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A
TREATISE ON THE
ASTROLABE
“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 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.”
PART I
Here 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.
Julius
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.
Augustus
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).
Form and Dates
• The Treatise is written in verse form. It
contains an introduction and two parts in
numbered sections. It is incomplete in that
it was intended to contain 5 parts in total.
• The Treatise on the Astrolabe is dated to
around 1391. This makes it one of
Chaucer’s latest works.
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 relevant
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.
PART II
HERE 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
“And for the more declaracioun, lo here the figure:”
National Library of Wales
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.