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(Christian) Portuguese mariner’s astrolabes which are heavy and have holes to prevent them from being blown in the wind.
It is impractical to use a Muslim astrolabe aboard a rolling ship. Not only does the motion cause inaccurate readings, but the observer could be hit in the eye by the astrolabe.
Muslims never figured out that if you drill a hole at the top of the astrolabe, suspend the astrolabe from a chain, and make the rete a very heavy frame-only device to prevent it from blowing in the wind, an astrolabe could be used on a ship to determine longitude (if you had a marine sandglass to tell the time). Such mariner’s astrolabes were first written about by a Christian named Ramon Llull. in Portugal. The mariner’s astrolabe was based on the original Greek design that used a different “climate plate” for each range of latitudes, not on the universal astrolabe of al-Zarqālī.
A mariner’s astrolabe also makes it more practical to measure the angle of the North Star to determine latitude.
If the North Star was not visible, astrolabes had engraved markings for the brightest stars and adjustments for the day of the astrological year and the time of night that permitted an estimation of which way was north.
Similar measurements could be done in the daytime by measuring the direction to the sun and its altitude in the sky.
The wet compass (magnetized object floating on water) was used in ancient China to point south to align buildings for feng shui reasons.
It is not known when the Chinese began to use the wet compass for land navigation, but it must have been before the idea spread from China to the Arabs.
Around the year 1000, al-Biruni added a marking to the box surrounding a wet compass, showing the direction to Mecca. This implies that Muslims had measured the latitude and longitude of Arab cities, probably by measuring the direction a camel was walking (by sighting the sun or using a wet compass) and estimating distance by how fast a camel could walk.
The earliest known use of a compass for ship navigation was around the year 1110 in China.
In 1269, a French Christian named Petrus Peregrinus of Maricourt was the first to write about the dry compass, balanced on a point.
HORARY QUADRANT TIME BY ANGLE OF THE SUN
The horary quadrant was invented in northern India or Nepal. It was used to determine the time of day from the angle to the sun. It sometimes had 2 sets of markings, so time in either type of hour ( 1/24 of 24 hours, or 1/12 of the time from sunrise to sunset) could be calculated. It was later used also by Muslims.
KINDS OF ASTROLABES
For use of astrolabes in astronomy, see the astrolabe page.
An excellent explanation of various types of astrolabes is at http://catalogue.museogalileo.it/indepth/Astrolabe.htmlhttp://catalogue.museogalileo.it/indepth/Astrolabe.html
GLOBES (AS EARLY MAPS OF THE EARTH)
Al-Idrisi did not build the first globe. Crates of Mallus built a terrestrial globe in the 2nd century BC, and a 2nd century AD Roman sculpture depicting a globe is in the Naples Museum in Italy. http://en.wikipedia.org/wiki/Globe
DIRECTION TO MECCA
Astronomers devised formulas for computing the direction to Mecca, based on the relative latitude and relative longitude.
9TH CENTURY MORE ACCURATE LATITUDE
In the 9th century, Muslims made more accurate latitude measurements of cities in northern Africa and the Middle East by measuring the angle to the North Star with an astrolabe, resulting in more accurate maps.
9TH CENTURY MERIDIAN
Abu Sa’id Gorgani in the 9th century wrote something about the meridian. My guess is he selected a Muslim city to be at zero degrees longitude.
LONGITUDE WAS (ON ONE OCCASION ONLY) CALCULATED FROM ECLIPSE DATA
Abū al-Wafā’ Būzjānī’s 10th century calculation of the difference in longitude between Baghdad and an observatory in what is now Uzbekistan, done by comparing the angles to the same eclipse of the moon.
DISCOVERY OF THE AMERICAS
Erdogan (the president of Turkey) and a small number of Muslim historians have made a dubious claim that Muslims discovered the Americas in the year 1172. The two bits of flimsy evidence are stories that Muslim sailors had visited land (probably the Canary Islands 100 kilometers off the west coast of Africa), and a second-hand report that Columbus had written in his diary that he had seen in the distance something shaped like a dome.
There are silly claims imagining that Arabic words were written onto stones in the 6th century in what is now the United States.
Muslims frequently imagine that something looks like Arabic words. For example, there is a claim that a NASA photograph of Pluto shows an Arabic phrase that means “Allah loves Mohammed”,
In the 1940’s an American professor of
zoology named Barry Fell claimed that ruins of native American structures in Colorado, New Mexico and Indiana were ruins of Islamic religious schools, but his claims were ridiculed by actual archeologists.
Archeologists in Rhode Island in the United States discovered what looks like a 9th century Arabic manuscript. Most likely the manuscript is a hoax.
Around the year 1400, Admiral Zheng He, who prayed to Allah at the mosque but also prayed for favorable winds at the temple of a Chinese sea goddess, sailed to Arabia and eastern Africa but did not reach the Americas. The ships were designed and built by non-Muslim Chinese.
Image by Georges Jansoone of 3 astrolabes at Museum of the Forte da Ponta da Bandeira; Lagos, Portugal, via Wikimedia Commons.
image credit https://commons.wikimedia.org/wiki/File:Lagos44.jpg#mw-jump-to-license