The Musée des Arts et Métiers in Paris has a large collection of astrolabes and these are rare as “hens teeth”. Today, we do not really understand the emphasis on astrology and astronomy in the past, but put simply, the heavens were considered quite literally heaven. The association of the phases of the moon and stars with tides, history and even menstrual periods was not lost on the ancients. They responded with astrology, an attempt to link the stars with events on earth. It reflected a worldview based on a highly esteemed Greek precedent and only lost credence during the “Age of Enlightenment” in the 18th century, giving way to our modern scientific, evidence based, worldview. The beautiful astrolabe shown above was made in Louvain, Netherlands in 1565 in the Arsenius workshop signed Regnerus Arsenius. An important university founded in 1423 helped to create the climate where the instrument workshop was founded in about 1530 and flourished through much of the century, directed at first by the cartographer Gerard Mercator (all modern maps use a “Mercator Projection”) in association with the mathematician and physician Gemma Frisius (famous for his Gemma rings and other things), and later by Gemma's 'nephew' Gualterus Arsenius. You can read more about this instrument here, it took about a year to make, it was delicate, complicated and expensive. There are only about 25 Arsenius astrolabes in the world, the one below is from the Galileo Museum in Florence, Italy.
An astrolabe is a two-dimensional model of the celestial sphere. The name has its origins from the Greek words astron and lambanien meaning “the one who catches the heavenly bodies”. An astrolabe is an instrument that once was the most used, multipurpose astronomical instrument. In the picture above you can see the scales more clearly, this is a planispheric astrolabe (only works at one latitude), designed not only to measure the altitude of the stars and planets and to calculate the time according to the position of the sun, but also to foresee the position of the stars and planets on a specific day of the year at a specific time for astrological purposes. The piece has a main body, called a mater, where a fixed latitude plate is placed with a projection of the celestial sphere on the equatorial plane at that latitude. The pointer or “rete” rotates on this ring, pointing with small, artistically designed spikes to the position of the biggest stars. A rule with sights, the alidade, is included to enable measurement of elevations of celestial bodies. An ecleptic circle marked with astrological signs shows the position of planets in different sectors of the sky. These astrolabes would often be supplied with several plates for every 5 degrees of latitude.
The time lapse photo above of the night sky really shows how the stars move and how the astrolabe mimics their movement. The center of all those rings is the north star, the angle between straight up and the north star is your current lattitude, in this case about 55 degrees. Similarly, on the astrolabe shown above, the latitude for the instrument is half way between the observer and the pivot point of the “rete”, meaning it is designed for about a 45 degree lattitude. Thus the rotation of the rete marked with positions of stars mimics the rotation of the actual stars.
The large picture above is the back of an Arsinius astrolabe which shows a universal astrolabe, first accurately described in 1550 by Juan de Rojas, because it can be used at different longitudes. In the Rojas version (to the right), the mater carries the equatorial-coordinates grid and the ecliptic, in the form of a diameter inclined to the celestial equator. The rete is replaced by a graduated alidade fitted with a graduated perpendicular cursor (picture to the right). These really never caught on, being hard to use and not very intuitive.
The astrolabe was widely used in Europe in the late Middle Ages and Renaissance, peaking in popularity in the 15th and 16th centuries, and was one of the basic astronomical education tools. A knowledge of astronomy was considered to be fundamental in education and skill in the use of the astrolabe was a sign of proper breeding and education. Their primary use was, however, astrological. Geoffrey Chaucer thought it was important for his son to understand how to use an astrolabe, and his 1391 treatise on the astrolabe demonstrates a high level of astronomical knowledge. Heloise and Abalard (see my post on Père Lachaise Cemetary) had a son they named Astrolabe in honor of the Arabic knowlege of astronomy.
Astrolabe made in 1460-1465 in Germany, used to teach astronomy.
Astronomical rings also known as Gemma's rings (published in 1534) are an another early astronomical instrument. The instrument above right made by Langlois consists of three rings, the meridien, the equator, and the declination ring. Above left made by Helye has only two rings with a central bar style. It may be considered to be a simplified, portable celestial sphere, or a more complex form of astrolabe. Additionally, the one on the upper right may be used as a sun ring and the one above right seems to have had a bar style that was removed. Both are from the mid 1700's.
