Largest stainless steel astrolabe ever made is ready in Kerala, South India. It has a diameter of 36 cm, thickness 10mm. And weighing about 8 kilograms.
What is an Astrolabe?
Astrolabe is an ancient scienfic instrument used find the apparent positions of the Sun and other important stars in the sky for any time of the day or night throughout the year. It was used both to make observations and to carry out calculations and was the
most widely used scientific instrument in the middle ages and into the early modern period. An astrolabe can be thought of as a form of computer; a multi function calculator. Its most common usage were solving of Solving problems of geometry ,Converting between
timekeeping systems , Calculating trigonometric functions , Basic surveying , and much more concerning astronomy and time keeping, but the full range of its capabilities is much larger: Alchemists, astronomers, astrologers, and educated individuals were used astrolabes.
Use of the astrolabe would have been taught in the course of advanced classes in the natural Siences and mathematics, the way slide rules used to be, and high end calculators are now. Most of the surviving astrolabes are constructed of metal, preserved by their higher value and duability. But there are also several surviving examples from the medieval period of astrolabes constructed of both paper and wood (and sometimes paper laminated to wood), so that we can assume that cheap versions where not uncommon.
Basics of Astrolabes
Understanding the various functions of the astrolabe is made easier if you have an understanding of the concepts behind its design, how they are used in the astrolabe and how the various parts work:
The Celestial Sphere .
The sky seems to be a hemisphere turned down on us and two hemispherers of day and night can be imagined as a large sphere with the fixed stars attached to it, with the Earth, stationary and nonrotating, at the center. The Sun, the Moon, and the planets all had their own concentric spheres between the Earth and the stars. As the various spheres rotate about the earth, different parts of the sky would become visible to a viewer standing in a given spot. Like the Earth itself, this celestial sphere has many fixed landmarks allowing the observer to find his or her way around:
The Celestial Poles and Equator
At due north, at the point about which the sphere appears to rotate, is Polaris, the North Star, or Pole Star. This point marks celestial north. Opposite it, invisible to viewers in the northern hemisphere, is celestial south. Between these two extremes is a circle marking the celestial equator. As these all lay directly above their earthbound equivalents, you can think of these points the way you think of Earth’s north and south poles, and equator; and just as you can define your position on the Earth using your latitude and
longitude, positions on the celestial sphere can be similarly defined.
The Ecliptic
The most obvious object in the sky is the Sun. As the Earth rotates (or as it would have been explained in the days when the astrolabe was current technology, as the Sun rotates around the Earth), the sun rises, moves across the sky from East to West and sets. Over the course of the year the Sun seems to move across the fixed stars, oscillating from north to south and back again. This set path in the sky that the sun moves along is known as the ecliptic and is marked by the zodiac constellations.
The Tropics
As the Sun moves along the ecliptic, it moves toward the north as summer approaches, and back south as winter comes. The circle marking the northernmost point of the sun's path is known as the Tropic of Cancer and the southernmost the Tropic of Capricorn.
The Celestial Year
There are 4 fixed events in the year, defined by the sun’s motion along the ecliptic.
The Solstices
The solstices mark the longest and shortest days of the year, these are the dates when the sun is at its most northerly and southerly limits.
The Equinoxes
The ecliptic intersects the equator, on two days of the year. These dates are the spring and fall equinoxes. The spring equinox occurs when the sun moves from the southern hemisphere to the northern; and the reverse is true for the fall equinox.
The Local Sky
In addition to the celestial sphere, the astrolabes function relies on knowledge of the local sky from the point of view of the user’s location. When you look up at the sky you can see half of the celestial sphere; the other half being blocked by the ground you are standing on. What part of the sphere is visible depends on where you are and the time of day and the time of the
year.
The Horizon
The horizon is marked, rather obviously, by the line where the sky meets the ground. Of course, your local terrain, mountains and such will alter what can actually be seen, but for the purposes of this manual imagine it as a smooth line all the way round you.
The Zenith and Nadir
The point highest in your local sky (i.e. Directly overhead) is the Zenith; its opposite point, directly under your feet, is called the Nadir. The horizon then
lies 90 degrees from both.
Almucantars
An almucantar is defined as a line of equal elevation above the horizon. For example: Imagine a line that lays 15 degrees above your local horizon, all around the sky. That would be the 15degree almucantar.
