A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Finding your latitude through double altitudes (and elapsed time).
From: Joel Silverberg
Date: 2007 Mar 31, 18:29 -0700
From: Joel Silverberg
Date: 2007 Mar 31, 18:29 -0700
I am having difficulty figuring out the logic behind the calculations for finding the latitude using the method of double altitudes. I am working from an 1833 edition of Bowditch. I do not know how far back this particular method goes ... I am sure it is earlier than 1833. This edition of Bowditch gives three methods. The first method is the one I am asking about. The second method is identified as basically the method of Douwes and the third method takes into account the possibility that the declination or polar distance of the object at the second observation may differ from that of the first observation. I'll summarize what the method does, and how the algorithm proceeds and then I'll ask my question(s). The method is as follows: determine the corrected altitude of the sun at two points in time, noting the elapsed time between the two observations. You do not need to know the actual time of either observation, simply how much time has passed between observation #1 and observation #2. A series of logarithmic calculations lead eventually to your latitude. The method in its simplest form assumes that the ship is stationary during the interval, but it is easily modified to take into account movement of the ship between the observations using a dead reckonning of the change in position between them. The details, as described in Bowditch are as follows: The sum of the log-csc of the elapsed time and the log-sec of the declination is the log-csc of arc or angle A. (The values of numerical examples in Bowditch indicate that he is not really using the the log-csc of the elapsed time as he claims, but the log-csc of half the elapsed time). The sum of the log-csc of the declination and the log-cos of A determines the log-csc of B. The sum of the log-cos of half the sum of the two altitudes and the log- cos of half the difference in the two altitudes is the log-sin of C. The sum of the log-cos of A, the log-csc of the half sum of the alititudes, the log-sec of half the difference in altitudes, and the log-cos of C determines the log-sec of azimuth angle Z. Z is named north if the zenith and the north pole lie on the same side of the great circle between the two observed points and is named south if the zenith and north pole lie on opposite sides of that great circle. B has the same name as the declination. E is the sum of B and Z if B and Z have like names and is the differnce if B and Z have contrary names. Finally the sum of the log-cos of C and the log-sine of E is the log-sin of the latitude. I am struggling to see what this elaborate rule does in terms of the spherical triangle P-X-Y , the spherical triangle Z-X-Y, and the arc P-Z (where X and Y are the positions of the sun at the time of the two observations, P is the north celestial pole, and Z is the zenith. (P-Z is the complement of the Latitude, P-X =P-Y = complement of the declination, Z-X is the complement of the first altitude, Z-Y the complement of the second altitude, and X-Y is the arc of the great circle between the two solar positions. With the availability of computers and calculators, it is no longer essential to view things in their logarithmic form. Direct solution of the trigonometric problem is now easy and it is far easier to see what the method is actually doing from a geometric or trigonometric viewpoint without the logs in the way. I assume that the calculations involving half the sum of the altitudes and half the difference in the altitudes derive from the trigonometric identity for converting sums and differences of trig functions to products of trig functions (so that logarithms may be used). If I reexpress Bowditch's method without using logarithms or the sum/ difference to product identity, the calculations are equivalent to solving the set of equations below. Let PD (polar distance) be the complement of the solar declination and let theta be the hour angle corresponding to the elapsed time between observations. Let H1 be the altitude of the first solar observation and let H2 be the altitude of the second solar observation. The rule is then captured by solving the following six equations: sin(A) = sin(theta/2) * sin(PD) [solve this equation for A] cos(PD) = sin(B) * cos(A) [solve this equation for B] 2 sin(C) * sin(A) = sin (H2) - sin(H1) [solve this equation for C] 2 cos(A) * cos(C) * cos(Z) = sin(H1) + sin(H2) [solve this equation for Z] E = Z + B (or E = Z - B as per rule) [solve this equation for E] sin(Latitude) = sin(E) * cos(C) [solve this equation for Latitude]. Can anyone help me understand what these angles and arcs represent. Where in the diagram are A, B, C, Z, and E ?? How do these calculations derive from the geometry of the situation ? It seems to me that A is the value of half the length of the great circle segment connecting the geographic position of the sun and the time of the two observations. Why he wants half of the segment I do not understand. I have been staring at these equations for weeks and can not see what B, C, Z , and E are. If there are auxilliary lines that have been added to the original triangles to convert the oblique spherical triangles to right triangles, then what are those lines, which triangles are being considered and how to A,B,C,Z, and E relate to these auxilliary figures? If anyone can untangle even some parts of this rule, perhaps it will put me back on track for figuring out the rest. I have seen a direct method explained in several sources, where the declination and elapsed time are used to compute the length of XY, then the law of sines is used to calculate one of the base angles in PXY. Now the three sides of ZXY are known and the law of cosines is used to find the angles. Subtracting the base angles gives enough information to find an angle in PZY (or PZX) and PY and ZY being known, the side PZ can be found by the law of cosines and the complement of PZ is the latitude. But I have not been able to see a connection between Bowditch rule and this direct method. I don't know if that is because there is no connection, but rather a totally different way of solving the problem, or if I'm just not seeing it. I hope someone can shed some light.... this is driving me crazy. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavListemail@example.com -~----------~----~----~----~------~----~------~--~---