Lunar Distance Revisited with a 1939 C.Plath Kriegsmarine Sextant

We happen to have on consignment a C Plath sextant made for the Kriegsmarine in 1939 Germany. It is a prized collector's item, but we wanted to demonstrate that it remains a top of the line instrument for practical celestial navigation, and likely will for another generation or two, and there is no better way to do this than to show it can be used for lunar distance measurements, considered the epitome of sextant sights.

These sights require measuring sextant angles and index correction to within 0.1' of arc, which is the practical limit for marine sextant sights. Not having done this for a while, I was reminded of several tips that we outline elsewhere, and will note again here. The distance measured was between the moon and Venus on two consecutive nights. We will see that the moon moves to the east of the stars and planets by about 12º per night (360º/30 days).

I did two sets of sights. One with the 4x40 scope that is stock for the instrument and one set where I replaced that with a 6x30 monocular scope (made by the modern C Plath company), which is preferred for this measurement, because it makes the edge of the moon sharper. The spread in the data were smaller with the 6x30 scope, but the resulting UTC and Lon found from the measurements was actually better with the 4x40. That was just an accident, as there is always some luck involved with these sights. The higher power scope should in the longer run get better results.

This early C Plath sextant is ideal for lunars in that it is high precision and very light weight (2 lb 10z), being made from an aluminum alloy that C Plath pioneered for sextant manufacture in that era. Lunar sights take longer than conventional sights, which reminded me of the ludicrous presumptions we periodically see in advertising claiming heavy brass sextants weighing almost twice as much are preferred for their inertia and stability doing sights! Those sextants weigh more than a half a gallon of milk. Hold a full milk carton up to your nose with your head leaned back a bit for a minute or so to get the picture. The lighter sextant is always preferred and always the top of line in sextants. 

For these sights, we even want more support if we can rig it. With an eye cup that lets us press the head against the sextant, and ideally a support to lean your elbow on. Then tune the scope to show the moon's edge as sharp as possible. The star or planet, however, will always be a point of light... in principle. In fact, that point of light will have a fuzzy halo around it when looking as close as we can, and that puts a limit on the accuracy. This could be optics or it could be we just needed some eye drops!

Here are results of the first set of sights taken from a pocket beach on Puget Sound (47º 40.5'N, 122º 24.5'W), 0.4 nmi due west of our office. These sights used the stock 4x40 scope from 1939. 

These were typed into Excel then plotted, and then fit with a straight line, which we can add with a button click, called "Add trend line." This produces a least squares fit of the data, with the equation showing.

On the other hand, we can just plot the data manually and then do the fit by eye, lay a transparent ruler across the plot such that it goes though or near the most points, leaving as many below as above the line. If any are notably far from the line, just remove them, as likely in error. The goal is remove the blunders and fit the random errors.

One way or the other, however, it is crucial to make the plot to find the best representative of the full set. One or two sights alone are not enough for this process, nor in fact for traditional cel nav sights.

Note that in the analysis, we do not have to take values that we actually measured. In principle we should take a value that is on the line. Ideally we would have at least one sight right on the line and we could use that one, but not in this case.

Many lunar clearing procedures (how we get measured time and longitude from these sights) do not work well for Venus or Mars (the two closest planets) nor for LD less than 10º or so, but the Frank Reed app online at fer3.com should cover these, which is what we used. (Sample output screen at the end here.)

Here an easy first choice might be to use a LD= 4º 8.0' at 10:01:56 PM PDT 6/17 = 05:01:56 UTC 6/18. That measurement (#1) leads to a discovered Lon error of 3.3' as shown in the list below, which corresponds to a time error of 13s.

Since we know the equation for the fit to the data, we can do maybe better since our choice was not exactly on the line. If we use the equation to find the precise time (#2) corresponding to 4º 8.0' it would be 10:02:11 PDT or time error of just 3s. But both analyses have to be considered on the fortuitous side. Generally it is considered good lunar work to find the correct time to within 30s, and these measurements, no matter how careful, can yield even larger errors.

For example, if we remove the 3rd and 4th sights, which are off the most from the fit, we get a new best fit line, and that yields a 4º 8.0' LD of 10:02:29, shown below as #3, which now has the best looking data fit, but the Lon and time errors are larger.