The Meridian
The last major landmark we will concern ourselves with is the meridian . This is an imaginary line in the sky, passing from the north celestial pole to the south celestial pole, and passing directly over your head (zenith). This line marks local noon, the sun's highest point above the horizon for any given day. Picture it as a line in the sky running from due north to due south directly overhead.
The Projections If you have done any work with maps, you will be familiar with the concept of projections. The Earth is a sphere; so creating a flat map of the curved surface involves projecting that Sphere onto a flat surface. This is why, on some maps, the continents appear very distorte near the poles. The astrolabe is based on what is known as the planispheric projection. In a planispheric projection, a spherical object is projected onto a plane surface by placing the origin of the projection at one pole of the sphere and projecting the points of the sphere onto a plane surface placed through its equator. The astrolabe is based on what is known as the planispheric projection. In a planispheric projection, a spherical object is projected onto a plane surface by placing the origin of the projection at one
pole of the sphere and projecting the points of the sphere onto a plane surface placed through its equator. , With its projection of the local sky. The top half of the plate contains the circular grid that is the projection of the local sky from the horizon (the bottom
most curve of the grid) to the zenith (the cross at the center of the smallest circle). Again, the outermost circumference of the plate is the projection of the Tropic of Capricorn, and its center marks the projection of the celestial pole. If you examine these projections, you will
notice that in both cases the projection is oriented the same, with the north celestial pole projecting as a point in the exact center of the projection. This allows us to overlay the projections, the celestial sphere over the local sky, and pivot it on the celestial pole By rotating the rete the user can then display the local view of the sky for any combination of time and day of the year.
The Parts of the Astrolabe
The Mater
The Mater is the main fixed part of the astrolabe; all the other parts connect to it. Permanently fixed to it are the Throne and the Limb.
The Throne
The Throne is attached to the top of the mater, and provides a means of suspending the astrolabe to take sightings. In use, a ring or cord would be attached to the throne, allowing it to hang freely and so allow measuring angles from the horizon accurately. Depending on the time, place and use of the maker, the throne might be anything from a simple bulge, to an impressively ornate decoration almost as large as the rest of the astrolabe.
The Limb
The Limb is the raised ring section on the front of the mater; it encloses a space that contains the plates and the rete. It is commonly marked with the hours of the day and/or a degree scale.
The Plates or Climates An astrolabe is a very precise instrument, but its accuracy is tied to a specific latitude because the projection of the visible sky change with the viewer’s
latitude. In the example below we see plates (also known as climates) for latitude 20 degrees north, 45 degrees north and 65 degrees north; as you can see the projection on the plate changes radically.
The Rete
The Rete is a cutout overlay that rests on top of the plates. It shows the projection of the celestial sphere. Unlike the plates, the rete is designed to turn freely.
The Rule
The Rule rests on top of the rete, and is designed to turn freely. It is used as a pointer during calculations and, depending on the origin of the astrolabe and the preference of the maker or owner, might be double or single ended; or not be present at all.
The Alidade
On the opposite side of the astrolabe from the rule is the alidade. This is a double ended
The history of the astrolabes
The history of the Astrolabe begins in the Hellenistic World of Alexandria. From there it spreads north into the Byzantine world and east through the Islamic world and into India. Later, knowledge of the astrolabe traveled west across North Africa and into Mus lim Spain. In the Latin middle ages, scholars from northern Europe traveled to Spain and returned with astrolabes and texts describing them. The history of the astrolabe is a history of technical, scientific knowledge embodied in instruments, texts, and practices. An Introduction to the Astrolabe introduces the reader to that history, tracing it from antiquity to modern day collections. The origins of the astrolabe remain uncertain. The earliest surviving instruments date from medieval Islam.
