Davis Mark-15 Plastic Sextant
Comes with plastic box, shown below, along with Starpath notes and forms for taking sights with plastic sextants. Sextant weight is 1 lb, shipping weight is 10.5 lbs
$179.00 ...item# 1840d15
Davis Mark 15 Plastic Sextant
This product when purchased here includes a complimentary Elibra ebook copy of How to Use Plastic Sextants—with Applications to Metal Sextants and a Review of Sextant Piloting by David Burch. The ebook must be read from a Windows PC. A serial number is provided at the time of sextant purchase. More info.
This model has many features of traditional sextants including 7 large sunshades, telescope (3 x 27), and micrometer drum vernier scale that reads to 0.2'. The Mark 15 has a standard 7" (18 cm) frame radius graduated from 120° to -5° and comes with a traditional half-silvered mirror, as well as the sight tube shown in the picture — although there would rarely be need for that device; normally you would use the telescope (the tube looks good in the picture, however!).
It comes with very good instruction booklet. All replacement parts (even the screws!) are readily available. Well cared for, this instrument will last a life time, and you could indeed circumnavigate the globe with this device as your guide — and our booklet on how to use it.
Please see our text on How to Use Plastic Sextants (PC ebook copy included with your purchase) and also the article on actual Sextant Sights Underway taken with this model of instrument. You will see that this instrument can be used with care, and most important with good procedures, to obtain accuracy of some few miles (3 to 5). This is in fact the same accuracy you would get from a metal sextant if not used properly, but with a good metal sextant this can be pushed to 0.5 to 1 mile.
With regard to actual accuracy, one cannot be very specific about this with a plastic device that obviously requires special care to protect it and proper procedures when using it. But one of the beauties of celestial navigation is that when you do complete a 3-star fix from well chosen stars, you will be able to evaluate the uncertainty of the fix. We go over this in our home study course. For rough rules of thumb, one might say that the Mark 15 and 25 should typically yield fixes with uncertainly of about or less than some 5 miles, whereas the Mark 3 would be more like about or less than some 10 miles. Again, when done carefully, both can be a bit better, but if not done well at all, or if the sights were taken and treated in the same manner as one might with a traditional metal sextant, then the uncertainly can be even larger.
We stress that you must be more careful when doing sights with plastic sextants than when doing them with a metal sextant. Think of measuring the length of a football field with a yard stick compared to a digital laser range finder. You can get useful results either way, but one method will take longer and require more care. That's a bit extreme for the analogy, but roughly the idea.
From our sextant articles you will see that the procedures required to optimize plastic sextant accuracy are more involved than those for using a metal sextant, but the actual physical sight taking itself is actually easier with the plastic instrument since they are so much lighter in weight. Also, these instruments are more than accurate enough for piloting fixes in coastal navigation (i.e. horizontal sextant angles) and for this application they might even be considered superior to metal sextants with regard to ease of use. In fact, even the Mark-3, will do this job of coastal piloting well enough in many applications. The reason plastic sextants work so well for coastal piloting is you only need tenths of a degree for a very accurate piloting fix whereas in celestial you need tenths of a minute.
Note this model does not have a light, but a simple dim light on a lanyard will solve that problem, if not even better than built in sextant lights.