Rubul Mujayyab is a scientific Instrument of Space and time used for timekeeping like Astrolabe. It is also known as a “sinecal quadrant” in the English-speaking world. The instrument could be used to measure celestial angles, to tell time, to find directions, or to determine the apparent positions of any celestial object for any time. Muslims used it additionally to know prayer time and to calculate lunar calendar. The name is derived from the Arabic “rub” meaning a quarter and “mujayyab” meaning marked with sine. According to David King, the greate science historian Rubul Mujayyab was invented by Muhammad ibn Mūsā al-Khwārizmī in 9th century Baghdad. The instrument is a quarter of a circle made of wood or metal (usually brass) divided on its arc side into 90 equal parts or degrees. The 90 divisions are gathered in 18 groups of five degrees each and are generally numbered both ways from the ends of the arc. That is, one set of numbers begins at the left end of the arc and goes to 90 at the right end while the other set the zero is at the right and the 90 is at the left. This double numbering enables the instrument to measure either celestial altitude or zenith distance or both simultaneously. At the apex where the two graduated straight sides of the grid pattern meet in a right angle there is a pin hole with a cord in it and a small weight on the free end with a small bead that slides along the cord. The cord is called “Khait” and is used as a plumb line when measuring celestial altitudes. It is also used as the indicator of angles when doing calculations with the instrument. The sliding bead facilitates trigonometric calculations with the instrument. Each part of the Instrument used for more than single purpose. Traditionally the line from the beginning of the arc to the apex is called “Jaibs” or sines and the line from the end of the arc to the apex is called “Jaib tamams”. Both jaibs and jaib tamams are divided in to 60 equal units and the sixty parallel lines to the jaibs are called sitheeniys or” sixtys “ and the sixty parallel lines to the jaib tamams are “juyoobul mabsootah”. The reason for sixty divisions along the Jaibs and Jaib Tamams is that the instrument uses the Sexadesimal number system. That is it is graduated to the number base 60 and not to the base 10 or decimal system that we presently use. Time, angular measurement and geographical coordinate measurements are about the only hold overs from the Sumerian/Babylonian number system that are still in current use. Like the arc, the Jaibs and Jaib tamams have their sixty divisions gathered into groups of five that are numbered in both directions to and from the apex. The double numbering of the arc means that the “Jaibs” and “Jaib tamams” labels are relative to the measurement being taken or to the calculation being performed at the time and the terms are not attached to one or the other of the graduated scales on the instrument.

Measuring A Celestial Altitude With The Quadrant On one of the straight edges of the quadrant there are two sight vanes which are called “Hadafatani”. Each of the vanes has a small hole or “pinnule” or atleast a noch at its center through which one sights a celestial object. Light passing from a star through both holes to the eye guarantees that the instrument is aligned with the star and that the plumb line will indicate the true altitude of the star when the instrument is held in a vertical plane.

It has been stated by several authors that two people are required to use the instrument successfully; one to take the sight and one to read the cord position on the arc. This is not entirely true as one can see from the photograph that it is quite easy to pin the cord to the face of the instrument with a finger of the left hand once the sight vanes are perfectly aligned with the star. A tthat instant one simply rotates the instrument in the right hand to read the position of the cord on the arc. However, it does help to have another person to write down the scale readings as they are taken. As any navigator knows one must take multiple readings and average the result in order to have any faith in the measurement. This is why the old wood cuts showed an assistant standing next to the observer. Not to read the scale but to record the results. Taking the altitude of the sun is a bit more difficult than sighting a star due to the blinding intensity of the solar disk. In this case one holds the instrument as one would to read the scale, that is facing the graduated face of the instrument. Then it is oriented with the hands in such a way that a ray of sunlight will pass through both sight vane pinnules and make a bright spot on the observer’s finger . As soon as the alignment is thus perfected the scale can be read by the observer at the point where it is cut by the cord.

Muslim Mathematician and Astronomer of 15 th century Jamaludheen Mardinee wrote a treatice on the Instrument and let me to post main text here translated.

In the name of God the Compassionate, the Merciful

World renounced Scholar Badarudheen Bin Muhammed from the tribe of Great Scholar Jamaludheen al Maradeeni And Allah may make his knowledge useful to all Muslims. All praise to the Lord of worlds and his peace be upon our Leader Muhammed and his Relatives all. And this is a treatiace about the use use of Sinical Qadrent ( Al Rubul Mujayyab) content a preface and twenty chapters. The preface is explaining it’s shape. It has a center and there is a hole with a chord in it and graduated with angle of Altitude is divided equally with ninety degrees written it’s numbers vertically and horizontally from the starting to the end. It’s beginning is in right direction of the observer and the strit line starting from the center and reaching at the starting of the arc is called “ Full Sines “ or Jaib al Tamam and the strait lines starting from it ending in the arc are called Juyoob al Mankoosa and the left line reaching from the centre to the arc is called the “ Sixties” or Sitheeniyy and the lines starting from it ending in the arc are called “Juyoob al Mabsoota” . Starting of all sines are from the centre and there is no confusion in it. We know the chord, plumbline view pointer from the Instrument.

Part One

The way of measuring Altitudes

You have to hold the Instrument with your both hands , tie a plumbweight on the chord make the empty side towards the Sun and move the Instrument till the shadow of the upper viewpointer

covers fully the lowerpointer , read the degrees and is the Altitude. Allah know

Part two

Finding the arc of sine and sine of arc

Count the graduation from the starting of the Arc of altitude till the degree we need to find sine and then enter the the end of last jaibs from juyoobul mabsoota and on the sitheeniyy you will get the jaibs of given altitude arc. Remember that the jaibs will never exeed sixty degrees. If you are counting jaibs equel as in sitheeniyy desired angle in the arc it is extending from the end towards the arc and what you are getting in the beginning of arc that is the jaib. Allah knows well.

Part III

Know inclination first end of each day to impose Put thread on the Sitheeniy Know with plumblineand put on twenty-four of its parts flat and then move the floss primary rest from the nearest Equinox to in the first arc height then down the Plumb weight to bow in the jaib al mabsoot if we find first inclination if you wish put thread on the sitheeniy and know jaib after nearest eqinox ofthem, and then move the plumb line to And then move the plumb line to the inclination starting, Twenty threee degrees and thirty five minutes in all countries we will get the value. Get down the plumbline into juyoobul mabsoot to arc find the first inclination also seen them fully Showing countries if it is in southern hemisphere must deduct it and if it is in northern hemisphere it is the end. The collected value of ninety is the latitude of the country in this case only. God knows. If the latitude and inclination in defferant take avarage and it is the sharp value.

Part IV

To know the latitude of the country and find the aim and then read the value and if it is ninety it is the latitude of the country. If it has inclination add full of it . In the case of indifferance in the values of take avarage of inclination and full aim and if they are unified that is the latitude of thecountry. God knows.


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