However, Greek and Syriac texts testify to a long theoretical and practical development that extends back to the second century BCE. The underlying mathematical principle of stereographic projection was described by Hipparcus chus of Nicaea (fl. 150 BCE). Less than two centuries later, Vi truvius (died post 27 CE) described a type of clock that depended on a similar stereographic projection. His suggestion that Eudoxus of Cnidos (ca. 408355 BCE) or Apollonius of Perga (ca. 265170 BCE) invented the rete or spider—the network of stars—almost certainly refers to the sundials he was discussing in the passage. Claudius Ptolemy (fl. 150 CE), the most famous astronomer from antiquity, wrote an extensive theoretical treatment of stereographic projection in his Plani sphaerium, which included a short discussion of a horoscopic instrument. Although he described an instrument that resembles an astrolabe, including both a rete and the stereographic projection of a coordinate system, Ptolemy’s instrument does not seem to have included the apparatus needed to make direct observations and thus to measure the altitude of the sun As early as the fourth century authors began composing manuals on the astrolabe.
Theon of Alexandria’s (fl. 375 CE) work “On the Little Astrolabe” is the earliest text to treat the construction and use and of the astrolabe. It became a model both in form and content for later literature on the astrolabe. After Theon, treatises on the astrolabes became increasingly com mon. Synesius of Cyrene (ca. 370415 CE) wrote a short work on the astrolabe and mentions a
silver planisphere that he sent to Paeonius in Constantinople. The Byzantine scholar Ammonius (died post 517 CE) reportedly wrote a treatise on the construction and use of the astrolabe. More importantly, Ammonius incorporated the astrolabe into his teaching, thereby introducing a number of people to the instrument. The oldest surviving treatise on the astrolabe comes from his most famous pupil, the mathematician and philosopher John Philopnus (ca. 490574 CE). In 530 he wrote a work entitled “On the use and construction of the astrolabe and the lines en graved on it.” Philoponus’ text offers a practical description of the astrolabe and surveys its most common uses. In the middle of the seventh century, Severus Sebokht of Nisibis, Bishop of Kennesrin in Syria, wrote a description of the astrolabe in Syriac. Sebokht’s exposition conforms to the patterns
established by Theon and completed by subsequent Authers. He eschewed theoretical discussions, concentrating instead on practical description and application. He greatly expanded the standard list of uses. Knowledge and production of the astrolabe spread from the Byzantine and SyroEgyptian context east through the Syrian city of Ḥarrān and into Persia.
HOW TO USE AN ASTROLABE?
Omer Abdul Rahiman Al Sufi, a medival mathematician and astronomer explained about 1000 mathematical and astronomical problems using an Astrolabe. Important of them are some calculations in trigonometry, spherical geometry and astronomy. Rising and seting of the Sun varies from place to place on Earth and this can be calculated for any day of the year using an Astrolabe. The plate is marked with almucantars (lines of altitude) and lines of azimuth (direction). The almucantars are 30 degrees apart and 0 (horizon) to 90 deg are marked. The lines of azimuth (direction) are also marked every 30 degrees The line below the horizon line is
the almucantar marking the end of twilight. The vertical line through the zenith is the Meridian. It marks a line passing overhead that runs North South. The Sun crossing the meridian marks local noon/midnight. The time scale on the mater limb is marked in degrees, each mark is 30 minutes, and the hours are marked in degrees . From the horizontal lines 1 to 90 degrees are marked in Arabic ABJD notations from both sides. How to find the Sunrise and Sunset for a given day.
Let the day be 21 st of July
Find the equalant day of zodiac calendar using the
calendar scale and alidada of back side of Astrolabe.
(in the Arabic Astrolabes time scale is marked with
degrees and ABJD notation of the Arabic
alphabets.)
A degree can be converted in to minuts by
multiplying with 4.
See the date is 29 of Cancer in the zodiac scale.
Put the rule on 29 of Cancer in the ecliptic of the
Rete of Astrolabe.
Rotate the rete alongwith the rule till the point align
with the horizon of Almuqandar on the left side.
Read the time on time scale and find it is 6.12 am.
To find Sunset time of July 21 rotate the rete and
rule to the right side of the west horizon of
Almuqantar and read the time on time scale and
find it is 5.48 pm.
How to find local coordinate of a given celestial body for a given date and time
Let the star be Altir ( Alpha aquilae ) and the date and time be 15 August 9 pm.
Find the zodiac date equalant to 15 August using calendar scales and Alidade in the back side of
Astrolabe.
See it is 23 of Leo in zodiac calendar scale.
Find the time 9 pm in time scale and rotate rule and rete together till it align with 9 pm marking.
Find Altair star in the rete and read it's positions of
altitude and azimuth.
