A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Personal equation in timed observations
From: Brad Morris
Date: 2018 Oct 23, 11:37 -0400
From: Brad Morris
Date: 2018 Oct 23, 11:37 -0400
Fascinating! Despite the best efforts of all involved, a systemic per observer effect remained. Particularly interesting was the affect of declination on the observation by the so called anticipators.
Such a systemic error will spill over into other facets of CN. Is the star really grazing just the limb, or does the observer expect it to be a pinch more occluded? Is that planet really grazing the horizon, or does the observer expect more light to be observed? Did the declination of the star affect the method of sextant rocking, forcing an induced error.
Does the presence of systemic errors preclude the use of the symmedian point (which demands no systemic errors)? Does it affect the probabilistic evaluation of the cocked hat?
The counterpoint argument will be: I know my personal error, and can thus account for it. In light of the affect of declination, can the counterpoint argument be sustained?
On Tue, Oct 23, 2018, 11:09 AM Paul Hirose <NoReply_Hirose@fer3.com> wrote:
In 1945 Popular Astronomy magazine ran a three-part series by Raynor L. Duncombe on the history of the "personal equation" in observations with transit instruments. He begins with the case of the Greenwich Observatory assistant who persistently observed meridian transits .5 to .8 second late relative to Maskelyne, who fired the man in 1796 for his "vicious" habit. However, by the early 19th century there was growing awareness of systematic individual differences in "eye and ear" transit timings by experienced observers. (The observer listens to the ticks of the clock and keeps mental count of the seconds while watching the star cross the vertical wire of the telescope.) For instance, a series of tests showed a 0.8 to 1.0 second offset in the transit timings of eminent astronomers Bessel and Struve. Development of the electrical art gave an alternative to eye and ear: the recording chronograph, in which a pen traced a helical path on paper, wrapped around a cylinder and rotated at constant speed. An electromagnet wired to contacts in a clock deflected the pen once per second. It also recorded the observer's tap on an electrical key at the moment of transit. This was an improvement. Early tests of personal equation were purely relative: one man with respect to another. But now absolute measurements were possible, via an apparatus which drove an artificial star across the field of view and recorded the true crossing of the central wire vs. the observer's key tap. Thus each observer's work could be made free of his personal equation. At least, that was the theory. But the growing body of chronograph data began to show a systematic error as a function of declination in the work of some observers, but not others. The fault lay in the lack of standardization in the tapping technique. Some observers were proactive — they thought it proper to anticipate the instant of transit and begin the muscular impulse early so the tap occurred exactly on the wire. Others were reactive. They waited until the star was on the wire, then tapped. Of course this tap was always late, but reaction time would be included in that observer's personal equation. It turned out the latter gave more consistent results. Proactive tappers had a tendency to develop a timing optimized for one particular declination. At any other declination the star would move at a different rate, and despite the observer's best effort the tap would be early or late. The next weapon against personal equation was the impersonal Repsold micrometer. In this the wire was movable, driven by a screw operated by the observer, who tracked the star continuously. A disc with electrical contacts was attached to the screw. This transmitted pulses to the chronograph. In some cases a motor rotated the screw, and the observer controlled its rate. With this device the transit instrument attained unprecedented accuracy. In fact, even recent JPL ephemerides depend in part on transit observations of the outer planets as far back as 1911. (This coincides with the introduction of the impersonal micrometer at the USNO.) Nevertheless, eye and ear is still the way we check a clock with a shortwave radio. When I find a time page on the Web dead accurate (or not) with respect to ticks from my radio, I wonder how much personal equation is present. Some of the values measured in the heyday of eye and ear are almost unbelievably large. http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1945PA.....53....2D&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1945PA.....53...63D&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1945PA.....53..110D&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES