2.North South Alignment from the Pole Star

North South Alignment from the Pole Star

North South Alignment from the Pole Star

How accurate do you want it?

Get away from the bright lights and dust of modern civilisation and aclear night sky returns to a magnificant spectacle. Watch for night afternight, and the rotation of the stars round the celestial pole becomes a focusof constancy in a changing and uncertain world.

If you live in a time where a star occupies that central focus, then asingle alignment gives true north. Otherwise you need to select the starnearest to the celestial pole and take two alignments, one east of the poleand one west of the pole as it rotates through the night. True north is thenmidway between these two alignments.

Set up tall trilegs (three poles joined at the top) on top of your highcentral point. Hang a plumb line from the top of the trilegs. This marksthe exact centre of the pyramid - where the diagonals would cross - eventhough this point is fairly inaccessible.

Now set an occlusion board vertically against the plumb line. The occlusionboard is a vertical board with a thin and straight metal (copper) edge.

The second item that helps to provide extreme accuracy is a pin hole viewer.This is a tiny pinhole in a metal foil sheet (copper or gold). (- I usedaluminium foil to test this out)

Now select a clear night when the heat of the day is no longer creatingturbulence in the atmosphere, and estimate where the southern edge of thepyramid is likely to be. Now look at the star at the celestial pole and youmove until the star is just beside the occlusion board on the central pinacle.Move your head to the left and the star dissapears, move to the right itreappears. Even if you just use one eye, there is a slight blur to the process.Now look through the pinhole and the occlusion board and the star image areboth sharp. Move slightly left and right. The star blinks out and then backwith a sharpness that can be suprising. It requires less than 1mm of movementto occlude the star, and this is the accuracy of this method!

Note - many people are not used to using their night vision and cannotconsciously focus their attention on the spot on the retina that is mostsensitive to dim light. You need to wait half an hour without any lighting,even street lights before your eye is fully adapted, and you need to practiceseeing at night before you can usefully use this method.

Set up a second trilegs to hold the pinhole foil and when the pinholeis exactly on the NS line, use a plumb line to mark this point on the ground.

Now set up a third trilegs on the level ground as close to the centralrock as you can, and now move this third trilegs until its plumb line isaligned with the central occlusion board as you look through the fixed pinholeon the established NS line.

You now have two points on accessible ground that are exactly on the NSaxis of the centre of the pyramid.

Join these points with a straight line.

Now use inelastic cord to draw circles to bisect this line, and then againdraw circles to produce a line that is exactly east west.

For extreme accuracy, use a (3m) measuring pole with a pin inset at rightanglesat each end to confirm the intersection points of the circles.

Now measure 1/2 the planned edge length of the pyramid out from the centralNS line to east and west. Again, using the measuring pole this should beaccurate to a mm or so.

We now have a point on the east side and a point on the west side forthe edge of the pyramid. We could again draw right angles, but it will bemore accurate to return to the celestial pole, as this removes any cumulativeerror.

Set up trilegs with the plumb line exactly on the east point we have measured.Set an occlusion board on this trilegs.

Now estimate where the south eastern corner will be and set up the pinholeviewer here.

This gives us a point that is exactly south, and we can line up trilegsand plumb line for the approximate northeast corner to give three pointsthat mark true NS for the eastern edge.

Repeat this process to mark the western edge.

We have now accurately marked the eastern and western edges of thepyramid to within a mm or so, but we have not yet marked the corners.

Marking the corners of the pyramid

Estimate where you think the corner should be and mark this point on theedge NS line.

Draw two intersecting circles to draw a line at rightangles to this point,and then two more circles to bisect this angle.

Now set your pinhole viewer on the estimated corner and a trilegs andplumb line on the diagonal and sight on the central occlusion edge. You aregoing to be off to one side or the other.

Now move both the pinhole and the trilegs on the diagonal the same distancefrom the estimated diagonal until they do line up with the central occlusionedge.

Now mark what appears to be the corner, and repeat the draw two intersectingcircles to draw a line at rightangles to this point, and then two more circlesto bisect this angle. Check that the plumb line of the sighting trilegs isexactly on this diagonal. If not, repeat the adjustment process from thenew diagonal.

Repeat for the other three corners.


You have located the corners of the pyramid to within a few mm without theuse of diagonals through the centre.

Your NS axis is the most accurate - within 1-2 mm.

Your EW axis is as accurate as you can produce a rightangle with a 3mmeasuring pole, and that is very accurate - I got an 'O' level in technicaldrawing!

This information is copyright Peter Thomson 2001-2004