The Midheaven is always aligned with north or south. However, the Ascendant and Descendant are not necessarily aligned with east and west, so the Ascendant is not necessarily at 90 degrees to the Midheaven. The Ascendant is the degree of the ecliptic that intersects the eastern celestial horizon at a specific time and location. For observers in the tropics, this will always be close to due east and the Ascendant will always be close to 90 degrees away from the Midheaven. As the observer moves farther north or south it becomes possible for the Ascendant to be farther and farther away from due east.