Circumpolar stars never dip below the horizon. Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface (except extremely close to the equator. Upon flat terrain, the distance has to be within approximately 2 km, although this varies based upon the observer's altitude and surrounding terrain). Generally, if a star whose declination is is circumpolar for some observer (where is either positive or negative), then a star whose declination is − never rises above the horizon, as seen by the same observer. (This neglects the effect of atmospheric refraction.) Likewise, if a star is circumpolar for an observer at latitude , then it never rises above the horizon as seen by an observer at latitude −.