The Case of the Clock Dial

Mark Pottenger

I recently came across a situation that nicely demonstrates the clichés “a little knowledge is a dangerous thing” and “lost in translation”. Most astrologers learning astrology these days don’t have to know a lot of math, since there are many computer programs available that do all the calculations you need starting with nothing but birth data. All atlases, ephemerides and tables of houses (or equivalents) are built into the programs. The danger of an astrologer with minimal math or astronomy training relying on a computer program is that the user might not notice a questionable result if the program is producing wrong answers. Beyond the usual sorts of bugs that creep into any program, there is at least one program in the French-speaking world that will produce wrong answers due to a misunderstanding of principles.

Most astrologers who deal with only living subjects aren’t concerned with Local Apparent Time (LAT), since it hasn’t been used in most of the world for over a century, but it is an issue for historical work. (See “Do You Mean Apparent?” in the Sag. 1986 Mutable Dilemma.) One of the few places LAT is still in active use is celestial navigation. Unfortunately, someone with some knowledge of navigation and astrology, but too little knowledge of the history of time changes, has pushed the mistaken idea that the “equation of time” needs to be used in modern charts. The equation of time is the difference between LAT and LMT (Local Mean Time), and should only be used in calculations where the original time was recorded in LAT (mostly pre-1800) to covert to the mean time used in all modern time zones and ephemerides. At least one computer program in the French world incorporated this mistake. The effect of this is to introduce an error of between 0 and 16 minutes of clock time (depending on date) into the chart.

I became aware of this problem from correspondence. The person promoting the error finally sent a copy of the “authority” justifying his adherence to his position despite several letters from me and others. The key sentence in a French translation of the Cambridge Encyclopedia of Astronomy was “Un cadran solaire ordinaire donne le temps solaire apparent, mais il est possible de graduer un cadran de manière à ce qu’il donne le temps solaire moyen.” The key to the translation loss problem is “cadran”, which can mean either dial or clock face. The encyclopedia passage was actually talking about sundials, but was mistakenly read as talking about clocks.

I have not received new correspondence on this issue for three months, so I believe the originator of the error has finally understood my (and others’) correction. I hope he has told any programmers he had convinced of his erroneous math about the correction, but I don’t know so. So, if you get any French astrology programs, run a chart with a November birth date (when the equation of time is largest) and check the cusps against a known good set of calculations done by hand or by another program. If the cusps are off by several degrees, the program probably includes the LAT error.