solar compass question

solar compass

I have almost no practical experience with a solar compass, but have seen one demonstrated and think I understand the principles.

The time dial can be adjusted in without having precise time available. It is quite important that it center the image in its direction, because it and the orientation interact somewhere between parallel and perpendicular. An error in centering the sun's image between the lines with the time dial will thus throw off the orientation.

There are two lenses and targets but only one set is used at a time. The other is used when the sun is of the opposite declination.

I recommend you get a copy of The Key to the Solar Compass, which is readily available from ebay or booksellers. It has more detailed instructions, including how to check the calibration.

solar compass

Ah, I see. Thanks Bill, I do have A KEY TO THE SOLAR COMPASS AND SURVEYOR'S COMPANION by William Austin Burt, a most excellent book, which I am trying to get my head around. As I understand it, the image of the sun can be made to fall between the equatorial scribed lines, but if the pointing sights are not in alignment with the observers Meridian, then the image of the sun will fall above or below the scribed lines.

I have been fascinated by the solar compass since I first learned of one.

David Ingram has taught me most of what I know about one as reading about it just made me more curious of how they actually worked. Before that, I just tinkered with one.

It is necessary to make adjustments throughout the day as time changes because earth spins making the sun to appear to move across the sky.

The computations for the settings can be done before going in the field. With a sheet of these in hand to get you through the day, making a slight adjustment based upon the time of day at each setup will keep you on bearing and with amazing accuracy.

If I remember correctly, when the instrument is set up and sighted property, you can loosen the compass needle and observe the actual magnetic declination of your location.

Sit in and participate at one of his field seminars when he gives you a chance to operate a solar compass or transit.

B-)

I wish you could purchase a modern version that would mount on a tribrach , perhaps with a clock drive for the hour arc, and a laser pointer rather than the sight vanes. This would allow the device to be even more compact. I am tempted to try to make one using stainless steel bevel protractors for the latitude and declination acrs. these very fine protractors have vernier scales which claim to have a 2" capability.

The one mirror (actually glass) is what the sun is picked up on and the time of day is not all that important as you can rotate the solar tube and when the sun follows within the two lines, the instrument is pointed North.

Thanks Keith, I made a rough working model today of a solar attachment for my spare Trimble, which works remarkably well considering. The first picture is the polar axis pivot (hour angle) which I made from an altered size 60 master chain link. It fastened nicely to the peep sight mount using the same screws which held the sight. The base of the wood declination arc swivels on the polar axis pin, and the long threaded screw adjusts the declination angle, (which I laid out with a protractor before slipping on the solar compass attachment). I had a small pointing scope off a telescope and I took of the eye lens leaving only the objective lens, which focuses the suns image at about 8 inches, at the target end I used the peep site as a target, and you can see the image of the sun centered on the target in the picture. Once the latitude of the instruments location is dialed into the vertical angle of the total station and the total station scope is pointed roughly North, then the declination arc is pivoted on its polar axis towards the sun until an image is detected, which will slew kitty corner across the target, either above or below the target. All that remains is to turn the horizontal motion of the total station until the image of the sun is centered on the target, when this is achieved the scope of the gun will be pointing due astronomical North.

Very ingenious. Very educational. It looks really good for a quick prototype. How accurate and repeatable is the azimuth it determines?

That will be the question, how repeatable will it be. The model is just to satisfy myself that I have the concept of the Solar Compass in hand. In order to get repeatable results, I will need to improve on the declination arc, so that I can dial in a consistent
declination angle. I have in the past made reference to Jerry Whal's ephemeris in order to obtain the declination value for any given day, and Burt's reference book has a handy table for extrapolating the declination for each hour of the day. Burt himself
cautions not to try to establish the North-South Meridian with the Solar Compass, within about 35 minutes either way of local solar noon, (giving the surveyor a good excuse to break for lunch ; )

Did you adjust his Ephemeris to Greenwich Apparent Noon? Roughly 12 hours ± difference for declination.

Jerry's website states "All values are for 0h GMT aka UT1." so yes a correction for longitude (time) would be required. Mr Whal's website does not give values as of yet for 2013.

http://www.cadastral.com/2013ephs.htm

Dave, Thanks! I had not noticed any update. Thank you also Mr. Wahl. This is a valuable resource.

Jerry's Ephemeris is based on midnight, the same as the old K&E, Lietz/Sokkia were. This needs to be calculated to reflect Apparent Noon to get the declination settings for use with the solar compass or the solar transit. The old BLM/GLO Ephemeris for use with the solar transit was in this format (Apparent Noon). It would show what I am trying to point out if you could look at one of these old BLM/GLO Ephemerises and compare it with the old K&E, Lietz/Sokkia, and Jerry's Ephemerises. I have to do this if I want to set up declination settings for my Gurley Solar Transit as the BLM/GLO Ephemeris has not been published since 1988.

It's not just a simple matter of changing Jerry's or the other Ephemerises data as published by 12 hours. The hourly change, Equation of time and Laplace correction all come into play.

Charles

It has been a few years since I have calculated the dec sheet for the day for the solar transit, but I used the KE info as it was easier to work with then the BLM info.

Just what I got used to and it worked.

And of course, I did not have the computer ability like now; my olivetti was not all that capable in the late 60's!

Charles

Keith:

It's been quite a few years since I have made up a Dec sheet too and I would have to go back and refresh my memory. I always used the BLM Ephemeris to plot the Dec sheet, as I recall it was easier than the K&E format and didn't require any calculation other than taking the hourly change times 10 to plot the 3:00 PM point to draw the Dec Line from the inital 5:00 AM Dec point and then to plot the refraction offset from the Red Book that was used to set the Dec on the Dec Arc on the solar transit.

Charles

Keith:

The Declination given in the BLM Ephemeris was Apparent Noon Greenwich which was 5:00 AM MST and could be plotted directly on the Dec sheet. The hourly change given in the Ephemneris was multiplied by 10 (hours) which was added to the Dec. given for Apparent Noon Greenwich to obtain the Declination for 3:00 PM to plot on the Dec sheet. A straight line from 5:00 Am to 3:00 PM was drawn and then the refraction correction from the Red Book was laid off from this line at the times specified in the Red Book which gave the Declination settings for the day to set on the Declination Arc on the solar transit. This format was much simpler than using the K&E or similar Ephemeris that needed to be converted over to the 12 ± hour difference.

Charles

I have no doubt that you are correct, but the KE worked for me.

There are no mirrors that I am aware of on a solar compass. The 'solar arm' which attaches to the declination adjustment has two lenses one used in summer and the other in winter on the other end of the arm about 3-4 inches away is the silver plate. That is so you don't have to have the declination arc go north and south from zero and the arm can just be rotated instead.

Now the smith solar attachment for a solar transit does use a mirror, but it is not concave that I am aware of. It simply reflects the sun's image up into the telescope. which is set at the angle of the latitude so that it is an instrument mechanically independent of the transits vertical circle. However the smith attachment is a bit more complicated to adjust because it has to be made parallel to the transit axis in 3 dimensions.

If the instrument is in reasonably good adjustment (settings for latitude and declination correct) The setting of the hour angle is not critical but has to be good enough only to get the sun's circle on the plate. The fine setting is then made by rotating the entire axis of the attachment so that the suns' image is between the lines.

Actually there is a slight refinement of that to correct for refraction which can be employed and is discussed by Burt in that little book. The correction for declination for refraction can be made on your declinatin chart which is what most in BLM did and is discussed in the '73 and earlier manuals using the tables from the red book. But for a solar compass it appears Burt is just using experience and mental computation to approximate it based on the rough hours from noon.

I have some videos if you are interested in some links perhaps.

Your design is similar to the burt attachment which is mounted on a scope with a bearing much like you created so that the transits vertical circle can be used to set in the latitude arc. A perfectly workable solution.

It does seem to me like your little post is going to have to be pretty precisely oriented at right angles to the scope or have some way of tweaking that.

- jlw

PS to last in regard to declination computation. The K&E or my tables can be used to plot the simple interpolation of declination for the local day. What has to be figured out is the total difference between solar noon and noon by your watch. Normal BLM procedure as I understand it was to have a watch set for local sun time.

The two corrections are 1) the equation of time for the day (better for near noon), and 2) the correction to the local meridian from the standard time zone meridian you are using on your watch which depends on where you are east or west of the standard time zone meridian.

If you set you watch to local meridian time you can avoid the second correction and not worry about the first over the day. By using standard time on your watch life is a little more complex. I will usually note the actual time of local meridian noon as well as the total difference between standard watch time and local sun time for the day and put it on my dec chart. That way you also know when local noon is for your noon checks and hour arc approximate settings.

Charts can be made plotting standard time and declination or local apparent time and declination once you come up with a precedure to add everything up correctly.

- jlw