Same great circle?

[degn]

How do you decide if three celestial objects are placed on the same great circle? One object is the galactic center. Need suggestions for both a mathematical and graphical visualization.

[callump]

If they all have the same Right Ascension ?

/callump

[daz]

This looks like a homework question to me....

[Psychobilly]

This looks like a homework question to me....

Certainly an Interesting question for a first post...

Peter...

[Tiki]

Find the great circle between two of the points and then see if the third point also lies on this great circle.

[Steve Ward]

Don't you just love those first posts that jump straight in with a plea to help with a homework question without so much as a "Hello" and basic introduction ..... 

Would love to offer up a solution , but I shan't in this particular case , sorry .

[acey]

If they all have the same Right Ascension ?

/callump

If they all had the same Right Ascension then they would all lie on a single great circle. However the converse is not true.

I agree with Tiki: find great circles through pairs and see if those two coincide. To do this you would want to convert from RA/Dec to polar co-ordinates.

But before going through all that labour - why would you want to do it?

[callump]

A yes, a great circle does not need to go through the poles.

Well it's a long time since I did celestial mechanics - no doubt my copy of Smart would reveal how to solve this (if i could find it).

Sounds like one of the exercises we had in 1st year astronomy, though...

/callump

[Paul M]

I'm not sure what the OP actually needs to know.

Has the "Galactic Center" been given as an arbitrary point or has someone mistaken it as having some relationship with geocentric coordinates ?

Anyway, not to bother. I doubt I could give a mathematical solution even if I was sure what was being asked  

[acey]

I assume by "galactic centre" the OP means the point in Sagittarius with co-ordinates

Right Ascension:        17 45 12
Declination:                -28 43 00.0

[Tiki]

Three points on a great circle is a special  case and the trigonometric identities will become much simplified. The cosine of one angle will equal the product of the sines of the other two angles.

http://en.wikipedia.org/wiki/Spherical_trigonometry

[Tiki]

Ignore the last post. I thought I had it.

[D4N]

The celestial sphere has a coordinate system just like Earths, dec is lat and RA is long. Use spherical trig to work out the initial bearing from object A to object B, repeat for A to C. If they are the same or 180 out then they are on the same great circle.

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