Sun Sign: Sagittarius. Mercury In Sagitta Rius. Sagittarian intellectualism is further enhanced when Mercury joins the Sun in this sign. There is a wide breadth of vision but very little ability to see fine detail, which is often overlooked. The Sagittarian broad and rather jokey sense of humour will be present, as will terrific optimism, enthusiasm, and a near fear of claustrophobic psychological or physical living conditions.