In the art of astrology, the sky is viewed as a giant sphere, which on paper translates into a circle. Astrological charts reflect this view, and are circular in shape, and operate on the ideas of cycles and turning. Because the chart is circular and involves ideas of movement, there are certain points on the circle, as seen on any chart, that have special significance. These points are evenly spaced around the chart, and are four in number, based around the intersection of the veritcal and horizontal axes of the sky. Each one of these points, called “Angles,” has a spcial significance. There are two Polarities of Angles. The first of these Polarities is the Polarity of Interaction (Self vs. Other), made up by the Ascendant and Descendant, which describe how you express yourself to others and how others express themselves to you.
The first Angle is the Ascendant, and the Sign that lies on this Angle is rising over the eastern horizon (the left of the chart). This is also called the Rising Sign, and is one of the three aspects of an astrological personality. It represents how your Sun Sign expresses itself to the rest of the world, and the mask that you wear when out in public; it is how you appear to others, and how your energy manifests.
The second Angle is the Descendant, and it lies opposite the Ascendant, setting over the western horizon (the right of the chart). The Sign here shows what it is that you seek when interacting with others; what you look for in your relationships, what qualities you seek in companions, and what energies manifest themselves in your relations with others.
The second Polarity of Angles is the Polarity of Function (Role vs. Actor). This polarity deals with how you as an individual interact with the larger world around you, and is made of the Medium Coeli (Midheaven) and the Imum Coeli (Undersky), which describe how you fit into larger society and how you as an individual fit into your own life.
The Medium Coeli is the Angle highest in the sky at the time of the chart, and so is also called Midheaven. This Angle represents your role in the world at large, and your utmost public face (not the Ascendant mask you wear, which appears even to close friends; this is the public self that even strangers can know, such as your internet profiles). It also represents how you fit into society as a whole, including your job, social circles, social class, race, gender, among other things.
The Imum Coeli is the Angle lowest in the sky at the time of the chart, and is also called the Undersky, as it was not visible during the time of the chart. It lies directly opposite the Medium Coeli, and represents your “backstage” and role, and corresponds to your private self, and how you express yourself outside of the public eye. It represents the emotions, feelings, and thoughts of you as an actor in the great social game, playing the role of the Medium Coeli.