinate System. Instead of the origin being the center of the Milky Way, the origin uses directions similar to latitude and longitude.  Depending on what you are investigating, you would use one system or the other. For example, if you were pointing out the North Star to your friend, it would be much easier to do it using the Equatorial system, because you could tell him to look north! Then, knowing your latitude on the Earth, you could tell him how far up from the horizon to look. But if you are investigating the shape of the galaxy, it would be much more useful to use galactic coordinates, because they use the center of the galaxy itself (and its flattened plane) for the reference points. So each object might have MANY different sets of coordinates, based on different origins and directions. You should be able to go from one set to another, and you can! DS9 leads the way....
  1. Open any image of interest...say the short exposure of the Centaurus Cluster.

  2. On the tool bar go to "WCS". Make sure there are two red dots, or a red checkmark, next to "WCS" and "galactic".

  3. Move the cursor over the object and observe the reading on the WCS display. Note the latitude and longitude.

  4. Now go the the Galactic Map and locate the Centaurus Cluster. What a coincidence, the Centaurus Cluster is in the middle of the Centaurus constellation!
Now you know where the Centaurus Cluster is in relationship to the center of the galaxy, but where is it with respect to earth?
  1. Change the coordinate system to "Equatorial" as shown below.

  2. Place the cursor and find the coordinates of the Centaurus Cluster in the display.

  3. So now you see that the Centaurus cluster would be about 41 degrees south of the Equator (the celestial one, right? If you extended the equator of the Earth into the sky, you have defined the celestial equator!)  and in a position very close to the autumnal equinox. Remember: alpha or Right Ascension is like longitude, and zero degrees longitude is at the vernal equinox, in the constellation of Aries. 12 hours RA is halfway around the celestial sphere, where the Sun would be at September 20th, and is in the constellation of Virgo. So, if you went to Virgo, and looked about 41 degrees south, you would find the Centaurus cluster of galaxies....
Now you can use Chandra to locate celestial objects in the sky relative to earth, and relative to the center of the galaxy.
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