A

TREATISE ON THE

ASTROLABE

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

•The Treatise on the Astrolabe is similar to The Man of Law’s Tale, because both emphasize structure,

seasons, weather, global location of things (longitude & latitude, and the concept of time.

* The Treatise is very socially and culturally relevent because it was a scientific document written

specifically for the instruction of those not familiar with academia (his ten-year-old son Lewis) and was

the first known technical manual in the English

language.

“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:”

1.No one in our region of England thoroughly understands the astrolabe;

2.I have found mistakes in other manuscripts about the astrolabe;

3.Your Latin is not quite good enough to be reading complex material in that language, and lastly,

4.You are only ten years old, and a simpler treatise written in plain English will help you understand the astrolabe better.

“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.”

“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 it3. Charts concerning latitude and

longitude4. Charts concerning the movements of

celestial bodies5. A general introduction into the theory

of astrologySo far, I have only the first two parts; I

will give you the others in time.”

PART IHere begins the description of your astrolabe:

1. 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”.

2. The ring can also go through a chain, from which the astrolabe hangs down from.

3. The astrolabe consists of plates piled on top of each other.

4. 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.

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.

10-11. 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.

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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.

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.

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.

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).

PART IIHERE BEGINS THE INSTRUCTIONS ON

THE USE OF THE ASTROLABE

1. By aligning the hands of the astrolabe with the date and month, you can find the longitude of the sun.

2. 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.

3. You may find the time of day through the sunlight or fixed stars or the degree of the stars ascending on the eastern horizon.

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.

5. The astrolabe can also be used to find the position of the sun if it is between two

almucantors, or celestial spheres.

6. It can also be used to determine the time

at daybreak or twilight.

7-12. The astrolabe can also figure the length of day, or “artificial day”, from the time of sunrise to

the sunset. The astrolabe can also figure the “vulgar day,” that is, the length of time between

daybreak and twilight.

It can also figure the equal lengths of hours in a year. It can tell the time, and the planetary hours

National Library of Wales

“And for the more declaracioun, lo here the figure:”

13. It can also figure the sun’s noon altitude, also

called the meridien altitude.

14. And to find the degree of the sun using the rete, or inscribed mark on the

astrolabe; 15. To find which days have the same length;

16-19. To find the coordinates of any star,

alone or in relation to other stars, or the movement of

any zodiac from the equator, or to find latitude.