This week, break apart an astrolabe to begin to understand this strange ancient device
The astrolabe remains possibly the most iconic instrument in the history of science for both its beauty and elegant functionality. At first glance, this strange flat device can seem incomprehensible. Somehow this device can tell time, aid in navigation, and even perform mathematical calculations with rings of numbers and some simple parts that can rotate in place. Compared to the telescope or even a sextant, the astrolabe requires more training to properly use.
The complexity of the astrolabe has largely relegated it to being a piece of art, mysterious and unknowable. However, the intricacy belies a treasure trove of complex and layered information. The best way to understand an astrolabe is to build the device up, piece by piece, to peel back the layers of information overlaid on top of itself. Could you read Shakespeare if it had been accidentally printed all on one page?
In addition to its visual elegance, the underlying math sets the stage for much of the pre-1800’s astronomy and navigation, including spherical trigonometry and the use of the relative positions of stars to determine position and time.
A casual observation of the night sky will reveal that over time the stars move in long arcs, rotating around a fixed point. In the course of a year, constellations like Orion will be high in the sky in one season and hidden away under the horizon in others. The stars themselves are not actually moving, instead the Earth is rotating beneath them. It takes 365 days for the Earth to fully orbit the Sun and 24 hours to complete a full rotation around its own axis. On a side note, for the extremely curious, the true length of a day—known as the sidereal day—is closer to 23 hour and about 56 minutes and the true length of a year—known as the sidereal year—is 365 days, 6 hours, and about 8 minutes since astronomy is actually messy. However, both the daily rotation and the annual rotation of the Earth are repetitive and as a result, predictable.
The position of the Earth during the year, during a given day, and the position of an observer on the planet will all impact what is visible in the night sky. Some stars are entirely hidden by the planet at all times of the year and some stars will always be visible. In the Northern Hemisphere, the Big Dipper is always visible, but its exact position depends on the time of day and year. In contrast, constellations like Gemini are considered winter constellations since they rise above the horizon and are only visible in the winter months.
Because of the sheer distance to the stars, they would appear almost perfectly fixed in their absolute position in the sky if the Earth did not rotate. Any change in their position (known as proper motion) is incredibly small, and can only be appreciated over thousands of years. So, the apparent position of a star like Vega will depend on the location of the observer (e.g. the city of Boston at 42.36° N, 71.06° W) and the exact date and time (e.g. January 19th at 3:14 AM).
At its core, an astrolabe is essentially a clock of the stars. It takes the position of the stars in the sky and translates them into a recognizable clock and calendar. Astrolabe-like devices were invented around the time of the ancient Greek astronomer Ptolemy in the 2nd century CE, but remained the steadfast companion of astronomers, engineers, and navigators for hundreds of years. Astrolabes could even be found aiding sailors well into the 18th century. An exceptionally versatile device, the 10th century Iranian astronomer al-Sufi wrote of a thousand possible uses for the astrolabe from astronomy to time keeping to navigation and surveying as well as serving dozens of religious purposes.
The information on an individual astrolabe can vary depending on the needs of its maker. However, for the sake of simplicity, the most astronomically significant pieces are: a “tympan” plate for a specific latitude, a “rule” to measure the declination of stars, a rotating “rete” or star chart to display the position of stars, an “alidade” to take the position of stars, and a decorative “horse” and “pin” to hold the pieces in place on the main body of the “mater”. The stationary mater serves the dual purpose of holding the tympan plate and rete in place as well as being engraved along the edge and back with important astronomical information.
An astrolabe is designed to describe a flat model of night sky with the rotating rete representing the daily rotation of stars. From the known position of a star, an observer can infer the exact time and, conversely, from an exact time, an observer can infer the exact position of a star.
To best understand how to use and construct an astrolabe, let’s first review some vocabulary.
Latitude is the distance from the North or South Pole at the top and bottom of the Earth and can be imagined as a horizontal ring around the Earth. The equator is a ring of latitude that runs around the center of the Earth and sits at 0° latitude. Any position north of the equator is considered positive latitude and any position south of the equator is considered negative reaching 90° at each of the poles. For example, the Tropic of Cancer is the latitude position 23.4° (sometimes written as 23.4°N and +23.4°) and the Tropic of Capricorn is in the south at -23.4° (also written as 23.4°S). The tropics represent the furthest position—north and south—that the Sun will appear directly above an observer at some time during the year. The tilt of the planet will cause the position of the Sun to gradually shift north and south along the Earth throughout the year. When the Sun is directly above the Tropic of Cancer in the Northern Hemisphere, around June 21st, this represents the summer solstice and is the longest day of the year in the Northern Hemisphere. Around December 21st, on the winter solstice, the Sun will reach the southernmost latitude position above the Tropic of Capricorn and have the shortest amount of daylight hours and longest night. The tropics are the direct result of the tilt of the Earth and a change in the tilt will shift the position of the tropics.
Longitude is the distance—east and west—from the prime meridian, a great circle running through the poles and Greenwich, England and is considered 0° longitude. Unlike the physical position of the North and South Pole on the top and bottom of the Earth, the prime meridian is an arbitrary position on the planet that has been agreed upon by mildly disgruntled consensus. Prior to the adoption of Greenwich in 1884, most countries had maps with a prime meridian that ran through their own capitals. For decades after its official adoption, France still stubbornly used the Paris meridian which required navigators using the Paris meridian and the English Nautical Almanacs to adjust all calculations to account for the 2° differences.
