Circle of Reflection

An instrument for measuring the angular distance of the moon from the sun or a star, and much superior for this purpose to a sextant.

ABC. The Earth

ZAK. Vertical plane passing through D, the moon, or d, a planet

ed GEDF. Circular arcs about C as a centre

C. The Earth's centre.

A. The Observer

KZ. Starry heavens

If at any time the moon be at D, she will be referred to the point H, by a spectator supposed to be placed at the centre of the earth, and this is called the true place of the moon; but the spectator at A will refer the moon to the place b, and this is called the apparent place of the moon; the difference Hb, or the angle HDb=ADC, is called the moon's parallax in altitude, which is evidently greatest when the moon is in the horizon at E, being then equal to the arc KI. When the moon is at Z (the zenith) there will be no parallax, the body being observed on the same line from the centre of the earth as from the earth's surface.

SEMI-DIAMETER,

1. Altitude of the sun's lower limb--semi-diameter added gives central altitude.

2. Altitude of the sun's upper limb--semi-diameter subtracted gives central altitude.

3. Altitude of the sun's middle--semi-diameter not considered.

4. Altitude of the moon's lower limb--semi-diameter added gives central altitude.

5. Altitude of the moon's upper limb--semi-diameter subtracted gives central altitude

6. Altitude of a star--possesses no apparent diameter.