#    UTC          LD            LD error   Lon error    Time error    Position error 

1    050156    4º  8.0'     +0.1'           3.3'             13s                    2.2 nmi

2   050211     4º 8.0'      -0.02'         0.7'               3s                   0.4 nmi    

3   050229    4º 8.0'      -0.2'           5.4'             22s                    3.6 nmi 

The main goal at hand for these sights were to show that right out of the box, the 1939 C Plath sextant we have on sale could do high precision lunars. These very good results, however, must still be considered fortuitous, especially since the index correction measurement was not as precise as we would want. It was set to zero using a direct view of Venus, which was not as sharp as it could be with more work. In the next set of sights, the IC was readjusted and measured more carefully with the solar index method.

________

For the second set, the next night, I changed to 4x40 to 6x30 and got these results for 6/18 PDT. The IC was 1.2' on the scale (see notes at the end here)

We see slightly better on the data scatter, and again if we were to take just one it would likely be the middle one right on the linear fit to the line, that leads to sight #4, which was not as good as the earlier session, though it seems the sights were better. In sight #4, the measured LD of 17º 37.7' was corrected for the IC ("If it's on; take it off!").

#    UTC          LD            LD error   Lon error    Time error    Position error 

4    054354    17º  36.5'     -0.5'           14.1'             56s                    14.1 nmi

5    054405    17º  36.8'    -0.3'             8.1              32s                    5.4 nmi  

In sight #5, to get a better fit, I removed the 3rd sight, which was off the most from the line, and then got a new slope, and used that slope to compute what time would correspond to 17º 38.0', the sextant value without IC = -1.2' applied. Note that with the third sight removed, the middle sight was no longer on the best fit line.

Excel keeps track of times in terms of fractions of 24 hr, i.e., x = (38.0+376.89)/927.58 = 0.44728). This sight is more typical of practitioners like myself who do these only infrequently. Experts can average about 15s. Note that even with fairly large Lon errors, the actual position errors are not that large at mid to higher latitudes. The position error is all Lon error since we rightfully assume that we can find accurate Lat with out accurate UTC.

Here is a sample output from the fer3.com lunar clearing solution:

The index correction was measured with the solar method using the 6x30 scope the following day. This is the preferred method for best accuracy. It requires a couple custom filters, which the article explains how to make. We have an app online that does the simple math to find the SD from two solar measurements.

So the IC was 1.2' On the scale. The actual SD at the time was 15.75', so this IC is likely accurate to within a few tenths of a minute, but normally one would take more sights than just two.

Later I will try to post more notes about using Excel to analyze cel nav sights.

Polar Wave Spectra

 

This is a place holder for soon to follow instructions on the use of polar wave spectra plots for understanding the seaway and for setting up optimum routing corrections for waves, including how to generate the plots with our new, free 

 Polar Wave Spectrum Plotter app

get the spectra data at this custom link we made

NDBC Buoys with Spectra Data

Thanks to the staff at NDBC for providing notes on the computation (References).

________________________

INSTRUCTIONS

1) Enter the NDBC buoy no, such as 41010

2) That will generate the link to the spectra data on the right

3) Click the link below that line to open that page in a new tab.

4) cmd+A or ctrl+A to select the full set of data, the cmd+C or ctrl+C to copy it

5) Paste it into the space provided

6) Click plot pasted data, which would typically load 5 days, with data every 30 min. This may vary with buoy.

7) The slide bar steps through the data.

Here is a sample of what the data should look like 

Not: period (sec) = 1/frequency(Hz)

