Reply To: Air Navigation

[mcaminos]

To Manxcat
Hi Joseph:

Bubble octants can be used anytime of the day you cannot see the horizon for meteorological reasons, or anytime at night that is convenient for you based on the starts, planets and moon available.

This is the great advantage of the bubble octants over regular marine sextants. However, if you are on a ship in the middle of mid to heavy weather (day or night) when the ships rolls and pitch all over, it would be quite a challenge to take a sight with the bubble moving all over the display. The same would apply for a plane under heavy turbulence. In relative calm seas or on the ground, that problem is gone and you can use the octant anytime, anywhere in the world.

The nautical almanac and sight reduction tables are 24 / 365, so you can use them anytime, with the respective interpolation when need it. The main difference between the nautical and the aeronautical almanacs, is that the nautical almanac gives the data by hour (from zero to 24 hs) and the aeronautical gives the every ten minutes, every hour, so it is easier to interpolate when need it.

Regarding the Mark V octant, it was designed in the 1930’s and weights around 4.5 pounds. It is a little heavy, but no problem to handle with both hands. This model, also has a natural horizon option, so you can also use it as a regular marine sextant.

By the way, in case you do not know why the bubble ones are called octants and the marine versions sextants, here goes the explanation:

it is because in the case of octants, the altitude range you can measure goes from 0 to 90 degrees but the range of the scale only moves 45 degrees (because of the double reflection principle). 360 degrees / 45 degrees range = 8 (octant).
The marine sextants can measure altitudes with a range from o to 120 degrees, but the range of the scale only moves 60 degrees (same principle of reflection): 360 degrees / 60 degrees range = 6 (sextant).

If you measure any celestial body with an altitude higher than 90 degrees (only with a sextant), it would mean that body is actually “behind you back” respect to the natural horizon you are using as a reference. Why you would do that ? In case the natural horizon on the same side of the body your are taking a sight of is not visible for any reason (weather or even the coast), you can use the opposite side horizon as a reference, as long as the body is at least with an altitude of 60 degrees or higher to respect to the body same side horizon.
With a bubble octant, you do not need to take any angle higher than 90 degrees, because you always have an artificial horizon as a zero reference for altitude. It the altitude of the body is higher than 90 degrees, just turn around towards the azimuth where the body is located. That is another great advantage of the octants.

I hope my explanations are clear enough, and please let me know if you have any questions.

Best regards, Martin