

A good sundial has the shadowing edge parallel to the earth's axis of rotation. The angle of rotation then is measured as the shadow move in a plane perpendicular to the earth's axis. This can be constructed by sticking a broomstick out from the roof on ones house at an angle with respect to the horizontal equal to the latitude one is at.  Next, use a compass corrected for local deviation to determine the angle in the horizontal plane to point the sick.  The stick then is mounted pointing true north or parallel to the earth's axis.  The shadow this makes at various times per day can be marked. The time each succeeding day for the shadow to get to this marked spot can then be measured.  Thus the earth can then be measured as it slows by each of us. 

In practice for those who are still around large buildings it is easy to use the roofline of a building or vertical poles (such as street light poles) around where one is located to produce a reasonably good sundial effect.  When one uses this type of shadow one can expect it to gain or loose time each day.  Making the earth look like it is slowing or speeding up. This is due to the 23-degree motion of the earth's axis from summer to winter and the fact that the roofline or pole is not parallel to the earth's axis.  The task then is to measure the acceleration or change in the difference between each day.  If we start to measure a rapid increasing slow down from day to day then we know we had better head for our remote survival site.  

If one finds a relatively tall building with at least one or two roof lines running parallel to north south direction and one is located below say 45 degree latitude then the roof line can be used to produce a good shadow to measure. If one is above 45-degree latitude then one should use a vertical pole.  The suns transit time can be measured as the time of minim shadow on the left and right side of the building when is looking north at the south side of the building. The sun's transit time can be found for each day at http://aa.usno.navy.mil/data/docs/RS_OneDay.html

As an example I took a 4-story north-south facing building and scrumptiously marked off 3-tick marks one minuet apart on a sidewalk at 10 min before the sun transit time (maximum height).  The shadow moves fast enough to measure to the nearest plus or minus several seconds.  I measured over the last 4 day an average of 10 sec/day slow down.  When I look up what the sun's transit time is doing over time it shows a 10-12 sec slow down for this time of year.  Transit time was listed the same as last year.  My transit time measurements are currently less than a minuet off from what it should be.  I consider this with in my error of measurements.  
