# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Interpolation for latitude**

**From:**Ark Shvetsky

**Date:**2009 Nov 12, 19:24 -0800

Greetings, First, I'm really grateful for a lot of very useful information and great explanation regarding mu humble question/mistake. I really do appreciate your time and your responses. Now, to the point: below is the copy of Bowditch , Article 1809 per Gary's recommendation. Now let me in a few brief sentences write the way I understand my mistake: As it was mentioned in one of the responses to my initial post, the correction for longitude is needed when dealing with nav., civil, twilight phenomena. I understand that the time of these phenomena are needed for pre-computation of the altitudes of the stars most suitable for the morning (f.e) star sights (or some planets also) and the time of civil twilight for the evening star sights. In the NA, time for all twilight phenomena are "corresponding" to the time zone meridian. So, if I'm at Long 88 deg. West I need to correct the time I took from the NA corresponding to my DR lat for the twilight phenomenon. As I know that each time zone equal 15 deg long.(or 1 hour in time) and each has its time zone meridian which is at 7.5 deg from "each end" of a time zone, I need to find the delta between my DR long and time zone meridian and convert this delta into time units, subtracting it (because 88 deg is east of 90 deg) from the time for this twilight phenomenon which I took from the NA corresponding to my DR Lat. Or , even simplier, like it was shown by Jerry in his attached worksheet calc. If my understanding is correct, I just need to practice to make it as "ingrained" algorithm for correct way to perform this type of calculation. Pretty much like anything else we want to be sure we do. From Bowditch, Art 1809 Local mean time (LMT), like zone time, uses the mean sun as the celestial reference point. It differs from zone time in that the local meridian is used as the terrestrial reference, rather than a zone meridian. Thus, the local mean time at each meridian differs from every other meridian, the difference being equal to the difference of longitude expressed in time units. At each zone meridian, including 0�, LMT and ZT are identical. In navigation the principal use of LMT is in rising, setting, and twilight tables. The problem is usually one of converting the LMT taken from the table to ZT. At sea, the difference between the times is normally not more than 30m, and the conversion is made directly, without finding GMT as an intermediate step. This is done by applying a correction equal to the difference of longitude. If the observer is west of the time meridian, the correction is added, and if east of it, the correction is subtracted. If Greenwich time is desired, it is found from ZT. Where there is an irregular zone boundary, the longitude may differ by more than 7.5� (30m) from the time meridian. If LMT is to be corrected to daylight saving time, the difference in longitude between the local and time meridian can be used, or the ZT can first be found and then increased by one hour. Conversion of ZT (including GMT) to LMT is the same as conversion in the opposite direction, except that the sign of difference of longitude is reversed. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList+unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---