[time-nuts] Measurements

[Brooke Clarke]

Hi:

It's even better. The procedure we learned in surveying class was to first zero 
the reading then lock the upper movement to the lower movement and aim at stake 
A and lock then lock the lower movement to the tripod.  Release the upper 
movement and turn to stake B and lock the upper movement.  Record this first 
estimate of the angle.  Unlock the lower movement and turn back to stake A (no 
change in the reading) and lock.  Now unlock the upper movement and turn to 
stake B (reading is now 2X) and lock.  Do it again (reading is now 3X).
http://www.prc68.com/I/Leitz115A.shtml
At this point plunge the scope, i.e. flip 180 degrees in vertical plane or in 
other words turn the scope upside down.

Do the above three more times ending up with a reading that's 6X the actual 
angle.  By knowing roughly the angle  you can figure out how many complete 
turns to add to the reading before dividing by six.

Plunging the scope has the effect of canceling some of the mechanical errors 
and results in more accuracy.  When all the angles in a closed polygon have 
been measured you know what the sum should be and if the measured sum is 
different then there's that much error in your readings.  For that class the 
result was 30 arc seconds off which was the spec on the transit used.  The next 
step is to spread the error over all the measurements, called an adjustment, 
resulting in an even more accurate result.

Most people on this list know of the "Longitude" problem and Harrison's 
contribution in making very good clocks.  But the "Latitude" was a much larger 
problem for science.

Ukiah, California was one of five "Latitude Observatories" that operated from 
1899 to 1962 (maybe the longest running science ever?) and the Zenith Telescope 
(ZT) was accurate to something like 0.1 arc seconds.  I'm still trying to learn 
more about how they did it.
http://www.prc68.com/I/UkiahObs.shtml
The Photographic Zenith Tube was developed for this application and later 
larger versions of the PZT were used to determine the time.  It turns out the 
the ZT gives about the same results and the PZT for measuring the latitude so 
the PZT was not adopted for the latitude measurements.

Have Fun,

Brooke Clarke
http://www.prc68.com