This is a drop dead gorgeous Bourda's reflecting circle on a brass stand from 1777 (no 2), created by Etienne Lenoir. It is capable of astronomical angular measurements that could be used even today. A Bourda's circle is a form of a sextant but more versatile, see my post on navigation instruments for a description. When hired by Jean-Charles de Borda around 1772 to work on the reflecting circle, Lenoir was about thirty years old and nearly illiterate. However, his intelligence and mechanical genius allowed him to perform work that few others could perform. As a result of this work, he became known as the pre-eminent maker of instruments for astronomy, navigation and surveying in France.
All of this astronomy required lots of instruments to do calculations. To the right you can see a mathematical instrument kit from the 1700's. I would imagine a navigator for a large ship would have similar instruments.
The nautical astrolabes used to measure the altitude of the sun differed from those used to measure the altitude of the stars. Although both these instruments are called astrolabes, the nautical astrolabe hardly resembles the planispheric astrolabe; besides the complexity of their shapes, the only common parts are the mater with graduated limb, the suspension ring, and the alidade. The holes in the sighting devices were smaller and instead of looking through them, the pilots used to hang the astrolabe at waist height and turn the diametrical rule until the light of the sun passed through the two holes in the plates and projected onto the floor or other surface. The altitude of the sun was then measured using the graduated circular disk. This process was given the suggestive name of “weighing the sun”. It was by weighing the sun with nautical astrolabes that the Portuguese and Spanish mariners explored the Atlantic, Indian and Pacific oceans. There are only about 80 of these in the world, this one by Sancho Guttierez 1563. Initially carried on almost all the great ships (particularly Spanish) from the fifteenth to the seventeenth centuries, the nautical astrolabe gave way instruments that were easier to handle and more precise.
Sancho Guttierez became official cartographer for the king starting in 1553 until 1574 when he died. His father Diego Guttierez was the official Spanish cartographer and instrument maker for the Spanish crown from 1534-1554. This was an incredibly lucrative business and a monopoly for the family which lasted until the death of Sancho in 1574.
This is a horizontal “double” sundial (it tells both time and the time of the year) from 1713 by Benjamin Scott. The thing sticking up is called a gnomon bar. Notice the scale on the plate used for determining the length of the shadow of the sun and hence the time of year. In simple terms, the noon sun is higher in the summer than winter and thus the winter shadow of the sun is longer than the summer shadow. This is the same principle as the Viking compass (see my post)
Although the Sun appears to rotate nearly uniformly about the Earth, it is not perfectly uniform, due to the ellipticity of the Earth's orbit (the fact that the Earth's orbit about the Sun is not perfectly circular) and the tilt (obliquity) of the Earth's rotational axis relative to the plane of its orbit. Therefore, sundials time varies from standard clock time. On four days of the year, the correction is effectively zero, but on others, it can be as much as a quarter-hour early or late. The amount of correction is described by the equation of time, shown in graphic form to the right. Sundial Noon is defined as when the Sun is directly overhead, not when a watch says it is noon. The difference here is the user's distance (east or west) from a time meridian. This difference can be as much as 30 minutes if you are exactly between two time meridians.
Prior to the invention of accurate clocks, in the mid-17th Century, sundials were the only timepieces in common use, and were considered to tell the “right” time. The Equation of Time was not used. After the invention of good clocks, sundials were still considered to be correct, and clocks usually incorrect. The Equation of Time was used in the opposite direction from today, to apply a correction to the time shown by a clock to make it agree with sundial time. Only after about 1800 was uncorrected clock time considered to be “right”, and sundial time usually “wrong”, so the Equation of Time became used as it is today, to correct sundials from clocks. The picture above is a sundial on the side of Waltham Abbey church.
The picture above shows a universal sun ring dial. It was likely invented by William Oughtred around 1600 and became common throughout Europe. The dial is suspended from the cord shown in the upper left; the suspension point on the vertical meridian ring can be changed to match the local latitude. The center bar is twisted until a sunray passes through the small hole and falls on the horizontal equatorial ring. This one was made by the famous London instrument maker John Dolland in the mid 1700's. Dolland was known for his 1757 invention which improved upon the achromatic objective lens by placing a concave flint glass lens between two convex crown glass lenses, thus seeing the rings of Saturn.
