How To Shoot The Sun

Manawatu Times, Volume LIV, Issue 6920, 28 May 1929, Page 10

How To Shoot The Sun

(To l!it; Editor.) Sir,—l noticed to-day “Compass's''' letter and thought he might bo glad to know the scientific method in the matter. Therefore i am giving details. The navigational method is to take a, sight: that is, to ascertain the altitude of the sun above the horizon, at the same moment noting the time by v, a 1011. A bearing of hire suit is theu taken with the prismatic compass. Another .sight is taken, and the altitudes and times proportioned to agree with the time of the bearing. The method now is fake zenith distance (altitude taken from DU degrees), co-declination (sun's distance in declination from the pole instead of the equator —that is either added to or subtracted .from 90 degrees according to whether the sun has north or south declination), and eo-latitude (latitude subtracted from 90 degrees for the same reason. This gives us the three sides of the spherical triangle and the formula is complicated. The three sides arc added together and divided by 2. A'ow, this problem is used daily to obtain iongiliule, and it. is used less frequently to obtain variation solely. But it. lias the two applications, and, certainly, “compass" does not ask how longitude is obtained, so id] explain regarding the variation only. Also “Compass" does not worry why such a thing is done, or formula explored. It would take very nearly one of your ]'ages to explain the thing fully. Take co-decliiialion. subtract if from, the half difference of the addition of tlie three sides. Take the logarithm (log.) eosecrants of laftiludo. and zenith distance, and the log sines of the lu:If sum sir,d the difference. Add these four together and half the result is the log cosine of half the tingle required —in this ease the azimuth. This is, the navigational and scientific method. Jl: ‘'Compass" desires, lie may purchase ;i. sexitant, a prismatic compass, an artificial horizon, the, nautical almanac, and, perhaps. Xoric’s

Epitome, aml :i guud set of tables. If lie perseveres lie can soon learn to find not only variation) but latitude and longitude also.

Mo speaks, however, of a simple method with a pocket, compass, f have never heard or read of one, but I've pondered over the matter and evolved a method which [ can recommend. The compass must be in points at least. The principle, is based on the fact that the sun is due north at noon. Doesn't this suggest a method! Isn't it obvious that if the position of the sun is taken at noon, the difference will be the variation ’ But there are two difficulties. The .first is that when the sun is about an hour amt a-lialf on either side of the zenith, it is too high to have accurate bearing taken, that i< bv the sight method. .My suggestion is to use the sundial principle. A

c!caii-cul, shadow thrown on a fiat surface by a perpendicular piece (if wire, say, will give tin' north and south line. The compass being laid on this line, the variation is easily obtained.. The s(ci>nd difficulty is when the clock .-trikes If at Ida Imrrston .North, the sun is not due north. It is to be remembered that the time is standard time, and if must be corcctcd to local time. The central meridian for New Zealand is 172 deg. fill see. E. The longitude of Palmerston North is 17.3 degrees, fi-3 see. K. The local lime a.t Palmerston North, then?is Id 1-fi mins, in advance of standard time. Apply this correction, and take the shadow for this time. That is my simple method. I haven’t used it myself yet, however. It is capable of being developed scientifically, and I have already done a little to that end. And lastly, blame “Compass’' and not my humble self for the length of this letter.—l am, etc., 11. W. KJ .DIN. Palmerston North. AI. a v _o.