Atmospheric refraction displaces all the heavenly bodies from the horizon towards the zenith. This is evident from Fig. 42. _OD_ is the horizon, and _Z_ the zenith, of an observer at _O_. Refraction would make a star at _Q_ appear at _P_: in other words, it would displace it towards the zenith. A star in the zenith is not displaced by refraction, since the rays which reach the eye from it traverse the atmosphere vertically. The farther a star is from the zenith, the more it is displaced by refraction, since the greater is the obliquity with which the rays from it enter the atmosphere. [Illustration: Fig. 42.] At the horizon the displacement by refraction is about half a degree; but it varies considerably with the state of the atmosphere. Refraction causes a heavenly body to appear above the horizon longer than it really is above it, since it makes it appear to be on the horizon when it is really half a degree below it. The increase of refraction towards the horizon often makes the sun, when near the horizon, appear distorted, the lower limb of the sun being raised more than the upper limb. This distortion is shown in Fig. 43. The vertical diameter of the sun appears to be considerably less than the horizontal diameter. [Illustration: Fig. 43.] 32. _Parallax._--_Parallax_ is the displacement of an object caused by a change in the point of view from which it is seen. Thus in Fig. 44, the top of the tower _S_ would be seen projected against the sky at _a_ by an observer at _A_, and at _b_ by an observer at _B_. In passing from _A_ to _B_, the top of the tower is displaced from _a_ to _b_, or by the angle _aSb_. This angle is called the parallax of _S_, as seen from _B_ instead of _A_. [Illustration: Fig. 44.] The _geocentric parallax_ of a heavenly body is its displacement caused by its being seen from the surface of the earth, instead of from the centre of the earth. In Fig. 45, _R_ is the centre of the earth, and _O_ the point of observation on the surface of the earth. Stars at _S_, _S'_, and _S''_, would, from the centre of the earth, appear at _Q_, _Q'_, and _Q''_; while from the point _O_ on the surface of the earth, these same stars would appear at _P_, _P'_ and _P''_, being displaced from their position, as seen from the centre of the earth, by the angles _QSP_, _Q'S'P'_, and _Q''S''P''_. It will be seen that parallax displaces a body from the zenith towards the horizon, and that the parallax of a body is greatest when it is on the horizon. The parallax of a heavenly body when on the horizon is called its _horizontal parallax_. A body in the zenith is not displaced by parallax, since it would be seen in the same direction from both the centre and the surface of the earth. [Illustration: Fig. 45.]