NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Astrofixes and Len Beadell
From: Kieran Kelly
Date: 2004 Apr 29, 17:57 +1000
From: Kieran Kelly
Date: 2004 Apr 29, 17:57 +1000
Greg Gilbert wrote: "1. Do the levelling devices on theodolites provide a better "horizon" than an artificial horizon used with sextants?" No definitely not. After using a theodolite several times the frustrating thing is that every time you touch the instrument the bubbles seem to move. Also if there is an error in the bubble it is quite a complicated procedure to allow for. The great advantage of a theodolite is that it gives azimuth as well as altitude and it is free from the "wobbly hands" of an observer. A mercury pan horizon conversely gives a true horizon distorted only by the negligible impact of local gravitation on the mercury. It is impossible therefore for the pool not to provide a "true" horizon. The problem with a mercury artificial horizon however is that it needs a glass cover known as a cloche and therein lies a source of error if the glass in not ground optically flat. That said, like a theodolite, an artificial horizon cloche error can be eliminated by simply taking sights with the cover in one direction then reversing it for another round of sights. Don't underestimate that fact that Dolland et al knew a thing or two about instruments and in all the time I have been using an 1850 Dolland AH I have detected not the slightest error that I could attribute to the cover. "2. When did theodolites become generally accepted for land based exploring, and replace sextants and artificial horizons?" Don't know but sextants were still being used in Australia right up until Madigan's expedition across the Simpson Desert in circa 1934. "3. Why didn't more early explorers use theodolites for astronomical observations and navigation? I understand that Thomas Jefferson used a theodolite in his own surveying practice, but did he equip the Lewis and Clark expeditions with one?" Don't know but may have been due to transportation difficulties. A theodolite is very difficult to pack on the back of a horse or a camel. It has long, very heavy legs that would be difficult to accommodate on a pack animal. Also a sextant is a much more robust instrument than a theodolite. Break the bubbles on a theodolite and it is useless. Not so a sextant. If it gets bumped you simply adjust the index error and carry on. "A question to Kieran Kelly, whose two books on Gregory and the Tanami Desert I really enjoyed: why did you use a sextant and artificial horizon when crossing the Tanami and not a theodolite? Was it a matter of historical accuracy in replicating Gregory's observation methods, or just a matter of weight and convenience when packing camels? Did Gregory use a theodolite on any of his Australian exploring expeditions?" A sextant is a very compact, rugged instrument as is an artificial horizon. During my last expedition, both were stored on the back of a camel for nearly six weeks and the index error on the sextant was the same at the end as it was at the start. Both sextant and artificial horizon fitted neatly into a hard plastic pannier used for transporting food and were surrounded by packing material such as dried peas, packets of beef jerky and freeze dried meals. There must be a way of tying theodolite legs on the back of a pack horse or camel but it would be a needless extravagance. Getting a shovel or shotgun to stay put is bad enough. I should imagine if you break the legs on a theodolite tripod the instrument becomes useless. (Or you carve more legs which sometimes happened in the field.) Furthermore there is the time factor. I would pull the AH out, pour in the mercury, slap the cover on, take the sights, pack it all up and put it away - no fuss - half an hour tops. Mucking around in the middle of the night, with no light, in the middle of a desert trying to get bubbles level and light the fire, cook a meal and tend to camels would have driven me nuts. Theodolites are for people who go exploring with vehicles not pack animals. Gregory used a theodolite on his professional surveying activities in Western Australia but never on his exploring trips and there is a big difference between surveying and exploration. To draw another analogy, Mason and Dixon were surveyors, L & C were explorers. They are two different professions trying to achieve two different ends although they both use the celestial bodies for navigation. Hope this helps and thanks for buying my books. Regards Kieran Kelly