The orbit of Mercury is the highest of all known planets, and also the shortest revelution. It completes three Mercurian years for every two Mercurian years. Mercury is bright when viewed from sunrise or sunset, ranging from −2.0 to 5.5 in the sky, but is not easily seen as its greatest is only 28.3°. Since Mercury is normally lost in the glare of the Sun, Mercury can only be viewed in morning or evening . Mercury has no atmosphere it has a minute atmosphere composed of sodium and helium.