This multiple sundial is by Roch Blondeau from the the mid 17th century. Multiface dials have the advantage of receiving light (and, thus, telling time) at every hour of the day. However, they are usually not self-aligning, since their various dials generally use the same principle to tell time, that of a gnomon-style aligned with the Earth's axis of rotation. If two or more dials that operate on different principles are combined, as in this sundial, the resulting multiple dial becomes self-aligning. In other words, the direction of true North need not be determined; the dials are oriented correctly when they read the same time. This is a significant advantage in portable dials and small sun dials such as this one.
This is a portable universal horizontal sundial, with a compass built in for orientation. This was made by Nicolas Bion in the late 1700's. He was a French mathematician and maker of globes and scientific instruments. His official title was Ingénieur du Roi pour les instruments des mathematique. Author of two important works : L'Usage des globes celestes et terrestres, et des spheres, suivant les différents systèmes du monde [Paris 1699] and Traité de la construction et des principaux usages des instrumens de mathématiques [Paris 1709]. Both works proved highly popular and were translated into a number of other European languages.
This is a German portable sundial with a compass and a string made by Ernst Christoph Stockert from Bavaria in the early 1700's. Notice the string can go from a lattitude of 34 to 56, which covers most of Europe. Stockert was a well-known manufacturer living in Nurenberg. There still is a compass manufacturer called Stockert in Nuremberg. These little papered wood sundials were very popular but it is a misconception that they are self aligning, the two dials will read the same no matter which direction it is pointed.
This is a “Card Dial of Universal Height” by Regiomontanus from about 1600. It is possible to determine the time with this card independent of lattitude.
The scales nearest the top of the dial are an arrangement of lines for latitude and date. The scale of dates is drawn with a zodiac calendar, where the year is divided into 12 “houses,” or “signs,” of 30 degrees each. The little lever with the holes in it at the top is called a brachiolus and it supports a long string with a plumbob at the end and a bead in the middle. The area to the right in the above picture is a list of the 12 months with their zodiac signs, shown in more detail to the right.
The explanation of how to use this device is a little complicated, but really simple in the end. A weighted string (red arrow) with a friction-fit sliding bead (blue dot) is attached at the movable end of the brachiolus (green dot). To prepare the dial for use, the brachiolus is adjusted to set the string's suspension-point at the appropriate latitude and date (the green dot in the above diagram). In the above figure, the green dot is at about 47 degrees lattitude and set for early in the 3rd month, March. Next the string is swung over to the same date on the second scale of dates on the right side of the dial, and the bead (blue dot) is moved along the string to the date point (see above). We can see in the above figure the bead or blue dot is set early in the third month on the right hand scale. Keep in mind that the bead (blue dot) must be set on the 12 hour line, not on the line between the zodiacal signs. This is the way to set the length of the string between the brachiolus' end (green dot) and the bead (blue dot) for the date and latitude. The dial is now ready to be used.
Pick up the whole thing and align the top with the aid of the two little spikes with the sun (see above). The bead or blue dot will indicate the time, read at the bottom. You will see there are two sets of numbers, the top set is for PM and the bottom set for AM. In our diagram above it is either a little before 5 PM or a little after 7 AM. Although it looks and sounds complicated, in actual use it is pretty simple. As you might expect, Regiomontanus (1436-1476) was a child prodigy, he went to university at age 11 and graduated the next year. His real name was Johannes Müller von Königsberg and he was a German mathematician, astronomer, astrologer, translator, instrument maker and Catholic bishop. He was internationally famous for all these talents, yet died at age 40.
A pantochronometer is a horizontal magnetic sundial, also called magnetic sundial. The rotating card or pointer is automatically pointing to the magnetic pole. The term, “pantochronometer” meaning combined sundial & compass, was first used late 18th century. In practice the needle acts as the gnomon, casting a shadow on a calibrated bowl underneath. The problem with these is that magnetic north is slightly different than true north, so an adjustment had to be made which is seen in the top of the above picture. Additionally, they were very limited with respect to lattitude. This one is from the 18th century, made by Humand.