See it is Altitude 45 degree and Azimuth 310
degree.
What is an Astrolabe?
Astrolabe is an ancient scienfic instrument used find the apparent positions of the Sun and other important stars in the sky for any time of the day or night throughout the year. It was used both to make observations and to carry out calculations and was the
most widely used scientific instrument in the middle ages and into the early modern period. An astrolabe can be thought of as a form of computer; a multi function calculator. Its most common usage were solving of Solving problems of geometry ,Converting between
timekeeping systems , Calculating trigonometric functions , Basic surveying , and much more concerning astronomy and time keeping, but the full range of its capabilities is much larger: Alchemists, astronomers, astrologers, and educated individuals were used astrolabes.
Use of the astrolabe would have been taught in the course of advanced classes in the natural Siences and mathematics, the way slide rules used to be, and high end calculators are now. Most of the surviving astrolabes are constructed of metal, preserved by their higher value and duability. But there are also several surviving examples from the medieval period of astrolabes constructed of both paper and wood (and sometimes paper laminated to wood), so that we can assume that cheap versions where not uncommon.
Basics of Astrolabes
Understanding the various functions of the astrolabe is made easier if you have an understanding of the concepts behind its design, how they are used in the astrolabe and how the various parts work:
The Celestial Sphere .
The sky seems to be a hemisphere turned down on us and two hemispherers of day and night can be imagined as a large sphere with the fixed stars attached to it, with the Earth, stationary and nonrotating, at the center. The Sun, the Moon, and the planets all had their own concentric spheres between the Earth and the stars. As the various spheres rotate about the earth, different parts of the sky would become visible to a viewer standing in a given spot. Like the Earth itself, this celestial sphere has many fixed landmarks allowing the observer to find his or her way around:
The Celestial Poles and Equator
At due north, at the point about which the sphere appears to rotate, is Polaris, the North Star, or Pole Star. This point marks celestial north. Opposite it, invisible to viewers in the northern hemisphere, is celestial south. Between these two extremes is a circle marking the celestial equator. As these all lay directly above their earthbound equivalents, you can think of these points the way you think of Earth’s north and south poles, and equator; and just as you can define your position on the Earth using your latitude and
longitude, positions on the celestial sphere can be similarly defined.
The Ecliptic
The most obvious object in the sky is the Sun. As the Earth rotates (or as it would have been explained in the days when the astrolabe was current technology, as the Sun rotates around the Earth), the sun rises, moves across the sky from East to West and sets. Over the course of the year the Sun seems to move across the fixed stars, oscillating from north to south and back again. This set path in the sky that the sun moves along is known as the ecliptic and is marked by the zodiac constellations.
The Tropics
As the Sun moves along the ecliptic, it moves toward the north as summer approaches, and back south as winter comes. The circle marking the northernmost point of the sun's path is known as the Tropic of Cancer and the southernmost the Tropic of Capricorn.
The Celestial Year
There are 4 fixed events in the year, defined by the sun’s motion along the ecliptic.
The Solstices
The solstices mark the longest and shortest days of the year, these are the dates when the sun is at its most northerly and southerly limits.
The Equinoxes
The ecliptic intersects the equator, on two days of the year. These dates are the spring and fall equinoxes. The spring equinox occurs when the sun moves from the southern hemisphere to the northern; and the reverse is true for the fall equinox.
The Local Sky
In addition to the celestial sphere, the astrolabes function relies on knowledge of the local sky from the point of view of the user’s location. When you look up at the sky you can see half of the celestial sphere; the other half being blocked by the ground you are standing on. What part of the sphere is visible depends on where you are and the time of day and the time of the
year.
The Horizon
The horizon is marked, rather obviously, by the line where the sky meets the ground. Of course, your local terrain, mountains and such will alter what can actually be seen, but for the purposes of this manual imagine it as a smooth line all the way round you.
The Zenith and Nadir
The point highest in your local sky (i.e. Directly overhead) is the Zenith; its opposite point, directly under your feet, is called the Nadir. The horizon then
lies 90 degrees from both.
Almucantars
An almucantar is defined as a line of equal elevation above the horizon. For example: Imagine a line that lays 15 degrees above your local horizon, all around the sky. That would be the 15degree almucantar.
