Maker

Troughton & Simms (London)

Model

N/A

Serial Number

4

Dimensions

Length 150mm, Width 80mm

Image

Description

For measuring the altitudes of stars or the sun up to about 60°, this artificial horizon was employed when it was impossible to see the actual horizon. At sea, the horizon is formed by the junction of sea and sky. On land, the true horizon is often obscured by hills or other topological features. For making accurate observations with a sextant of the altitude of the sun or of a star an artificial horizon was required. In order to obtain the altitude of a celestial body, the angle between the sun or star and its reflection in a plane, level surface was measured and divided by two. The instrument consists of a shallow wooden tray about 150 mm by 75 mm and a turned wooden bottle for the mercury. The wooden tray is set on the ground or on a stand in an approximately level position and filled with the mercury. When the instrument is not in use, the mercury is kept in the bottle shown. The corner of the tray has a small hole and socket so that the mercury may be poured back without any danger of spilling it. A roof with sloping faces of glass plates of uniform thickness (shown on the left) covers the tray during the observations, in order to prevent rippling by wind striking the mercury surface. This artificial horizon gives very good results within its limits, its main disadvantage being that it is rather bulky and heavy.

History & comments

The basic principle employed by this instrument is that 'the angle between a distant point as viewed directly and its reflected image seen in a pool of mercury is equal to twice the angle of elevation of the point.' (laws of reflection of light).

Conservation

Mahogany case

Condition

Good

Remarks

• Artificial horizon is often used with a sextant. In navigation, therefore, angles of elevation of the sun are obtained by observing the vertical angle between the horizon of the sea and the sun, a correction, depending on the height of the sextant above sea level, being applied to correct the observed angle for the angle of dip between the observer and the visible horizon.
• Inscription "Sydney Government / No.4"
• Catalogued by T. Ko
• Updated by F. Pall

Dates

Manufactured in 1920 (approx.). Catalogued in 1997