THE MOON

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This is the newest post in category ASTRONOMY&NAVIGATION.

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Greeks measuring angles during different phases of the moon attempted to measure the ratio of the distance to the sun and the distance to the moon. Their calculation, which was copied by Muslims, was off by a factor of ten.
Because al-Battani assumed that heavenly bodies traveled at constant, his calculated percentage variation in the distance between the Earth and the sun was about double the variation.he observed.
Al-Battani accurately wrote down the difference in time between 2 consecutive eclipses of the sun. Edmond Halley around the year 1693 did the same, discovering the moon was taking a longer time to go once around the Earth than it had around the year 1100.
Al-Biruni had read works of the 5th century Hindu astronomer Aryabhata who claimed the planets and the moon do not have their own light.
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THE SUN AND PLANETS

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Muslim astronomers and astrologers extrapolated where in the sky planets would be on future dates.
They may have read an Arabic translation of Archimedes citing Aristarchus of Samos writings. Muslim astronomers disagreed over whether Aristotle or Ptolemy was correct.
Abū Rayhān al-Bīrūnī (973 – 1048) learned the astronomy of India by interrogating Indian scholars whom the Muslims had imprisoned.
Nearly all Muslim astronomers agreed that the Earth does not rotate.
Muslim astronomers thought that the stars were not much further away than Jupiter..
Al-Zarqālī discovered that the apparent elongated path of Mercury did not agree with Ptolemy’s theory, but avoided admitting that the path was not composed of circles.
Al-Biruni’s data would later be used by Copernicus, who believed the Earth and other planets traveled around the sun.
Neither the Muslims nor Copernicus knew that planets sped up as they got nearer to the sun.
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STARS AND GALAXIES

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Shortly after 1424, Ulugh Beg’s catalog had slightly less than the number of stars in Ptolemy’s 2nd century catalog, but was slightly more accurate in measuring the locations because the Muslims used larger instruments.
Muslim astronomers saw the cloudy regions of the sky which we now know are.galaxies, but did not know what they were.
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MARINE SANDGLASS

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Abū Ishāq Ibrāhīm al-Zarqālī (1029–1087) published an improved estimate of the length of the Mediterranean Sea that was only 67% of the estimate made many centuries earlier by Ptolemy. Al-Zarqālī did not say how he made the estimate of longitude.
Al-Zarqālī could have had someone record star positions before departing, record star positions after arriving, and cabin boys could have turned over primitive marine sandglasses to record elapsed time. Thus he would have known the difference in local time between the 2 locations.
Marine sandglasses were hung from a low ceiling to keep them vertical, and were sealed to prevent moisture from entering.
Europeans did not have glass-blowing technology until centuries later, but Muslims could have learned glass blowing from the Jews in Israel.
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NAVIGATION AND EXPLORATION

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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.
A Christian in Portugal first 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.
MUslims did not discover America

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PIRI REIS 16TH CENTURY MAP

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The Piri Reis map is a naval world map compiled in 1513 for the Ottoman Empire, showing Muslim lands and the coasts of Europe and North Africa and part of the eastern coast of South America. It copied from ten Arab maps, a map of Columbus, and 4 maps from India acquired from Portugal. A false claim has been made in modern times that it proves Antarctica had had been explored. Another false claim is that it shows continental drift of South America away from Africa.
Muslim websites claim that this map was far superior to any other maps of that era. An article at en.wikipedia rejects this claim.
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MAPS PRIOR TO PIRI REIS MAP

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The map of Ahmed ibn Sahl al-Balkhi showed the Indian Ocean as connected to the Pacific Ocean.
A map of al-Biruni showed Spain being a short distance from China.
Abū al-Wafā’ Būzjānī calculated the difference in longitude between Baghdad and an observatory in what is now Uzbekistan, by comparing the angles to the same eclipse of the moon.
The sea captain Ahmad Ibn Mājid devised a route from India to Malaysia.
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CIRCUMFERENCE OF THE EARTH