The ecliptic plane is the flat plane of the orbit around the Sun and from the perspective on Earth determines the apparent path that the Sun makes through the sky during the year. The Sun will appear higher in the sky as summer approaches and lower as winter looms. If the Earth had no obliquity and was aligned with the ecliptic plane, the Sun would always rise to the same height in the sky, regardless of the time of year, but instead, the Earth is tilted along its own axis. The axial tilt along the ecliptic plane is known as the obliquity and can vary (because, again, astronomy is messy) but for the sake of simplicity is 23.4°.
The celestial sphere is an imaginary sphere with an infinite radius centered on an observer on Earth. Objects in the sky, like stars, lie on the surface of the sphere. Distance along the celestial sphere is measured as an angular distance and angular distance measures the curved distance along the surface of a sphere.
Along the celestial sphere, declination is a celestial latitude where the sphere, with Earth at its center, is vertically split into degrees. 0° declination is the celestial equator and +90° and -90° represent the North and South Celestial Poles. Right ascension is a celestial longitude where the celestial sphere is horizontally split. Rather than being measured in degrees, right ascension is typically measured in hours, minutes, and seconds. The 360° full rotation of the Earth is converted into a 24 hour “day” where one hour of right ascension is a rotation of 15°.
Some astronomical designations are relative to an observer rather than a fixed position in space. The zenith and nadir are the points directly above and below an observer respectively. Altitude is the height from the observer to a point in the sky. Azimuth is the distance—north and south—from a point relative to the observer. The meridian of an observer crosses vertically above and through the zenith and nadir. The almucantar are circles that run horizontally and are parallel to the horizon and are centered on the observer. The almucantar circle at 0° is considered the horizon.
An astrolabe works by squashing the celestial sphere, peppered with stars in the night sky, onto a flat circle. In the process, some information could be lost. The distance and angle between two points in the sphere will not immediately match the angle and distance on the flattened sphere. To account for this, astronomers and artists employed spherical projection.
Spherical projection involves constructing circles along projected points to ensure that arc length and angles between positions in a sphere are maintained. Since what is visible in a night sky depends on an observer’s location, each latitude requires a unique plate to represent its distinctive night sky. It is common for an astrolabe to be constructed with a few different plates that can be traded out through a voyage. Part of the purpose of the horse and pin is to allow for the astrolabe to be easily disassembled to swap in a new plate.
An astrolabe is made up of many pieces that all slot together in a synergistic elegance.
The rule, sometimes also called a ruler, is a rotating piece on the face of the astrolabe. The lines across the rule represent declination in the sky. It is used to match the observed declination of stars in the sky along the edge of the astrolabe.
The rete is a rotating piece on the face of the astrolabe that represents the position of stars. Rete is latin for net and is also sometimes referred to as a star chart or star net. The rete is cut away—often in beautiful and elaborate ways—to allow the plate beneath to be more visible. The pointers along the rete indicate the positions of bright stars in the night sky that will be used to determine time. Within the rete, a small offset circle represents the ecliptic to track the path the Sun takes across the sky throughout the year.
Tympan plates, also known as latitude plates, represent the sky above a specific latitude. The lines across the face are layered with information about the arcs for the almucantar, azimuth, and horizon in a grid.
The mater is latin for mother and is the body of the astrolabe that holds all the pieces on the face of the device. The raised rim along the edge of the mater is the limb and is split into 24 sections every 15° to represent the right ascension and 360 marks for the full 360°. The hollowed-out center of the mater is known as the womb and securely holds the tympan plates in place.
On the back of the astrolabe, the alidade is a long rotating bar. A scale of 360° runs along the outer limb to measure the altitude of stars using the two small holes in the rotating alidade to sight stars.
Horse and Pin
The horse sits on the face of the astrolabe and connects to the pin on the back to hold all the parts together. These pieces can be simple, but are often an opportunity for artistic creativity. The horse got its name because the piece is commonly designed to look like one.
The front of the astrolabe is a combination of the mater with an engraved raised limb and the face of the tympan plate.
The decorated throne holds the ring that the astrolabe is suspended from. The throne connects the ring to the body of the astrolabe. Often the throne is a place for elaborate designs and art, but its main purpose is to hold a ring in line with the meridian so the astrolabe can be suspended in the air by the observer to study the stars.
A circle is 360° degrees and represents the complete rotation of a day and complete rotation of a year. Along the front limb of the astrolabe, the 24 equal hours (0-23) represent the value of the right ascension measured in hours, minutes, and seconds.
The position in the night sky around which the stars appear to rotate are the celestial poles, in the north and south. For an observer in the Northern Hemisphere, the North Celestial Pole is the top almucantar circle at 90°.
The observer’s meridian is an imaginary circle that passes through both the northern and southern celestial pole as well as the observer’s zenith and nadir.
The azimuth arcs are a series of vertical circles that all run through the North and South Pole and represent the horizontal angular distance measured clockwise from the observer’s meridian. The azimuth is the horizontal direction of the star.
The almucantar arcs are horizontal circles that run parallel to the horizon from an observer’s perspective. The almucantar represents the vertical direction of a star so if two stars rest on the same almucantar then they have the same altitude.
From an observer’s perspective the horizon is where the Earth meets the sky and is a 0° almucantar arc.
Unequal Hours Arcs
Also known as seasonal hours since while the Earth is taking a full 24 hours to rotate, the number of daylight and nighttime hours can vary depending on an observer’s latitude.
The back of the astrolabe contains alidade and scales for measuring the altitude of stars and converting that position into a date and time.
The year can be split into a calendar year with months and days by accounting for a small offset from the center of the astrolabe. The offset can be used to determine how the Gregorian calendar and zodiac calendar are split along the outer circle.
Zodiac houses split the ecliptic every 30° into 12 equal parts throughout the year: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces.
Calendar Months and 365 Days
A full orbit around the Sun takes the Earth about 365 days which divides the year into days which are each slightly less than a degree.