Components of the Astrolabe
My astrolabe is composed of a double rule, rete, mater and alidade. On the mater I have drawn the Montreal sky (45°N, 73°W). This projection of the sky includes almucantars and azimuths lines. Almucantars represent the degrees above the horizon while azimuth lines show compass directions.
On the limb of the mater you can find the 24hours in a day written in roman numerals.
The rete illustrates the stars and constellations in the sky. The sun’s annual path, the ecliptic, is also on the rete; it is the off-center circle with the zodiac date. The rete can display the local view of the sky at any date and time in the year.
How to Use the Astrolabe
Finding the Zodiac Date
We can find the zodiac date by using the back of the astrolabe. By aligning the alidade to the day and month of the year, it then points to the equivalent Zodiac Date.
Finding the Location of a Star
The astrolabe must first be placed parallel and in front of your eye. Once the star, sun or object can be seen, the sights on the rule should be pointing towards the object. By keeping the rule in place and bringing the astrolabe back down, the location of the object can be read off of the azimuth and almucantar lines.
The location of the sun can also be found just with the date and time. If we align the rule with the zodiac date on the rete and the time of day on the limb, it will be pointing towards the sun’s angle and location.
Finding the Time
The time of day can be determined using the date and locate the angle of the sun (or a specific star). The date can be translated on the back of the mater and the angle of the sun or star can be read off the front with the rule pointing directly at it.
Next align the zodiac date on the rete with the location of the sun.
Finally, the rule can be aligned with the zodiac date on the rete. By doing so, the rete is pointing directly at the time located on the limb of the mater and the orientation of the rete shows the local sky.