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Muslims successfully repeated the Eratosthenes measurement by use of angles of shadows at noon at two locations far from each other.
Al-Biruni misinterpreted and fudged the results of his “angle to the horizon from a hill” method of calculating the circumference of the Earth because he did not know that air had a lower density at the top of a hill, causing considerable refraction of the light.
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EARLY ASTROLABES

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The theory of the astrolabe was written about by Hipparchus in the 2nd century BC and added to in later centuries.
True astrolabes were built by Greeks by the 7th century. The Greek-invented astrolabe had a plate and a rotating part called a rete. It was used by astrologers to measure the angle to stars, “wandering stars“ and the sun in the sky. It could determine the time in the day or at night, and was used in land navigation to determine directions and latitude.
In the 7th century, when what is now Syria and Iraq were part of the former eastern Roman Empire that had become the Byzantine Empire, the Christian bishop Severus Sebokht (575 – 667), who lived in what is now Iraq, wrote in the Syriac language a treatise on the astrolabe.
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LATE ASTROLABES

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Early 9th century Muslim Persian Al-Khwarizmi’s sine quadrant could determine the apparent positions of any celestial object for any time. It consisted of a quarter of a circle divided by marks numbered both 0 to 90 and 90 to 0. It had a plumb line and a grid that looked like graph paper. He added to the Greek-invented astrolabe markings to show the times of the day to pray, and the direction to Mecca.
Ibn Yunus (950-1009) used a 14-meter long astrolabe to obtain very accurate measurements of star positions, planetary conjunctions and lunar eclipse …
Al-Majriti (? – 1008) may have taught his daughter Fatima how to make astrolabes. He also adjusted astronomy tables for the latitude of Spain.
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TUSI COUPLE

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The theory that the Muslim astronomers borrowed from Ptolemy predicted that the apparent relative diameters of the sun and moon as seen during eclipses of the sun would vary twice as much as is actually observed. The reason for this error is that they did not know that gravity causes the smaller body to travel faster when it is nearer to the larger body.
The Tusi couple does not make any sense, but accidentally nearly agrees with measurements of the changes in the apparent relative diameters of the sun and moon as seen during eclipses.
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PTOLEMY’S MODEL

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Ptolemy was a Greek-speaking scientist in the Roman colony of Alexandria Egypt. The model of Ptolemy states that all heavenly bodies travel in perfectly circular paths at constant speeds around a point that is near the Earth but not exactly at the Earth. Since Ptolemy did not believe in oval paths and did not know that the planets traveled around the sun instead of around the Earth, he used the concept of the epicycle to make a circular adjustment to the circular paths.
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CALENDARS AND ASTROLOGY

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The length of the year depends upon whether you are measuring the tilt of the Earth that causes the seasons (tropical year) or are measured relative to the stars in the sky (sidereal year).
The sidereal year is about 20 minutes longer than the tropical year.
Ptolemy in the 2nd century AD in the Greek- speaking Roman colony of Alexandria Egypt compared his data with data of Hipparchus and 2 other astronomers from about 300 years before Ptolemy, and calculated the difference between the tropical year and the sidereal year, obtaining a value that was about 70% of the true value now known to be the rate of the precession of the equinoxes.
Other astronomers, both Greek and Muslim, had difficulty determining which year was longer.
When Muslims built very large astrolabes, they were able to make precise measurements of the length of both kinds of years.
Very large astrolabes were useful in making precise astrological predictions of future events.
In the 9th century, al-Farghani wrote about the astrology concept of lunar mansions,
Omar Khayyam copied the Hindu sidereal calendar, changing the astrological constellations.
The Islamic year AH calendar is a complicated topic beyond the scope of this post. It is a lunar calendar. 103 lunar years is approximately 100 solar years of the Gregorian calendar.
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BABYLONIAN ASTRONOMY AND OBSERVATORIES

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Muslim websites falsely claim that Muslims built the first astronomical observatories.
We know that astronomical observatories existed in Babylonia in pre Islamic times because of the Babylonian writings on clay tablets about their observations.
The ancient Babylonian calculation of the length of the month was off by only nine-tenths of a second per month.
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