International Nautical japanese sextant

[Sailor Sailor]

Does anyone know anything about a japanese company called International Nautical?

I bought a used 1976 sextant off e-Bay. It seems very well made, and included tools and a locking wooden box. It looks somewhat like a Tayana. An internet search reveals no information about the company.

There is a photo album on my profile page with several photos.

[SeanPatrick]

There are many companies that make or made "Tamaya-like" sextants. Most seem to be decent copies. What I find odd about yours is that it only has two horizon shades and two index shades. Most sextants have three and four, respectively.

It looks nice, but it's impossible to say how accurate it might be (if at all) without being able to test it. You could, of course, test it yourself. I'd start with the standard checks: index mirror perpendicularity, side error, index error. The standard index error check (Sun limb tangency) will result in a measure of the Sun's apparent angular diameter, which can be cross-checked with an almanac. After that, I'd probably start taking some sights with it.

Your sextant could be every bit as good as a Tamaya ... or it might be best used as a paperweight or desk ornament. One thing that bothers me is that the certificate has a date of 1976, but from what I can see in the photo, the instrument looks like it was made yesterday. Either it was never used and stored very well, or something is amiss. Unfortunately, there are many eBay listings for "sextants" that are, in reality, just 'toys' at best. Again, it's tough to say without being able to examine it.

[Sailor Sailor]

Thanks. I think this one is an actual instrument, not a display item. I did see many on eBay that were designed for display only. The previous owner was a delivery captain, he learned on this one, but never really used it. It sat on a desk in its box. It looks almost new, with a little wear. The previous owner bought it at Fawcetts Boat Supplies, a reputable marine store in Annapolis, while he was working there.

I did some checks when I received it and the optics and alignment are good so far. It has an adjustable scope. The gearing on the micrometer and its movement on the arc seem excellent. It seems like a high quality instrument, although it is slightly smaller - 7" arc.

[Sailor Sailor]

Originally Posted by SeanPatrick

... The standard index error check (Sun limb tangency) will result in a measure of the Sun's apparent angular diameter, which can be cross-checked with an almanac...

In the Almanac is that the difference between a lower limb sight and an upper limb sight in the Altitude Correction Tables for the Sun?

For example, in the Oct-Mar. column, at my approximate latitude, it shows LL +15.0' UL -17.3', so the Sun's apparent angular diameter is 32.3' . I assume each correction on either side equals half the diameter of the sun.

[SeanPatrick]

No, half of the apparent angular diameter of the Sun (the "semi-diameter") can be found on each set of daily pages, on the right side of the opening at the bottom of the Sun column. For example: today's opening (October 10, 11 & 12) reads "SD 16.0 d 0.9". So the [average] semi-diameter of the Sun on those three days is 16.0', and the apparent angular diameter is 16.0 · 2 = 32.0'.

The "d 0.9" is the rate of change of declination of the Sun and is not important here.

The altitude correction tables are not dependent on latitude, but are instead dependent on the altitude of the body at the observer's location (wherever that may be). They include the semi-diameter, but also the effects of refraction and probably parallax in altitude. The refraction correction is always subtracted and the parallax correction is always added.

Refraction ranges from about -34.0' at the horizon to -0.0' at the zenith (atmospheric conditions can change refraction dramatically). Parallax in altitude varies according to the distance of the Earth from the Sun and can be as high as +0.2' at the horizon to +0.0' at the zenith.

So, if we took a sight of the Sun today at an altitude of 37°00.0' at the lower limb, the corrections would be +16.0' for semi-diameter, -1.3' for refraction and +0.2' for parallax. 16.0' - 1.3' + 0.2' = +14.9'. If we had shot the upper limb, the correction would be -16.0' for semi-diameter, -1.3' for refraction and +0.2' for parallax. -16.0' -1.3' + 0.2' = -17.1'. These numbers don't exactly match the almanac for several reasons, but probably mostly due to rounding. They're probably also using a larger number for SD, since the table has to cover six months at a time.

[SeanPatrick]

Also, If you'd like to calculate these values for yourself, here are the formulas and examples with the values from above:

• The sine of the horizontal parallax of the Sun = radius of the Earth / distance of the Earth from the Sun
  • asin((6378 / 149597870.7) / 0.99791) = 0°00'08.8124"
• The sine of the SD of the Sun = radius of the Sun / distance of the Earth from the Sun
  • asin((695700 / 149597870.7) / 0.99791) = 0°15'59.2312"
• Refraction = -0.0167° / tan(H + 7.32 / (H + 4.32))
  • -0.0167 / tan(37 + 7.32 / (37 + 4.32)) = -0°01'19.2708"

[Sailor Sailor]

Originally Posted by SeanPatrick

The standard index error check (Sun limb tangency) will result in a measure of the Sun's apparent angular diameter, which can be cross-checked with an almanac.

I measured the Sun's apparent angular diameter close to sunset last night at 35', the Almanac reads 32' (2 x SD 16.0). This morning with a challenging waning gibbous Moon, I measured the fullest part at 34', the Almanac reads 29.8 (2 x SD 14.9). So, unless I am not good at taking shots, or I am missing some correction, it is a few minutes off. I am not sure why the error on the Moon shot would be greater than the error on the Sun shot.

With the micrometer at 0 degrees 0', the two halves of the Moon lined up well in the sights as a single object.

It appears more accurate than my Davis Mark3 lifeboat sextant, which measured the Moon at 20' this morning.

