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circles on the sky [message #65879]

Fri, 27 March 2009 10:53

Christopher Thom
Messages: 66
Registered: October 2006

Member

Hi all,

I'm drawing some points on a sky map. The map is <0.5deg across, so I
thought a flat approximation would be ok...this may not be true. Around my
central point, I draw a circle using a flat geometry relation [x =
x0+r*cos(theta); y = y0 + r*sin(theta)], but i see a point outside the
circle that I expect to be inside.

I expect this point to be inside the circle, because the radius of the
circle (in arcsec) is *greater* than the great-circle angular distance
from the centre of the circle to the point.

So...I'm thinking that my flat-geometry assumption is false. My question:
can anyone point me towards forumlae/code that will calculate this circle
on the sky (i.e. all points which have a fixed great-circle distance from
the centre)? I'm using the astro library gcirc.pro to calculte my
great-circle angular distances...I kind of want the "inverse" of that
routine, I guess.

Or...is there a better way to do it? [Or maybe my bug is elsewhere?]

cheers
chris

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[Message index]

		circles on the sky By: Christopher Thom on Fri, 27 March 2009 10:53
		Re: circles on the sky By: pgrigis on Tue, 31 March 2009 15:45
		Re: circles on the sky By: Christopher Thom on Tue, 31 March 2009 15:27
		Re: circles on the sky By: JDS on Tue, 07 April 2009 07:17
		Re: circles on the sky By: pgrigis on Tue, 31 March 2009 14:20
		Re: circles on the sky By: Christopher Thom on Tue, 31 March 2009 14:07
		Re: circles on the sky By: pgrigis on Tue, 31 March 2009 13:00
		Re: circles on the sky By: Kenneth P. Bowman on Tue, 31 March 2009 12:34

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