South Celestial Pole - Is this right?

[DIYman (Doug)]

Hi everyone

I am setting up my mount and my software charting tool shows the SCP as being at 12 01 47 -89 55 32. (I presume that is today's actual position of the SCP based on the J2000 ephemeris -is this correct?)

I would really appreciate someone running their planetarium program and cross checking the coordinates of the SCP. It would be great to be confident in knowing exactly where the SCP is within a minute of arc or even closer. Cheers Doug.

[mithrandir (Andrew)]

Originally Posted by DIYman

Hi everyone

I am setting up my mount and my software charting tool shows the SCP as being at 12 01 47 -89 55 32. (I presume that is today's actual position of the SCP based on the J2000 ephemeris -is this correct?)

I would really appreciate someone running their planetarium program and cross checking the coordinates of the SCP. It would be great to be confident in knowing exactly where the SCP is within a minute of arc or even closer. Cheers Doug.

The SCP is technically located at any RA and declination -90 but that isn't where a planetarium program will have the RA lines all converging.

CdC tells me
Mean J2000 location is RA: 12h00m20.000s DE:-89d55m39.00s
Mean today location is RA: 23h50m31.592s DE:-89d59m41.58s

[DIYman (Doug)]

Andrew, thanks for the quick response - much appreciated.

I notice that with the RA, our results (for today) are on opposite sides of the virtual SCP (where the coordinates merge). Can this be right?

I also notice that there are 4 minutes difference in Dec (for today) which is surprising.

And I thought it was a simple matter of just printing out a chart with a half degree field and marking the actual position of the SCP.

[mithrandir (Andrew)]

Doug, it's all to do with precession.

The true SCP is always at dec -90, but astronomy uses coordinate systems fixed at various dates with B1950 and J200 being the most commonly seen. The coordinates would have to be precessed backwards to determine at what date they had dec -90 and hence were pointing at the true SCP.
As for jumping from RA 12 to RA 23, when you are that close to the pole tiny changes in position can cause large changes in RA while making little difference in dec.

[DIYman (Doug)]

Andrew
Thanks for bearing with me on this question. I understand that the J2000 coordinates for the SCP was only valid for the year 2000 but am after the actual position of the SCP today. I wish to plot the SCP's position in relation to the background starfield. (I find it interesting the differences in coordinates given to the SCP by our programs - mine is Skytools Pro 3.0.)