This week, build an astrolabe’s main body—mater—with common rings for the front outer limb
Mater: Front Limb
Within the main body of the astrolabe, known as the mater, the tympan plates are secured in place with a pin through the center. Unlike the plates, which vary based on latitude, the mater surrounding the plates remains uniform across all latitudes.
The entire center of the mater is a circular depression known as the “womb” where the tympan plates and rete are securely fastened in by a pin. While the womb is hidden below the plates, it is a common place for a creative instrument maker to include engravings.
Often, the engravings are pragmatic, displaying mathematical diagrams to calculate sine, cosine, and tangent. Islamic astrolabes often contain an engraved gazetteer, a list of cities and their associated latitude and longitude coordinates. These coordinates can be used to determine the direction towards the Kaaba in the heart of Mecca—the qibla—which all Muslims turn towards during their daily prayers. The practical importance means Islamic astrolabe makers will often also include the inclination of the azimuth of the qibla (the inḥirāf) or the direction of the azimuth of the qibla with reference to the four cardinal directions—northeast, southeast, southwest, south of the qibla, and north of the qibla—known as jihat. Combined with a compass, these calculations can be used to determine the direction a Muslim should turn towards during prayer and other various religious obligations.
For other instrument makers, the womb can be a place for creativity, and might be extravagantly decorated but, just as often, it is left unornamented.
Raised along the outer edge of the mater lies the front limb. The limb can be engraved with any number of concentric rings. However, the most common are for equal hours and various rings to measure the altitude—height—of stars which can, in turn, be used to determine time.
The front limb encircles the womb (excluding the throne at the top of the astrolabe).
Altitude 360°:
The simplest marks on the limb split the ring into 360° with one tick mark for each degree.
Above the tympan plates, a ruler is held in place by the top pin. The rotating ruler can be aligned on the face of the astrolabe to point along the outer limb. While holding an astrolabe, an observer can then rotate the ruler until it aligns with a star in the sky, resting the ruler anywhere along the outer altitude circle.
Altitude (5°):
Sometimes, the granular nature of 360° can be difficult to use, whether simply from user error or the result of low light. So, it is common to also have a ring for altitude where it is measured either every 10°, or (as below) every 5°.
Equal Hours:
Clocks split the day into 24 equal hours. As opposed to unequal hours—where the day’s length depends on how much sunlight a region has—equal hours are always the same length. Equal hours measure how long it takes for the Earth to complete one full rotation along its axis, from midnight to midnight the next day. On Earth, that is 24 hours (give or take).
Along the front limb, equal hours split the full 360° ring into 24 equal parts, so each segment is 15°.
In the northern hemisphere, the numbering follows a clockwise order from 1 to 24.
The direction (either clockwise or counterclockwise) for the ordering of equal hours depends on the hemisphere in which the astrolabe is made. In the northern hemisphere, the sun rises in the east and sets in the west, but will travel through the southern portion of the sky.
Clockwise is defined by the movement of a sundial’s shadow as the sun rises and sets in the northern hemisphere. A sundial in the northern hemisphere is positioned to point to the north. In the morning, the sun rises in the east, casting a shadow in the west.
As the day progresses, the shadow will move right (clockwise), until it points north at noon when the sun is at the highest point in the sky.
Until the evening when the sun finally sets, casting the shadow to the east.
A sundial can only cast a shadow during the day, but the clockwise direction can be extrapolated for the full 24 hour day, eventually forming a complete circle. Equal hours are defined by each hour in the day lasting for the same length of time, so each segment is of equal size.
Sometimes, astrolabes will split the day and night periods into two ranges from 1 to 12.
For both historic and aesthetic reasons, equal hours are often depicted with roman numerals instead of Hindu-Arabic numerals.
Combined together, each ring of the front limb is raised along the outer edge of the mater and forms the main body of the astrolabe.
Southern Hemisphere:
In the Southern Hemisphere, equal hours are measured counterclockwise (or anticlockwise). This is a quirk of history, but also the result of astronomy. While the sun still rises in the east and sets in the west in the southern hemisphere, as it moves, it will appear to move in the northern, instead of southern, portion of the sky.
A sundial in the southern hemisphere is positioned to point to the south. So, in the morning, while the sun rises in the east, the shadow will be cast in the west.
However, as the day progresses, the sundial will move right, but now counterclockwise, and point south at noon.
And, finally, the shadow will rest in the east as the sun sets in the west.
The movement of a sundial’s shadow is reversed in the southern hemisphere, instead moving counterclockwise. As a result, the numbering for equal hours are also rearranged in counterclockwise order.
Equal hours can still be split into day and night 1 to 12 hour portions, but they are still engraved in counterclockwise order.
The same remains true for equal hours engraved as roman numerals in the southern hemisphere.
A complete front limb for an astrolabe in the southern hemisphere will look very similar to an astrolabe in the northern hemisphere. The only difference will be the counterclockwise ordering of the outer ring for equal hours.
