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Showing posts from October, 2019

Fit-Slope Method to Analyze Sextant Sights

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We have been teaching what we named the Fit-slope Method for analysing sextant sights since 1978, and it has appeared in all of our textbooks since then. Consequently we were pleased and honored that this method was referenced in the latest edition of Bowditch's American Practical Navigator, which in turn led me to realize that we do not have an explanation of this method in public that is easy to reference, although I did post a note related to it.  That note, and the Bowditch description, are short, and do not emphasize the key aspect of the method. This note is intended to remedy that.

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For any measurement, we always get a more realistic value by making several independent measurements and then averaging the results—or maybe even applying more sophisticated statistical analysis such as least squares.  This is no different in celestial navigation when we measure the sextant height of a star from which we can compute a line of position (LOP) on the chart. We do not want to…

Satellite Cloud Images — Underway Sources

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Satellite data are crucial to modern marine weather. The most direct use we have comes from scatterometer wind measurements that give us surface wind speed and direction worldwide. We get near-live measurements somewhere close enough to us to be helpful for navigation planning every few hours.  Google ASCAT to get to the main page of graphic data. We can also get the ASCAT data in GRIB format.

The ASCAT data are from satellites in low polar orbits. The earth rotates below them, so they eventually look down upon all points on earth in their typical 103 minute periods. Other satellites in these low orbits measure sea surface temperature and the height of the sea surface, which can be used to compute ocean currents. We also can get the resulting ocean current forecasts in GRIB format.

Another type of satellite is in a much higher orbit specifically chosen so it circles the earth at exactly the same rate that the earth rotates below it. These are called geosynchronous satellites. They are…

Squall Forecasting in Puget Sound... Maybe.

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We had a very pleasant surprise visit in the office today from sailing and weather expert Bruce Hedrick, during which the discussion of local squalls came up, which reminded me of an example from a couple months ago, which at the time seemed to me a peek into the future of squall forecasting.

This one occured on Sept 16, 2018,  during the J&J race from STYC. It started with a bike ride home in the afternoon noting what fine sailing weather we had as a race was going on in the Sound, off Shilshole Bay. Then once at home I saw the same fleet hit by a sudden squall, which was gone in 15 min or so.

Perhaps one of Bruce's racing column readers was sailing that race and can tell us more about what they observed. This took place between about 1:15 and 1:30 PM, on Sunday, Sept 16.

At the time I made a video to document the observation; this note is a summary of that video, which was longer than needed... too much talking!  Plus a couple new notes. The boats affected that I saw were at…

Navigation Exercise: Crossing Currents

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In an ongoing article about crossing the Equatorial Countercurrent (to be posted shortly) we confront a classic navigation exercise in optimal routing through and across currents. More often than not, there are external factors that dominate how we choose to cross a current, but it is valuable to have in mind the fundamental results for comparison.

The problem is illustrated below. The situation looks idealized, but as it turns out, the values are fairly realistic—we are for now disregarding the performance of the vessel (assuming a constant speed on any heading) and also ignoring the constant equatorial currents of 1.0 kts flowing the other direction on each side of the countercurrent. We will come back to that reality, but as first guess we assume that does not affect our solution to this problem.  The vessel in this case is a custom single-handed rowboat, on its way to Cairns, AU, after departing from Neah Bay, WA about 5 months earlier.



Figure 1.Pacific Equatorial Countercurrent of…

Florida Gulf Stream: An Exercise in Sources

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... Well, that title is going to upset some folks right a way. Many feel that the name of the strong current off the east coast of Florida should be called the Florida Current. However, navigation and marine weather publications that make the claim to that name, very shortly move on to call it the Gulf Stream themselves—just as we will, and just as Benjamin Franklin did when he named the current system, 200 years ago.

We consider here a detail (our speciality!) of this current flow that stems from questions that came up in our online class about Question 16 in our online Weather Course Quiz 3: When sailing from West Palm Beach Florida toward Grand Bahama Island located some 70 nmi to the east, how far offshore would you expect to first run into the Gulf Stream Current?  And we give a Hint that such information is in the Coast Pilot, among other sources, including charts, and weather maps, and that we were not talking about the axis of the current, but rather when would we first start …