[SeanPatrick]

You have to be careful when shooting objects near the horizon. Refraction can do some pretty crazy stuff at low altitudes. (E.g., Fata Morgana.) And the Sun appears large enough in our sky that the lower limb can be refracted more than the upper limb, causing the Sun (and Moon) to have a slightly "squashed" appearance.

Sights of the Moon always have to be done with care, because very rarely is the Moon 100% full. And if it's not, you may be shooting a "false limb" - what is actually the terminator line between the illuminated and non-illuminated sides of the Moon. Even at what we call "full Moon", the Moon is usually something like 99% illuminated because of the geometry. For this reason, the Moon is not recommended for index error checks. And when doing things like timing occultations, one must even take into account the mountains and valleys on the Moon's surface.

Besides all that, how you measure your index error is important. The standard procedure [using the Sun] is to set the micrometer close to zero and bring the two images of the Sun into contact so that the limbs are just tangent to each other and note the reading. Then, you do the same with the opposite limbs. This will result in one positive and one negative value. Finally, you add the two values together algebraically and halve the result. The sign of the index error is the same as the sign of the larger of the absolute value of the two measurements. So, if your first measurement is +33.2' and your second measurement is -30.0', then your index error is (33.2' - 30.0') / 2 = +1.6' (or 1.6' "on the arc"). The apparent diameter of the Sun is the average of the absolute values of the two measurements. Using the above example: (33.2' + 30.0') / 2 = 31.6'. The semi-diameter is half of that value: 31.6' / 2 = 15.8'.

Note: Special care must be taken when reading the negative value as the micrometer needs to be read backwards.

You also need to be aware of something called "backlash error". This occurs when you turn the micrometer one way, and then the other when taking a measurement. Slight imperfections in the way the worm screw and rack fit together will result in a delay of movement when the direction of the micrometer turn is reversed. This will introduce an error into the measurement. For this reason, it is recommended that you only turn the micrometer in one direction when taking a shot. IOW, if you go too far trying to get the limbs tangent and the images overlap, move the micrometer until the images are well separated again and then bring them back into tangency. Don't just back off the micrometer until they are tangent again. This applies to any measurement you take with a sextant.

Lastly, although Davis plastic sextants are adequate for actual navigation, they are much more prone to varying index error than a metal sextant. I have found that the error in my own Davis Mk. 15 can vary just by how tightly I am holding the handle. These plastic sextants are much more likely to distort with temperature and other changes than metal sextants. For this reason, when using a plastic sextant, it is advised to check the index error between every shot. (!)

[Sailor Sailor]

Thanks. Yesterday, I took a midday shots with the Japanese sextant that was accurate at 32' positive and -31', so I am pleased with its performance. I assume the micrometer reading at negative 31' is actually 29'.

[Jackg]

I have an International Nautical sextant I bought new from Weems and Plath in Annapolis in 1974. It looks just like yours but has more shades.


It's accurate. Pre-GPS I used it to navigate across the Atlantic a couple of times, trips to Bermuda, Caribbean, etc. I still use it for fun and to stay in practice. A few months ago I shot a couple of sights while sailing in the lower Chesapeake and was within 1.5 miles of my GPS position.


I use HO 229 and find that it takes just a few minutes to reduce a sight and plot it on plotting sheets. I don't use a calculator or other electronics (other than an SSB to check time) as that takes some of the fun out of it.

[SeanPatrick]

In the above image, one would normally read this as 29.4'. But ... the index mark is below 0°. So, you have two options: 1) read the micrometer "backwards" - so 'pretend' that the numbers on the micrometer (and vernier) go the other way, or 2) read the micrometer normally and subtract the value from 60. Either way, you should get an actual value of -0°30.6'.

[double u]

...if one is not exercising regularly with the sextant (& by this I mean: at sea, sailing, wind & waves...not just on the balcony), the accuracy of the instrument will matter very little...

[Al Stewart JM]

Hello!

I also have an International Nautical sextant... a 1975 full-size "Professional" model with circular 50mm diameter horizon mirror (traditional / half-silvered), variable polarizing shades on horizon and index, and 4 x 40mm scope. Your pictures show the "Mini-sextant" model, which was perhaps 7/8 size.

From my reading, the "International Nautical Company" was owned / linked to Weems & Plath (Annapolis). The sextants were made for them in Japan by another company; they are therefore very similar to other sextants with different brand names including Simex, MAC, Saura, etc.

The International Nautical sextants were well-made, and I certainly had good results in practice with mine. FYI, following text from an article I came across in "Motor Boating & Sailing" magazine, June 1974...

... finally, there are two Japanese sextants from International Nautical Company. Both are made of brass and have the accouterments of professional instruments. The lower priced Mini-sextant is a bit over two pounds - slightly less than the Plath Yachtsman. Its arc is graduated from -5° to 105° and the micrometer drum reads to 1'. The scope is 3x26, and there are two horizon and two index shades. Price is under $200, with a wood case. The Professional Model, about $50 more, is a pound heavier with a 4-power scope, and a pound and a half more with a 7x35 monocular. The filters are rotating and polarized, and both drum and arc can be lit for night use. The graduation of arc is -5° to 125°, and the micrometer vernier reads to 0.2'. It seems a serious, rugged instrument, and both it and the mini-sextant deserve serious consideration by skippers who really mean it, as well as by navigation schools.