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Solar "North"

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#1 Tonsch



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Posted 23 June 2007 - 04:55 PM

Hi there... Newbie question.

I have been using my PST for almost a month now.  I've mounted it on a German Equatorial Mount and align the mount loosely to north.  Question:  Is there a simple relationship between north declination on the drive, and Solar North?  I am trying to figure out how to describe the position of a prominence on the Sun's limb.  I seem to be disoriented as to which side of the sun is 'up' as I observe morning/noontime/afternoon.

Any easy answer? Or is there a good textbook that I have to buy?

Thanks and Cheers


#2 leinbach


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Posted 24 June 2007 - 09:31 AM

I'll try for an incomplete answer.  Try to align the mount as close to true north as possible. Make sure the tripod is level, then adjust the latitude of the head to your geographic latitude. I think most equatorial mounts will have a latitude scale on them. You can use a compass to roughly align the mount in azimuth to true north, if you set the compass to account for your local magnetic declination (angle between true north and magnetic north).  You can get your declination off the WEB if you do a search for magnetic declination.

Now, you can find the geographic north direction of the image as follows. Center the Sun's image in the eyepiece. Rotate the telescope slightly about the declination axis, moving the telescope in a northerly direction.  Regardless of whether the image is inverted or not, the southern limb of the Sun will disappear from the field of view first, as the telescope is moved to the north.  Likewise, you can find the east or west limb of the Sun, by first centering the Sun in the eyepiece, then moving the telescope about the polar axis only. Moving west will make the east limb disappear first.

Since the Earth's orbital plane is tilted with respect to the Sun's rotation axis, our geographic north does not correspond to the Sun's north direction. As I recall, the Sun's north direction can be as much as +- 7 degrees from geographic north. Furthermore, the Sun's north pole can be tilted away from, or towards the Earth. All of this information is available as a function of the day of the year, in tables, or in plots of the heliospheric coordinates on a disk (Google Stonyhurst Disks).


Harold Leinbach

#3 Tonsch



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Posted 27 June 2007 - 05:13 AM

Thanks,  that is basically what I was looking for. that the Sun's 'north' is basically the same as our polar north with a +/- 7 degree variance.  It should be a bit easier to be oriented as ss961 is making its appearance.

Thanks again!

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