The Meridian
The last major landmark we will concern ourselves with is the meridian . This is an imaginary line in the sky, passing from the north celestial pole to the south celestial pole, and passing directly over your head (zenith). This line marks local noon, the sun's highest point above the horizon for any given day. Picture it as a line in the sky running from due north to due south directly overhead.
The Projections If you have done any work with maps, you will be familiar with the concept of projections. The Earth is a sphere; so creating a flat map of the curved surface involves projecting that Sphere onto a flat surface. This is why, on some maps, the continents appear very distorte near the poles. The astrolabe is based on what is known as the planispheric projection. In a planispheric projection, a spherical object is projected onto a plane surface by placing the origin of the projection at one pole of the sphere and projecting the points of the sphere onto a plane surface placed through its equator. The astrolabe is based on what is known as the planispheric projection. In a planispheric projection, a spherical object is projected onto a plane surface by placing the origin of the projection at one
pole of the sphere and projecting the points of the sphere onto a plane surface placed through its equator. , With its projection of the local sky. The top half of the plate contains the circular grid that is the projection of the local sky from the horizon (the bottom
most curve of the grid) to the zenith (the cross at the center of the smallest circle). Again, the outermost circumference of the plate is the projection of the Tropic of Capricorn, and its center marks the projection of the celestial pole. If you examine these projections, you will
notice that in both cases the projection is oriented the same, with the north celestial pole projecting as a point in the exact center of the projection. This allows us to overlay the projections, the celestial sphere over the local sky, and pivot it on the celestial pole By rotating the rete the user can then display the local view of the sky for any combination of time and day of the year.
The Parts of the Astrolabe
The Mater
The Mater is the main fixed part of the astrolabe; all the other parts connect to it. Permanently fixed to it are the Throne and the Limb.
The Throne
The Throne is attached to the top of the mater, and provides a means of suspending the astrolabe to take sightings. In use, a ring or cord would be attached to the throne, allowing it to hang freely and so allow measuring angles from the horizon accurately. Depending on the time, place and use of the maker, the throne might be anything from a simple bulge, to an impressively ornate decoration almost as large as the rest of the astrolabe.
The Limb
The Limb is the raised ring section on the front of the mater; it encloses a space that contains the plates and the rete. It is commonly marked with the hours of the day and/or a degree scale.
The Plates or Climates An astrolabe is a very precise instrument, but its accuracy is tied to a specific latitude because the projection of the visible sky change with the viewer’s
latitude. In the example below we see plates (also known as climates) for latitude 20 degrees north, 45 degrees north and 65 degrees north; as you can see the projection on the plate changes radically.
The Rete
The Rete is a cutout overlay that rests on top of the plates. It shows the projection of the celestial sphere. Unlike the plates, the rete is designed to turn freely.
The Rule
The Rule rests on top of the rete, and is designed to turn freely. It is used as a pointer during calculations and, depending on the origin of the astrolabe and the preference of the maker or owner, might be double or single ended; or not be present at all.
The Alidade
On the opposite side of the astrolabe from the rule is the alidade. This is a double ended
The history of the astrolabes
The history of the Astrolabe begins in the Hellenistic World of Alexandria. From there it spreads north into the Byzantine world and east through the Islamic world and into India. Later, knowledge of the astrolabe traveled west across North Africa and into Mus lim Spain. In the Latin middle ages, scholars from northern Europe traveled to Spain and returned with astrolabes and texts describing them. The history of the astrolabe is a history of technical, scientific knowledge embodied in instruments, texts, and practices. An Introduction to the Astrolabe introduces the reader to that history, tracing it from antiquity to modern day collections. The origins of the astrolabe remain uncertain. The earliest surviving instruments date from medieval Islam.
However, Greek and Syriac texts testify to a long theoretical and practical development that extends back to the second century BCE. The underlying mathematical principle of stereographic projection was described by Hipparcus chus of Nicaea (fl. 150 BCE). Less than two centuries later, Vi truvius (died post 27 CE) described a type of clock that depended on a similar stereographic projection. His suggestion that Eudoxus of Cnidos (ca. 408355 BCE) or Apollonius of Perga (ca. 265170 BCE) invented the rete or spider—the network of stars—almost certainly refers to the sundials he was discussing in the passage. Claudius Ptolemy (fl. 150 CE), the most famous astronomer from antiquity, wrote an extensive theoretical treatment of stereographic projection in his Plani sphaerium, which included a short discussion of a horoscopic instrument. Although he described an instrument that resembles an astrolabe, including both a rete and the stereographic projection of a coordinate system, Ptolemy’s instrument does not seem to have included the apparatus needed to make direct observations and thus to measure the altitude of the sun As early as the fourth century authors began composing manuals on the astrolabe.