YYYY MM DD hh mm bands
freq bandwidth energy r1 r2 alpha1 alpha2
2026 06 27 22 50 46
0.033 0.005  0.000 999.00 999.00 999 999 = 30s period
0.038 0.005  0.000 999.00 999.00 999 999
0.043 0.005  0.000 999.00 999.00 999 999
0.048 0.005  0.000 999.00 999.00 999 999
0.053 0.005  0.000 999.00 999.00 999 999
0.058 0.005  0.000 999.00 999.00 999 999
0.063 0.005  0.000 999.00 999.00 999 999
0.068 0.005  0.000   0.27   0.53 120 112 = 14.7s period
0.073 0.005  0.043   0.43   0.49 116 116
0.078 0.005  0.100   0.59   0.18 104 100
0.083 0.005  0.100   0.38   0.21 116 176
0.088 0.005  0.143   0.35   0.27 112 156
0.093 0.005  0.315   0.63   0.35 108 108
0.100 0.010  0.529   0.85   0.68 108 108
0.110 0.010  0.958   0.87   0.67 124 124
0.120 0.010  1.430   0.88   0.61 120 124
0.130 0.010  1.301   0.91   0.76 128 128
0.140 0.010  0.758   0.84   0.55 124 124
0.150 0.010  1.115   0.91   0.73 112 112
0.160 0.010  0.601   0.85   0.63 128 132
0.170 0.010  0.601   0.86   0.57 128 128
0.180 0.010  0.586   0.88   0.66 128 124
0.190 0.010  0.529   0.82   0.46 132 132
0.200 0.010  0.300   0.69   0.08 144 168
0.210 0.010  0.300   0.80   0.42 136 140
0.220 0.010  0.157   0.75   0.38 128 136
0.230 0.010  0.129   0.68   0.19 152 148
0.240 0.010  0.200   0.72   0.23 136 156
0.250 0.010  0.086   0.61   0.46 124  96
0.260 0.010  0.100   0.67   0.05 140 128
0.270 0.010  0.043   0.73   0.33 128 136
0.280 0.010  0.057   0.72   0.30 132 120
0.290 0.010  0.043   0.61   0.10 156 172
0.300 0.010  0.057   0.76   0.41 156 160
0.310 0.010  0.014   0.55   0.21 132  84
0.320 0.010  0.043   0.69   0.25 160 184
0.330 0.010  0.014   0.66   0.42 160 180
0.340 0.010  0.029   0.55   0.14 144 188
0.350 0.010  0.014   0.66   0.30 148 124
0.365 0.020  0.014   0.41   0.21 164 196 = 2.7s period
0.385 0.020  0.000   0.48   0.46 208 220
0.405 0.020  0.000   0.57   0.47 144 136 
0.425 0.020  0.000 999.00 999.00 999 999
0.445 0.020  0.000 999.00 999.00 999 999
0.465 0.020  0.000 999.00 999.00 999 999
0.485 0.020  0.000 999.00 999.00 999 999

All 9s means no data for those specs. We only plot 4s to 25s.

Back soon with notes on what the plots show and how we can use them.

Periods of 3 to 8 sec are waves (global mean ~ 5s) ; periods over 10 sec are swells, with global mean of about 11s... but with large variations on ocean and season (~8 to 18s). 

Note that not all NDBC buoys have spectra data. Ones we know about are in the interactive link above.

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References

NDBC Technical Document 03-01:
Nondirectional andDirectional WaveData AnalysisProcedures 

See also:  https://www.ndbc.noaa.gov/faq/measdes.shtml Section on Spectral Wave Data.

See also SOFAR Spotter Buoy Reference Manual

Notes on S-100 and the Eventual S-101 ENC Display Standard

Background

At present all ENC content and structure (internationally) are based on the IHO S-57 standard, and how these ENC should be displayed is determined by the IHO S-52 standard. ECDIS systems are required to follow the S-52 standard precisely, which does include a few display options, whereas most ECS strive to follow those standards, but they are not required to. Some apps are far from the standard, others, like qtVlm, are remarkably close to the standard.

The change from S-57 to S-101 ENC will be part of a huge change in overall electronic navigation, described as S-100, the Universal Hydrographic Data Model, which includes: 

S-101: Electronic Navigational Charts (ENCs)

S-102: Bathymetric Surface

S-104: Water Level Information

S-111: Surface Currents

S-412: Weather Overlay

Note that the S-100 based product specifications include more parameters than listed here, and indeed their item numbers do not all start S-1xx, as noted in this weather data component. Indeed, the system is intended to encompass all aspects of coastal navigation and hydrography.

 

The new system is much more GIS oriented with various new overlays. Except for the S-101 ENCs themselves, the other components of S-100 are well underway already, with tremendous promise. With digital soundings and tide heights everywhere on the chart, we are then aware of the actual water depths everywhere, which can also be forecasted continuously over time.

So far the S-412 weather products do not add to what racing sailors have been using for years with GRIB formatted model forecasts overlaid on their charts, but this will make the data more universally available to commercial and governmental vessels in the future. Unfortunately, the GRIB format we are all accustomed to, created specifically by the WMO for weather work, will be replaced in S-100 by a new h5 format, that so far none of the popular ECS apps or ECDIS can read! 

