NavList:

Hi Bruce,

Still not entirely sure how you're going about this but I think we're getting warmer. Are you saying that you tried to use the Moon for a traditional time sight to get longitude? If so, you realize that the Moon was nearly South at the time. Time sights become rapidly sensitive to errors in altitude (or latitude) when the object approaches the meridian. Time sights were normally observed when the celestial body was as near as possible to East or West to avoid this issue. With the Moon at an azimuth of 165°, you would expect that an error in altitude would be multiplied by about a factor of 5 in the determination of the longitude. You can figure this out by looking at the equivalent LOPs. Draw an LOP on a "chart" to represent your latitude as input. It's a straight line running east-west, right? Now draw an LOP inclined to that at 15°. This is perpendicular to the Moon's azimuth of 165°. The longitude from a time sight calculation is simply the longitude where the two LOPs cross. But notice what happens if you "wiggle" either of these LOPs. The crossing point shifts by a large distance east-west for a relatively small change in EITHER the input latitude OR the observed altitude. For this specific case, the ratio of the change in longitude to a small change in latitude is just about 5-to-1. The ratio of the change in longitude to a small error in altitude is about 5.1-to-1. Notice that an error of 0.3' in altitude would yield a 1.5' error in longitude. By contrast, if the celestial body were due East or due West, a small change in latitude would have no effect on the longitude. A small change in the observed altitude would have a 1-to-1 impact on the longitude, as you would expect.

-FER

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