Theon of Alexandria’s (fl. 375 CE) work “On the Little Astrolabe” is the earliest text to treat the construction and use and of the astrolabe. It became a model both in form and content for later literature on the astrolabe. After Theon, treatises on the astrolabes became increasingly com mon. Synesius of Cyrene (ca. 370415 CE) wrote a short work on the astrolabe and mentions a
silver planisphere that he sent to Paeonius in Constantinople. The Byzantine scholar Ammonius (died post 517 CE) reportedly wrote a treatise on the construction and use of the astrolabe. More importantly, Ammonius incorporated the astrolabe into his teaching, thereby introducing a number of people to the instrument. The oldest surviving treatise on the astrolabe comes from his most famous pupil, the mathematician and philosopher John Philopnus (ca. 490574 CE). In 530 he wrote a work entitled “On the use and construction of the astrolabe and the lines en graved on it.” Philoponus’ text offers a practical description of the astrolabe and surveys its most common uses. In the middle of the seventh century, Severus Sebokht of Nisibis, Bishop of Kennesrin in Syria, wrote a description of the astrolabe in Syriac. Sebokht’s exposition conforms to the patterns
established by Theon and completed by subsequent Authers. He eschewed theoretical discussions, concentrating instead on practical description and application. He greatly expanded the standard list of uses. Knowledge and production of the astrolabe spread from the Byzantine and SyroEgyptian context east through the Syrian city of Ḥarrān and into Persia.
HOW TO USE AN ASTROLABE?
Omer Abdul Rahiman Al Sufi, a medival mathematician and astronomer explained about 1000 mathematical and astronomical problems using an Astrolabe. Important of them are some calculations in trigonometry, spherical geometry and astronomy. Rising and seting of the Sun varies from place to place on Earth and this can be calculated for any day of the year using an Astrolabe. The plate is marked with almucantars (lines of altitude) and lines of azimuth (direction). The almucantars are 30 degrees apart and 0 (horizon) to 90 deg are marked. The lines of azimuth (direction) are also marked every 30 degrees The line below the horizon line is
the almucantar marking the end of twilight. The vertical line through the zenith is the Meridian. It marks a line passing overhead that runs North South. The Sun crossing the meridian marks local noon/midnight. The time scale on the mater limb is marked in degrees, each mark is 30 minutes, and the hours are marked in degrees . From the horizontal lines 1 to 90 degrees are marked in Arabic ABJD notations from both sides. How to find the Sunrise and Sunset for a given day.
Let the day be 21 st of July
Find the equalant day of zodiac calendar using the
calendar scale and alidada of back side of Astrolabe.
(in the Arabic Astrolabes time scale is marked with
degrees and ABJD notation of the Arabic
alphabets.)
A degree can be converted in to minuts by
multiplying with 4.
See the date is 29 of Cancer in the zodiac scale.
Put the rule on 29 of Cancer in the ecliptic of the
Rete of Astrolabe.
Rotate the rete alongwith the rule till the point align
with the horizon of Almuqandar on the left side.
Read the time on time scale and find it is 6.12 am.
To find Sunset time of July 21 rotate the rete and
rule to the right side of the west horizon of
Almuqantar and read the time on time scale and
find it is 5.48 pm.
How to find local coordinate of a given celestial body for a given date and time
Let the star be Altir ( Alpha aquilae ) and the date and time be 15 August 9 pm.
Find the zodiac date equalant to 15 August using calendar scales and Alidade in the back side of
Astrolabe.
See it is 23 of Leo in zodiac calendar scale.
Find the time 9 pm in time scale and rotate rule and rete together till it align with 9 pm marking.
Find Altair star in the rete and read it's positions of
altitude and azimuth.
See it is Altitude 45 degree and Azimuth 310
degree.