This one very cool, it is a moondial. A volvelle is basically a circular slide rule or analog calculator, often used in astrology and old time astronomy to calculate things. This volvelle, in conjunction with a sundial, can be used as a lunar clock to determine time at night. In use, the inner disc is set to show the current phase of the moon through the window. After determining the hour angle of the moon (with a separate sundial used as a moondial) and setting the lunar dial of the volvelle to that hour angle, one can find the location of the sun, hence ascertain the time at night. This instrument is different from the nocturnal that employs the pole star and selected others to determine time at night. These two-fold ivory sundials are called Dieppe-style because they were originally developed and mass produced by Charles Bloud in the northern French city of Dieppe (Normandy) from 1666 on. The one shown above is a Bloud sundial.
Another way to measure time at night is the nocturlabe. This one is from 1730 by A Hoevenar. A nocturnal or nocturlabe in French is an instrument used to determine the time based on the position of a certain star in the night sky relative to the north star. It is closely related to the sundial. A nocturnal is typically a navigational instrument. To use it, you first set the month and day with the inner dial. The most commonly used reference stars are the pointer stars from the Big Dipper (Ursa Major or Great Bear) or Kochab from the Little Dipper (Ursa Minor or Little Bear). In the nocturlabe shown above, you can use either, just choose the right pointer (LB or GB). I have uploaded this picture to Picasa and you can download it and make your own nocturlabe, it would make a great kids project. Just go to https://picasaweb.google.com/lh/photo/DAuB, cut out the pieces and glue to cardboard (use a crayon to color brown), use an eyelet found at any fabric store to hold the pieces together.
Then just line it up so you can see the north star through the hole in the middle and move the big arm so that it lines up with the chosen stars. Then just read the time off the inner scale. You can do the same thing with a full astrolabe or even a sextant but it requires some calculations and this little instrument does it for you. This method determines local time which is useful for determining what time it is where you are, but does not tell you anything about your longitude.
Since the Earth rotates at a steady rate of 360° per day, or 15° per hour, there is a direct relationship between time and longitude. If the navigator knew the time at a fixed reference point when some event occurred at the ship's location, the difference between the reference time and the apparent local time would give the ship's position relative to the fixed location. Finding apparent local time is relatively easy as we have shown above. The problem, ultimately, was how to determine the time at a distant reference point while on a ship. The concept of using a clock to measure longitude can be attributed to our now well known friend Gemma Frisius.
I am going to finish this up with pictures of three very cool clocks on display all with astrological features. The earliest really expensive clocks often had astronomical attributes added, like the phases of the moon. The watch shown above is a double faced Abraham-Louis Breguet from 1785 (see my post on chronometers). It has dials for hours, seconds, day of the week, day of the month, month of the year and something that goes from -15 to +15 which I can't figure out. The other side has astrological displays such as the phases of the moon and at least 7 other functions. What a beauty!
The next clock is a skeleton pendulum clock made by Jean-Simon Bourdier from about 1800. It too has hours, seconds, days of the week, days of the month and phases of the moon. Bourdier established his shop in Paris in 1805 and made a number of complex clocks; some with astronomies and equations as well as those fitted with flutes and singing birds. His clocks were housed in the finest and most elaborate cases suitable to furnish a number of royal palaces. They can be found all over Europe.
Finally this clock from the famous Antide Janvier dated 1805. Javier gained a reputation as a maker of ingenious and complicated clocks, including many astronomical clocks and clocks showing the tides. He eventually became Louis XVI's royal clockmaker. After the French Revolution he spent time in prison because of this royal association and then fell on hard times; his hardships were increased by the death of his wife in 1792. He sold his watches, equipment and designs to Abraham-Louis Breguet. This date of 1805 would make this clock a very unusual piece since it was after Janvier had sold off his business to Breguet. In any case, a beautiful clock with obvious astronomical features.
Well, there is a lot more but I am getting tired, I will continue the Musée des Arts et Métiers in another post on some very different subjects. Who knew there were so many kinds of sundials, and I didn't even present them all. As always, hope you enjoy reading as much as I enjoyed writing.
If you are really interested in astrolabes, sundials and compasses, and after writing this I must admit I am, try learning more in these links;
Museum of the History of Science: http://www.mhs.ox.ac.uk/
Museo Galileo; http://www.museogalileo.it/en/index.html
Sundial Primer (they have free paper kits for making sundials); http://www.mysundial.ca/tsp/tsp_index.html
Regiomontanus, Apian and Capuchin Sundials; http://www.dse.nl/~zonnewijzer/reg-cap.
At Home Astronomy (more home paper kits); http://cse.ssl.berkeley.edu/AtHomeAstronomy/index.html