Java applet page:
The traditional astrolabe shown here
can display the arcs which,
together with the horizon and the meridian,
divide the heavens into
the 12 astrological Mansions of Heaven.
The equinoctial, universal and spherical astrolabes
displayed with this program do not have this facility.
During medieval times,
it is probable that most astrolabes
were intended to be used for astrological purposes.
Many astronomers had no belief in astrology and said so,
but others found it profitable to know
how to use an astrolabe for astrological purposes.
Of greatest importance was knowing the positions of the Mansions of Heaven at the time of a significant event, primarily at the date and time of someone's birth. The Mansions of Heaven were also known as the Astrological Houses.
There have been four common ways to divide the heavens for this purpose. Of these, the two which can be displayed here on the traditional astrolabe show the Mansions according to Campanus and Regiomontanus.
Gazing at the sky, the division according to Campanus is the easiest to imagine. The prime meridian is the arc from the east position on the horizon via the zenith to the west position on the horizon. If you divide the prime meridian into six, and imagine an arc from the north position on the horizon through each of those (five) points to the south position, you have divided the sky into six of the 12 Campanus Mansions. The other six Mansions are beneath the horizon.
To visualise the Mansions according to Regiomontanus, you must first visualise the arc of the equator which, of course, is the arc followed by the Sun across the sky during the days of the two equinoxes. It passes from the east position on your horizon, via a point which is equal to your colatitude above the south position and on to the west position. (Your colatitude is equal to 90 degrees minus your latitude.) Divide this arc into six, and again imagine arcs through these (five) points from the north to the south positions on the horizon. Of course, if you live on the equator, the Sun moves from the east to the west along the prime meridian during the days of the equinoxes, so the division of the sky is exactly the same with both the Campanus and Regiomontanus methods.
At the bottom left of the button panel, there is a button marked 'Mansions'. If you click repeatedly on this button, you will first see four arcs appear across the astrolabe, and underneath the button you will see the word, 'Regiomontanus'. If you click on it again, you will instead see four different arcs, and underneath the button you will see the word, 'Campanus'. Another click will reveal arcs and wording for both 'Regiomontanus and Campanus', and a final click will hide the Mansion arcs, when there will be no words underneath the button. The cycle will then repeat. The "2" key on the keyboard has the same effect. There is also a menu item:
The horizon arc and the meridian (the vertical line from the North to the South through the zenith) were also Mansion divisions, making six lines and arcs in all. These arcs all crossed at the southern point of the horizon, and divided the sky into the 12 Houses or Mansions of Heaven.
Other astrological information could be found using the astrolabe. After setting the astrolabe for the date and time of interest, the position on the ecliptic circle where it crossed the eastern horizon was known as the ascendant and its crossing point with the western horizon was known as the descendant. Mid-heaven was the arc from the south to the north on the horizon via the Zenith, which on an astrolabe was the vertical line from the top down to the north point on the horizon arc. The position on the ecliptic circle where it crossed mid-heaven was known as the degree of mid-heaven.