Every navigator has, at various times, been confronted with the situation where a badly needed observation of the sun or other body was rendered temporarily impossible due to an obscured horizon. It is then that one prays for a bubble sextant that may be used with satisfactory results on a surface craft. Under such circumstances the horizon, while obscured directly beneath the body, may be quite clear at the opposite sector. Such conditions are frequently met when a vessel has encountered hazy or intermittently foggy weather.
It often happens that at evening twilight, the star most desired for an observation is located above that sector of the horizon opposite the setting sun, where a good observation is rendered impossible due to a poor and rapidly disappearing horizon. Possibly an overcast sky during twilight has prevented any evening star sights, and, as is so often the case, several bright stars pop out after the horizon has disappeared. At the same time the full moon has illuminated the horizon with sufficient clarity for an observation on the opposite sector from the star which it is wished to observe.