Known optical instruments for measuring angles, such as sextants and octants, comprise support means in the form of a frame; an eyepiece mounted on the support means; a first reflector, known as a "horizon glass", mounted on the support means in alignment with the axis of the eyepiece; an adjustment member, known as an "index bar", pivotally mounted on the support means; a movable reflector mounted on the adjustment member so as to face towards the first reflector; and measuring means comprising an angularly graduated "arc" for measuring movement of the adjustment member. The support means comprise first and second arms extending divergently from a hub and interconnected at their free ends by an arcuate limb. The eyepiece and first reflector are respectively mounted on the first and second arms and the adjustment member carrying the movable reflector is pivotally mounted in the hub at the junction of the first and second arms.
To use an instrument of this type to measure the inclination of a celestial body, such as the sun or a star, it is necessary to hold the instrument with the optical axis of the eyepiece aligned with the horizon so that the horizon may be viewed through the eyepiece and a non-reflective portion of the horizon glass and then, with the pivotal axis of the hub arranged horizontally, above the optical axis of the eyepiece, to measure the angular movement of the adjustment member from a datum position in which the movable reflector is parallel to the first reflector to a position in which light from the celestial body is reflected from the movable reflector to the first reflector and back along the optical axis of the eyepiece so that the celestial body can be viewed through the eyepiece. This angular movement is equal to half the angle of inclination of the celestial body from the axis of the eyepiece. This is consistent with the law of optics whereby the angle between the first and last directions of a beam of light which is reflected twice, by two plain reflective surfaces, is twice the angle between the two reflective surfaces. By reference to navigational tables, it is possible to establish from such inclinations the positions from which measurements have been made.
Although an octant, in which the first and second arms are inclined at 45.degree., is capable of measuring 45.degree. movement of the adjustment member and therefore 90.degree. of inclination from an axis extending from the eyepiece to the horizon, this is not sufficient for measuring all inclinations if the instrument is used by an operator standing on an elevated platform such as the bridge of a ship. In this case, the axis extending from the eyepiece to the horizon dips below horizontal and so the maximum elevation which can be measured is less than vertical. To overcome this disadvantage, it is necessary to use the more expensive sextant in which the first and second arms are inclined at 60.degree. and which is capable of measuring inclinations of up to 120.degree. from the optical axis of the eyepiece.
In any case, to be of value, conventional sextants and octants have to be accurately made and are thus often too expensive for all but the most serious and professional navigators. Moreover, these instruments are particularly difficult to use, and cannot be used at all when the horizon is obscured by haze or at night. The elevations of stars must therefore be measured during twilight, at dusk and dawn, when both the stars and the horizon are visible. Clearly, on very many occasions when the horizon is indistinct, the great precision of navigational instruments such as sextants and octants is unnecessary in view of the unavoidable error in aligning the optical axis of the eyepiece with the horizon.
One known technique for modifying a conventional sextant or octant so as to make it possible to measure inclinations when the horizon is not visible is to provide the instrument with a spirit level and complex optical means which enable an operator to view an artificial horizon when the instrument is held so that the optical axis of the eyepiece extends horizontally. However, the resultant instrument, known as a "bubble sextant", does not work satisfactorily and has never been widely accepted by navigators. The modification increases the complexity of the instrument with attendant difficulty in keeping the instrument in accurate operating adjustment, and the cost is considerably higher than for conventional sextants and octants.