1. A
TREATISE ON THE
ASTROLABE

2. The Treatise on the Astrolabe is incomplete.
Sources & Manuscripts
Aside from The Canterbury Tales, The Treatise on the Astrolabe has the most surviving manuscripts of Chaucer’s work, amounting to 34.
The Treatise on the Astrolabe is incomplete.
Some versions appear to have been altered slightly by scribes, perhaps because of a desire to make it more scientific and organized.
Although the original manuscript has not been ascertained, the oldest versions are believed to have been the versions that contain many diagrams

3. “Little Lewis, my son, I have prepared a lesson on the astrolabe for you, because I see that you are interested in science. The reasons I have prepared my own treatise in English on the subject are as follows:”
According to the MED, tretise has a variety of meanings. For the purpose of our presentation, I think the most relevant meaning would be “a work of instructional or informative character”. It does seem a bit formal to call it a treatise, especially since it is directed at his son. I wondered why “lesson” wasn’t used so I looked that up in the MED and “lessoun” is clearly derived from French with the specific meaning of something to be read, studied, and learned by a student. It also has the added meaning of a sermon or other kind guidance as to prevent certain kinds of behavior. (Kevin)

4. No one in our region of England thoroughly understands the astrolabe;
I have found mistakes in other manuscripts about the astrolabe;
Your Latin is not quite good enough to be reading complex material in that language, and lastly,
You are only ten years old, and a simpler treatise written in plain English will help you understand the astrolabe better.

5. “My son Lewis, as a philisopher once said, “he wrappeth him in his frend, that condescendeth to the rightful preyers of his frend,” or in other words, “a good friend helps out his friend” which is what I am trying to do by giving you this treatise.”

6. “Don’t be envious of my knowledge of science; all that I have written here is not my own work, but compiled from other manuscripts. But considere wel, that I ne usurpe nat to have founde this werk of my labour of olde Astologiens, and have hit translated in myn English only for thy doctrine; and with this swerd shal I sleen envye.
The treatise has 5 parts:
1. A general description of the astrolabe
2. How to use it
3. Charts concerning latitude and longitude
4. Charts concerning the movements of celestial bodies
5. A general introduction into the theory of astrology
So far, I have only the first two parts; I will give you the others in time.”

7. PART I Here begins the description of your astrolabe:
The astrolabe has a hole in it, with which to put your thumb through, for measuring height, or, as I will from now on call it, “altitude”.
The ring can also go through a chain, from which the astrolabe hangs down from.
The astrolabe consists of plates piled on top of each other.
A line crosses the astrolabe: the upper part is called the south line or meridionel, and the lower part is called the north line or the line of midnight.

8. 5. There is also a line running down, creating a cross like this: 6
5. There is also a line running down, creating a cross like this: 6. The left side is west, and the right side is east. 7. The astrolabe is divided into 90 degrees.
8. The astrolabe is divided into the 12 zodiac signs.
9. The astrolabe is also divided into 365 days.

9. 10-11. The astrolabe is also marked with months of the year
The astrolabe is also marked with months of the year. These are mostly named after Lords of Rome. Julius Caesar, for example, stole 2 days from February, which has 28 days, to put into his month of July. Augustus Caesar has 31 days in his month of August, too.
Augustus
Julius

10. 12. Next to the A B C lines, there is a scale made up of two squares that is divided into 12 points. The top part is called the Umbra Versa and the bottom part is the Umbra Recta (or Extensa). 13.The alidade (see photo) can be used to determine the sun’s position during the day or the stars’ positions at night. 14. A pin holds the rete and plates to the mater. It acts like the North Pole of the Astrolabe.

11. 15. The womb side of the Astrolabe into four quarters by a cross, just like the other side. 16. The womb side is divided exactly the same way as the other side (see number 7). Chaucer makes it clear that he has said this twice “Now have I told the twyes” 17. Under the rete, the plate is engraved with three circles: Cancer, Aries/Libra, and Capricorn. These three circles or events are important because: Cancer is the summer solstice; Aries/Libra are the equinoxes (where the division between day and night are the same); and Capricorn which is the winter solstice.

12. 18. There are concentric circles engraved on the astrolabe called almuncantars (on diagram Altitude arcs). The zenith is the center of the smallest circle and should be the point directly above your head.

13. 19. Azimuths make right angles with the zeniths
19. Azimuths make right angles with the zeniths. They can be used to find the zenith of the sun or any other star. 20. Twelve divisions under the azimuths indicate planetary hours. 21. The zodiac plays an important role in using the astrolabe. Zodia is a Greek word meaning “beasts” in Latin. Either the sun takes on the characteristics of the beasts when it enters the signs, or the stars are arranged like the beasts, or the planets take on the beastly characteristics when they move through the signs. There can be other effects felt as the planets move through the signs (for example, if a hot planet moves through a hot sign or a cold planet moves through cold sign). Each of the twelve signs also governs parts of the body (Aries=head, Taurus=throat, Gemini= arms and armpits, etc).

15. HERE BEGINS THE INSTRUCTIONS ON THE USE OF THE ASTROLABE
PART II
HERE BEGINS THE INSTRUCTIONS ON THE USE OF THE ASTROLABE
By aligning the hands of the astrolabe with the date and month, you can find the longitude of the sun.
By aligning the holes in your astrolabe with the sun’s light, the direction in which the rays flow through will tell you the altitude of the sun.
You may find the time of day through the sunlight or fixed stars or the degree of the stars ascending on the eastern horizon.

16. 4. The rising star, or ascendent, is very auspicious
4. The rising star, or ascendent, is very auspicious. The alignment of the stars and planets plays an important part in a person’s birth, depending on the fortune of the planet in the ascendent The astrolabe can also be used to find the position of the sun if it is between two almucantors, or celestial spheres It can also be used to determine the time at daybreak or twilight.

17. 7-8. The astrolabe can also figure the length of day, or “artificial day”, from the time of sunrise to the sunset. It can also figure the equal lengths of hours in a year The astrolabe can also figure the “vulgar day,” that is, the length of time between daybreak and twilight. The same manere maystow worke, to knowe the quantitee of the vulgar night. And for the more declaracioun, lo here the figure:

18. National Library of Wales