Ciao Angelo, You asked:
>To all sundialer >if a wall is not lighted at 12 am, how can I know its >declination whitout >magnetic compass ? (12am is obviously the local hour). >Thankyou to all >Angelo Merletti Whenever the sun does conveniently shine there, mark the position of the shadow cast by a plumb line at your wall, and record the Standard time that you make that mark. For the day of your observation, calculate your local noon hour in Standard time. Take the difference in time between local noon and the time of the west wall mark. Divide this difference (in minutes and decimal fractions) by 4 min./degree. This will give the hour angle from noon in degrees, H. Find the sun's azimuth angle, Z, which is the horizontal angle of the sun at the time you marked the shadow, by using the following equations: (1) tan M = tan D/cos H (2) tan Z = (cos M)x (tan H)/sin(L-M) (3) tan A = cos A/tan(L-M) M is an intermediate value just used for calculation. H is the sun's hour angle when you marked the shadow. L is your latitude, and D (use + or - sign) is the sun's declination for that day. Z is measured from true south, eastward for time before local noon, westward for times after local noon. (The third equation solves for A, the altitude, or sun's angle above the horizon, in case you need it.) When using a calculator with trigonometric functions to do such calculations, I find it best to convert all times to minutes and decimal fractions of minutes, and all angles to degrees and decimal fractions of degrees at the start. This is also a useful form for designing and making a dial. Good luck with your dialing, Bill Maddux.
