For advanced students and professionals
THE WHEEL CHART
In the wheel chart, the planets are placed in their zodiac positions. The zodiac stretches in a circle through all 360 degrees of the surrounding sky, and this circle is divided into sections of 30 degrees — the familiar 12 signs. Planet positions are measured within signs by degrees, minutes, and seconds of circular arc. Each degree contains 60 minutes of arc and each minute of arc contains 60 seconds of arc. For example, I might tell you that my Moon (Moon at birth) is in the sign Taurus (2nd sign). More exactly it is in the 23rd degree of Taurus. It is actually at 23 degrees and 28 minutes of the sign Taurus.
The chart wheel is usually divided into 12 sections called houses. The houses are numbered (counterclockwise) from 1 to 12. The pie-like lines that divide one house from the next are called house cusps. For example, the cusp of the 1st house is the horizontal line on the left-hand (east) side of the wheel.
Keep in mind that our Earth makes a complete turn on its axis once in 24 hours. The chart wheel represents the space surrounding where we were born. It is as if we were standing outside with the sky above us (upper part of chart) and the Earth beneath our feet (lower hemisphere). As the Earth turns, it brings each of the 360 degrees of the zodiac overhead (or to any part of the wheel) once in 24 hours — a new degree of the zodiac every four minutes.
As you can see, the symbols for the planets have been placed in the part of the sky (the house) in which they were at the time of your birth. For example, if you were born in the daytime, then the symbol for the Sun will be somewhere in the upper half of the wheel. If you were born at noon, the sun would be almost right overhead or at the top of the chart, while if you were born at midnight it would be at the bottom (4th house) of the chart.
The wheel is also seen as two hemispheres — a top and a bottom. The upper part of the chart (houses 7-12) represents that part of the sky that was overhead and above the horizon at the time of our birth. It has to do with the world of thoughts, ideas, ideals, and planning. The lower hemisphere (houses 1-6) marks that part of the heavens that we have under us — and cannot see — below the horizon and on the other side of the Earth from us. It has to do with experiences, embodiments, incarnation, and so forth. In other word, the sky above and the Earth below.
(This is the text from the supplement of terms and concepts included with the Astro*Talk Profile , created from the author, Michael Erlewine.)