| Updated: 24 april 2001 | To the Software table of contents |
| Introduction 1 2 3 4 5 6 7 8 (reversal 1) 9 10 11 12 (reversal 2) 13 14 15 (summing up) | |
Three spheres are used to define the ascendant and MC:
The earth and its axis and equator (shades of grey in the
picture). It is the rotation of the earth around its axis that makes the MC and
ascendant move through the zodiac. The rotation is counter-clockwise, i.e.
from west to east.
The plane of the ecliptic and its axis. It is titled
approximately 23°26.5' from the earth's axis. It is this plane that is
divided into the twelve zodiac signs.
The heavens as seen by the observer on a specific point on
the earth (shades of red in the picture). The inclination of the plane of
the horizon depends on the geographical latitude of the observer. This
picture is drawn for 70° northern latitud.
As the earth rotate, the plane of the horizon will rotate and intersect the
plane of the ecliptic at different points. These points are the ascendant
and descendant.
The picture show the position of the plane of the horizon at the sidereal time 6:00:00. This plane intersects the eclitic at 0° Aries and 0° Libra. The rotation of earth will cause 0° Libra to rise above the horizon. Thus 0° Libra is the ascendant and 0° Aries the descendant. At this time the ascendant is exactly to the east. It will again be exactly to the east when it's 0° Aries. At other times it will be somewhere between south to north on the eastern side of the meridian circle.
The culminating degree is one of the two intersections of the meridian circle with the ecliptic. At this sidereal time 0° Cancer and 0° Capricorn are intersected by the meridian circle. The MC is 0° Cancer because it is above the horizon. On sub-polar latitudes it will always be exactly to the south (on the northern hemisphere).
The chart below is erected for a place at 70°00' northern latitude when the sidereal time is 6:00:00. Depending on the software you use the chart might have to be calculated for a time differing some seconds from 12:56:45 CET (11:56:45 UT).
The ascendant and the MC behave as usual: they are moving counter-clockwise through the zodiac.
The house system of Alcabitius is used because it works in polar regions, while many other systems collapse at geographical latitudes north of the northern arctic circle and south of the southern arctic circle.