# NavList:

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

**Re: Silicon Sea: Leg 71 (belatedly)**

**From:**Bill Noyce

**Date:**2001 May 29, 10:45 EDT

> 1) At the log reading of 89.6Nmi, what is the DR position? > -- ------------------------------------------------------- Mid-Latitude Sailing using L=13d 20' TC = 263.7 dLat = 9.8' S dep = 89 nm W DLon = 91.5' W DR position = 13d 15.8' N 63d 1.6' W > 2) At the DR position what is the celestial phenomena that is starting > at this DR Zone Time(ZT)? > -- ------------------------------------------------------------------- 18:27:00 + 10.3 hrs = 04:45 ZT 30 May At this time and position, I find the sun's altitude is -13d 58' -- still 2 degrees short of the start of Nautical Twilight. But if we're actually 1.5 degrees east of here, that would explain most of the difference. > 3) What is the FIX position? > -- ------------------------- Using the above DR position, Body Fomalhaut Polaris Deneb Vega GHA 36d 3.0' 342d 53.1' 70d 20.8' 102d 8.4' Decl S 29d 36.9' N 89d 16.1' N 45d 16.8' N 38d 47.0' Hc 39d 50.8' 13d 23.4' 57d 23.5' 47d 5.2' Az 149.1d 0.7d 350.4d 313.9d Int 58.2' T 14.8' A 32.4 A 75.0' A Pretty long intercepts, and a poor cut. I used Ageton's method, and for Polaris found why it's not recommended for LHA near 90 degrees with high declination -- you end up trying to find an angle whose sine is about 1.0, and since the function doesn't vary rapidly there, the angle isn't established with any precision. So instead of using sin K = sin decl / cos R I tried cos T * sin R cos T * cos d cos K = cot T * tan R = ------------- = ------------- sin T * cos R cos R Is this Sadler's modification to Ageton's method? Anyway, I tried again with an AP of 13d 0' N 61d 30' W and got intercepts of Fomalhaut: 0.3' A Deneb: 0.3' A Mars: 4.6' A Venus: 0.1' A Vega: 0.1' A and except for Mars, it looks like we're there. I wonder if I made an obvious mistake here: Mars GHA 114d 23.9' decl S 25d 47.5' K S 38d 42.0' Hc 25d 33.1' Az 232.8d > 4) What is the TC and Dist from our FIX to the Aruba Gap > (13d 50.0'N 70d 00.0'W) > -- ------------------------------------------------------ Mid-Latitude (using 13d 25') dLat = 50' N Dlon = 510' W log(dep) = 1.69897 TC = 275.7d Dist = 499 nm > 5) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap? > -- ------------------------------------------------------------- TVMDC, West is Best, add 11d: 286.7d > 7) What is the anticipated Speed Made Good(SMG) to Aruba Gap? > -- ---------------------------------------------------------- Current essentially right on our tail, 9.3+0.8 = 10.1 kts > 6) What is the time enroute to Aruba Gap? > -- -------------------------------------- 2 days + 1h 24m