A treatise on astronomy / by Sir John F.W. Herschel.

  • Herschel, John F. W. (John Frederick William), 1792-1871.
Date:
1833
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on astronomy / by Sir John F.W. Herschel. Source: Wellcome Collection.

    34/450 (page 18)
    less obtuse, or, in other words, the apparent angular diameter of the earth diminishes, being nowhere so great as 180°, or two right angles, but falling short of it by some sensible quantity, and that more and more the higher we ascend. The figure exhibits three states or stages of elevation, with the horizon, &c. corre- sponding to each, a glance at which will explain our meaning; or, limiting ourselves to the larger and more distinct, MNO PQ, let the reader imaginen NM, MQq to be the two legs of a ruler jointed at M, and kept ex- tended by the globe N mQ between them. It is clear, that as the joint M is urged home towards the surface, the legs will open, and the ruler will become more nearly straight, but will not attain perfect straightness till M is brought fairly up to contact with the surface at m, in which case its whole length will become a tangent to the sphere at m, as is the line vy. (24.) This explains what is meant by the dip of the horizon. M m, which is perpendicular to the ge- neral surface of the sphere at m, is also the direction in which a plumb-line* would hang; for it is an ob- served fact, that in all situations, in every part of the earth, the direction of a plumb-line is exactly perpen- dicular to the surface of still water ; and, moreover, that it is also exactly perpendicular to a line or surface truly adjusted by a spirit-level.* Suppose, then, that at our station M we were to adjust a line (a wooden ruler for instance) by a spirit-level, with perfect exactness ; then, if we suppose the direction of this line indefinitely pro- longed both ways, as X M Y, the line so drawn will be at right angles to Mm, and therefore parallel to «my, the tangent to the sphere at m. A spectator placed at M will therefore see not only all the vault of the sky above this line, as X Z Y, but.also that portion or zone of it which lies between X N and Y Q; in other words, his sky will be more than a hemisphere by the zone YQ XN. It is the angular breadth of this redundant zone—the angle Y MQ, by which the visible horizon appears de- * See this instrument described in Chap. II,