Because of the h5 format, it takes extra work to view these new data sets. We have notes on this elsewhere, along with custom conversion apps. 

But the question at hand is the charts themselves and what will govern how they should be displayed, keeping in mind that we are looking ahead, and largely just answering a question that might have come up. S-101 charts are likely to be 5 or so years out, and hopefully by that time the rest of the S-100 suite will be well underway.

Here are notes on the S-101 ENC display standards as presented at the moment.

S-101 absorbs the display standard into itself — it will not be a separate numbered standard.

Unlike the S-57 family (where S-57 = data, S-52 = display, S-58 = validation, S-63 = encryption, as four separate documents), the S-101 ENC Product Specification covers content, structure, data encoding, and metadata for S-101 ENC data, and the same specification also includes the portrayal (display) requirements for use within ECDIS. So portrayal/display rules are baked directly into S-101 as its Portrayal Catalogue, rather than living in a standalone "S-52 equivalent."  The S-101 Portrayal Catalog is analogous to the S-52 Presentation Library.

Conceptually, S-101's portrayal does the same job S-52 does today.

S-101 is similar in content to the current S-57 object catalogue and S-52 presentation library, but implements the dynamic constructs prescribed by the S-100 framework. The key structural difference: in S-101, the relationships between features, attributes, and enumerants are defined in a single feature catalogue, and the portrayal catalogue links those feature catalogue elements to their graphical representation — both built through a machine-readable registry rather than being hard-coded into ECDIS software.

Why this matters practically.

Under S-57/S-52, catalogue updates (new symbols, new display rules) are embedded in ECDIS software and can take up to five years to roll out via software updates, whereas under S-100/S-101 the feature and portrayal catalogues are versioned in a continuously-adapted registry, letting updates happen via catalogue update rather than a full software upgrade cycle. 

There is active work explicitly comparing the two. 

The IHO's S-101 Project Team has a working document titled "Allowable Differences Between S-52 and S-101 Display," and earlier project team materials describe an explicit S-52 to S-101 portrayal gap analysis, along with a list of S-52 symbols no longer required in S-101 and proposals to retire some symbol definitions — so the transition is being managed carefully, symbol-by-symbol, rather than a clean swap.

Summary.

There won't be an "S-52 for S-101" as a separate cross-referenced document the way mariners are used to. The display rules will be part of S-101 itself (its portrayal catalogue), with the additional change that the rules can be updated from within the S-101 ENC itself.

Details of the above topics and S-100 more generally are at the IHO web site.

Shapefiles for qtVlm

May 31, 2026 update: added US states and bumped this note forward,

A powerful function of qtVlm is its ability to show shapefiles in a very convenient manner. They can even be configured to include links to live data. One example is the UK shipping forecasts that we made for qtVlm some time ago, which we put here at the top of the list.  But we need a list because there are many of these floating around that can be very useful for navigation, and I am beginning to loose as many as I find. Just found a couple neat ones for the Gulf Stream, which motivated setting up this index.

(1) UK Shipping forecasts
Note this is a special type of shapefile in that Starpath has made code that lets you get live forecasts at each zone. Normally shapefiles load static data. If we want overlays that update automatically we need to have links to images or KML files.

(2) Add elevation contours to an ENC

(3) Add north and south walls to the Gulf Stream plus Add eddies with ID 

The above Navy data were typically updated every 36 hrs, but at present (4/30/25) it is nearly a month old. So we need to keep an eye on this.  There is a lot of chaos in ocean and weather data delivery these days. A sample below:

These shape files (two are loaded here) are to be overlaid onto either the RUCOOL SST images or one of the model forecasts for the current, or overlay onto the Navy Gulf Stream Analysis to annotate what they show. These eddies will coincide exactly with what are on the Analysis maps.  Note too that these shapefiles have to be downloaded  each time they are new, which is typically every 36 hr. They are identified by day of the year, ODate, ie in 2025, 90 = Mar 31.... however, as of May 4, 2025 we are seeing only erratic updating on Navy GS products, so their fate is uncertain.

(4) Up dated US Forecast zones

(5) US state borders with state ID

The zip file US_states.zip brings with it two sets of shape files. One is the outlines only where there is a tool tip showing the state, but it is not always clear which side of the boundary it refers to. The second shape file solves that problem by planting a mark at the centroid of each state with the states name.  The download brings both files into qtvlm, but you can choose to show either one or both. To show both, you need to load both individually.