OF PARALLAX AND REFRACTION.

Aftronomy is fubject to many difficulties, befides thofe which are obvious to every eye, When we look at any ftar in the heavens, we do not fee it in it's real place; the rays coming from it, when they pafs out of the purer etherial medium, into our coarser and more dense atmofphere, are REFRACTED, or bent in fuch a manner, as to fhew the star higher than it really is. Hence we fee all the ftars before they rife, and after they fet; and never, perhaps, fee any one in it's true place in the heavens.

There is another difference in the apparent fituation of the heavenly bodies, which arises from the stations in which an observer views them. This difference in fituation is called the PARALLAX of an object,

The parallaxes principally used by astronomers, are those which arife from confidering the object as viewed from the centers of the earth and the fun, from the surface and center of the earth, and from all three compounded.

The difference between the place of a planet, as feen from the fun, and the fame as feen from

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the earth, is called the parallax of the annual orbit; in other words, the angle at any planet, fubtended between the fun and the earth, is called the parallax of the earth's or annual orbit.

The diurnal parallax is the change of the apparent place of a fixed ftar, or planet, of any celestial body, arising from it's being viewed on the furface, or from the center of the earth.

The fixed ftars have no diurnal parallax, the moon a confiderable one; that of the planets is greater or lefs, according to their distances.

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Let HS W, fig. 2, plate VIII. represent the earth, T the center thereof, o R G part of the moon's orbit, Pr g part of a planet's orbit, Za part of the starry heavens. Now to a fpectator at S, upon the surface of the earth, let the moon appear in G, that is, in the fenfible horizon of S, and it will be referred to A; but if viewed from the center T, it will be referred to the point D, which is it's true place.

The arc A D will be the moon's parallax; the angle S G T the parallactic angle; or the parallax is expreffed by the angle under which the

femidiameter TS of the earth is feen from the

moon.

If the parallax be confidered with respect to different planets, it will be greater or lefs, as thofe objects are more or lefs diftant from the earth; thus the parallax AD of G is greater than the parallax a d of g.

If it be confidered with refpect to the fame planet, it is evident that the horizontal parallax (or the parallax when the object is in the horizon) is greatest of all, and diminishes gradually, as the body rifes above the horizon, until it comes to the zenith, where the parallax vanishes, or becomes equal to nothing. Thus AD and a d, the horizontal parallaxes of G and g, are greater than a B and a b, the parallaxes of R and r; but the objects O and P, seen from S or T, appear in the fame place Z, or the zenith.

By knowing the parallax of any celeftial object, it's distance from the center of the earth may be eafily obtained by trigonometry. Thus if the distance of G from T be fought, in the triangle STG, ST being known, and the angle SGT determined by obfervation, the fide TG is thence known.

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The parallax of the moon may be determined by two perfons obferving her from different ftations at the fame time, fhe being vertical to the one, and horizontal to the other. It is generally concluded to be about 57 minutes of a degree.

But the parallax moft wanted, is that of the fun, whereby his abfolute diftance from the earth would be known; and hence the abfolute diftances of all the other planets would be alfo known, from the second Keplerian law.

As one of the principal objects of astronomy is to fix the fituation of the feveral heavenly bodies, it is neceffary, as a first step, to understand the causes which occafion a falfe appear, ance of the place of thofe objects, and make us suppose them in a different fituation from that which they really have. Among these causes, REFRACTION is to be reckoned. By this term is meant, the bending of the rays of light as they pafs out of one medium into another,

The earth is every where furrounded by an heterogeneous fluid, a mixture of air, vapour, and terreftrial exhalations, that extend to the regions of the fky. The rays of light from the fun, moon, and ftars, in paffing to a spectator upon

upon earth, come through this medium, and are fo refracted in their paffage through it, that their apparent altitude is greater than their true altitude.

Let A C, fig. 3, plate VIII. reprefent the furface of the earth, Tit's center, BP a part of the atmosphere, HE K the fphere of the fixed stars, A F the fenfible horizon, G a planet, G D a ray of light proceeding from the planet to D, where it enters our atmosphere, and is refracted towards the line DT, which is perpendicular to the furface of the atmosphere; and as the upper air is rarer than that near the earth, the ray is continually entering a denfer medium, and is every moment bent towards T, which causes it to describe a curve as D A, and to enter a spectator's eye at A, as if it came from E, a point above G. And as an object always appears in that line in which it enters the eye, the planet will appear at E, higher than it's true place, and frequently above the horizon A F, when it's true place is below it at G.

This refraction is greateft at the horizon, and decreases very fast as the altitude increases, infomuch that the refraction at the horizon differs from the refraction at a very few degrees above

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