187. The Sun and Moon appear bigger in the Horizon than at any
considerable height above it. These Luminaries, although at great
distances from the Earth, appear floating, as it were, on the surface of
our Atmosphere _HGFfeC_, a little way beyond the Clouds; of which, those
about _F_, directly over our heads at _E_, are nearer us than those
about _H_ or _e_ in the Horizon _HEe_. Therefore, when the Sun or Moon
appear in the Horizon at _e_, they are not only seen in a part of the
Sky which is really farther from us than if they were at any
considerable Altitude, as about _f_; but they are also seen through a
greater quantity of Air and Vapours at _e_ than at _f_. Here we have two
concurring appearances which deceive our imagination, and cause us to
refer the Sun and Moon to a greater distance at their rising or setting
about _e_, than when they are considerably high as at _f_: first, their
seeming to be on a part of the Atmosphere at _e_, which is really
farther than _f_ from a spectator at _E_; and secondly, their being seen
through a grosser medium when at _e_ than when at _f_; which, by
rendering them dimmer, causes us to imagine them to be at a yet greater
distance. And as, in both cases, they are seen[46] much under the same
Angle, we naturally judge them to be biggest when they seem farthest
from us; like the above-mentioned house § 186, seen from a higher
ground, which shewed it to be farther off than it appeared from low
ground; or the hay-rick, which appeared at a greater distance by means
of an interposing Fog.
[Sidenote: Their Diameters are not less on the Meridian than in the
Horizon.]
188. Any one may satisfy himself that the Moon appears under no greater
Angle in the Horizon than on the Meridian, by taking a large sheet of
paper, and rolling it up in the form of a Tube, of such a width, that
observing the Moon through it when she rises, she may, as it were, just
fill the Tube; then tie a thread round it to keep it of that size; and
when the Moon comes to the Meridian, and appears much less to the eye,
look at her again through the same Tube, and she will fill it just as
much, if not more, than she did at her rising.
189. When the full Moon is in _perigeo_, or at her least distance from
the Earth, she is seen under a larger Angle, and must therefore appear
bigger than when she is Full at other times: and if that part of the
Atmosphere where she rises be more replete with vapours than usual, she
appears so much the dimmer; and therefore we fancy her to be still the
bigger, by referring her to an unusually great distance; knowing that no
objects which are very far distant can appear big unless they be really
so.
[Illustration: Plate IIII. _J. Ferguson delin._ _J. Mynde Sculp._]