Celestaire also has, I believe, an Astro Compass which can locate compass points accurately using the sun. Basically you set the device for the LHA and Dec and rotate it until the sun casts a shadow between two locating bars. The Astro Compass in the catalog is a bit pricey but I picked one up used for very little. Anthony
> -----Original Message----- > From: Slawomir K. Grzechnik [SMTP:[EMAIL PROTECTED] > Sent: Tuesday, February 09, 1999 6:01 AM > To: sundial@rrz.uni-koeln.de > Subject: Re: Magnetic North > > Hello Fernando and other Fellow Dialers > > In spite of all the varieties of errors magnetic compasses were and still > are very useful. Like Fernando I also used to possess more than one as I > could not resist buying a nice thing for about $10. My offsprings worked > hard however to bring that number down to two or three. > > Now in sports stores you may buy really good and robust compasses produced > in Sweden and Finland. They are about $10, not very big but very accurate. > The dial is marked every 2 degrees so with some skill, and when making a > series of bearings, you may reach 1 degree accuracy. Do not hold such > close > to your eye but simply press its back side to your stomach while keeping > the thing horizontally and try to take a bearing by turning yourself in > the > direction of your target. After few exercises you will gain confidence in > this method. These compasses are filled with liquid which greatly > stabilizes them and are guarranted to endure low pressure up to 5000 m > above sea level, which is not always true. At least one of mine got > bubbles > in Yosemite at about 3000 m above sea level only (BTW, quite properly you > reach this elevation driving your car as in America you should). These > compasses are so cheap that it does not pay to send them back to producer > if they break in spite of lifetime warranty. Their magnetic needle is so > thin that it is physically nearly impossible to magnetize them otherwise > than along the needle axis. > > When you buy a compass check first of all if the needle moves freely and > if > it stablilizes very fast. Then put the compass on any table inside the > store, note the direction of the magnetic, rotate the compass few times > and > see if the needle comes back exactly to the same direction every time. > > Generaly good compass should be sensitive, stable and quiet, just like a > good wife. > > A more expensive, excellent hand bearing compass you may order at > > http://www.celestaire.com/catalog/c_page34.html > > Other remarks. Yes, compass shows its own compass North, rather than > magnetic North, let alone the true geographical North. Nearby objects made > of steel bring in the compass error that deflects compass readings from > magnetic readings. The only way to know this error is to measure it. In > case of dialers this is no big deal as most of us know the geographical > position and are cabable of calculating azimuths of celestial bodies at > any > given moment. Charts of magnetic variation are published, they also give > variation changes with time so you may calculate variation for the given > year. Even good touristic maps provide information on magnetic variation > and its changes. After some practice all those things become natural. > > All best > > Slawek > > P.S. The Celestaire Company I gave the URL to, has quite a choice of > marine > and air navigation instruments and publications with prices generally > lower > than at marinas and coastal stores. The most funny thing is that the > company is located in Kansas, right in the middle between the two Oceans. > Their strategic planning was really flawless. About a year ago I bought a > sextant from them and am really happy with it. > > At 10:34 PM 2/8/99 +0200, Fernando Cabral wrote: > >Silas > > > >I did not know until recently, when I found I have more than 30 > compasses: > >I am a compass collector. Just for the fun of it, I started comparing > different > > > >makers, models, styles. At first it came as a suprise to find that > different > >compasses will show completely different readings. 2, 3, 5 or even 10 > degrees > >east or west are quite common. > > > >Also, using the same compass you can get quite different readings as > >a consequence of temperature (I guess), pressure (altitude) and, > especially, > >how you hold it in your hand. That is, if it is completely level or not, > >inclined to one or the other direction, so on and so forth. > > > >Temperature and pressure will affect it especially if it has air bubles > in > >the liquid it is supposed to be suspended. I think the temperature will > >also affect the way the needle pivots around it axle because of the > friction. > > > >Also, I've found that sometimes, when you turn your compass 180 degrees, > >it will not show 180 degrees but some degress more or some degrees less. > > > >The bottom-line is: there are too many factors influencing the reading. > >As if that wasn't enough, a layperson can not use it properly, can > >not even take parallax into acount, can not take into account local > >forces like iron in the floor or in the wall. > > > >Here in Brazil we say that if counsels were good we would not give > >them for free, we would charge for them. :-) but here my free advice > >goes: don't ever ask your client to read a compass. 999 times in 1000 > >they will give you completely wrong readings. In the 1 case left > >reading will be within 2 or 3 degress off. > > > >- fernando > > > > > > > >-- > >Fernando Cabral Padrao iX Sistemas Abertos > >mailto:[EMAIL PROTECTED] http://www.pix.com.br > > mailto:[EMAIL PROTECTED] > >Fone: +55 61 321-2433 Fax: +55 61 225-3082 > >15º 45' 04.9" S 47º 49' 58.6" W > >19º 37' 57.0" S 45º 17' 13.6" W > > > > Slawek Grzechnik > 32 57.4'N 117 08.8'W > http://home.san.rr.com/slawek
