seeing, in astronomy, the clarity with which stars and other celestial objects can be observed. It is primarily determined by the atmosphere of the earth. The most obvious phenomenon is twinkling, when the brightness of a star seems to fluctuate. Known to astronomers as scintillation, twinkling is caused by thermal motion of the air, which swirls air layers of different temperature and density. This motion causes minute alterations in the path of light from a star because different densities of air will bend light by different amounts. Twinkling is most obvious near the horizon because the light path from a star passes through more of the atmosphere. Since a planet is a disk and not, as a star, a point source, it will not usually show twinkling, but undulations across its surface can be viewed when it is near the horizon owing to the same effect. In addition, the atmosphere is denser at the bottom than at the top and thus continually bends a ray of light from a star more and more toward the vertical. As a result, all stars except those directly overhead appear to be closer to the zenith than they actually are; this is most pronounced for stars near the horizon. This effect causes the sun (or moon) to appear elliptical when it is rising or setting because its bottom edge is raised more by the refraction of the atmosphere than its top. Astronomical observatories are located in areas where seeing is good, usually on mountains where they are above some of the more turbulent layers of the atmosphere and also removed from cities' lights. Astronomers consider the seeing excellent when the star image covers 0.5” of sky or less. Some observatories use adaptive optics, in which telescope optics are adjusted instantly by computer to correct for seeing effects.