At long last, here is a way to effectively practice taking sights without having a natural (sea) horizon, and without spending hundreds of dollars for a Professional Bubble Horizon. With this, you can use the same sextant you will take to sea, and use it in a normal manner to take sights from your backyard. It differs from the more expensive professional bubble horizon in that it is not as accurate, is unlighted, and has less light gathering.
How it works The scope is of zero-magnification, and is mounted on the sextant in place of the regular telescope as shown above. An ordinary spirit level is reflected by a mirror to appear upright in the opaque left side of the scope. The right side admits a view of the outside world as reflected by the sextant's index mirror (when using a whole horizon mirror, a horizontal view is also seen superimposed). The right side is divided by a horizontal hairline. The sextant is held such that the bubble appears alongside the hairline (as shown), and the celestial body is brought down to the hairline by movement of the micrometer drum. A reading is then taken and the time noted in the normal manner. The problem of focusing simultaneously on both the celestial body and the much nearer bubble is solved by using a slit aperture in the eyepiece. This acts like a lens to keep the bubble in focus without altering the image of the celestial body.
Capabilities The scope is unlighted which prevents its use after dark. The bubble is exposed to ambient light, which is adequate for use during daytime, and the normal star observing period (twilight) when both stars and outside features are discernible. A sextant installation correction (SIC) is initially determined, and is applied to subsequent observations. The slit aperture slightly reduces the eye's natural light gathering to the extent that faint stars may be more difficult to ascertain. Test results with the Practice Bubble Horizon show repeatable observations to an accuracy of 2 minutes of arc under ideal conditions. Although this is quite good, and rivals that of many more expensive bubble horizons; its accuracy cannot be relied upon for all lighting conditions. Accordingly, we recommend this product for practice purposes only, and not for serious navigation.