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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celestial_map_signs_of_the_zodiac_and_lunar_mansions

16th century Ottoman Empire painting of signs of the zodiac and lunar mansions

LENGTH OF THE TROPICAL YEAR
As the Earth travels in its orbit around the sun, [for people living in the northern hemisphere] in winter the north pole points somewhat away from the sun, but in the summer the north pole points somewhat toward the sun. The time from the day and hour and minute when it is pointing most away from the sun until the day and hour and minute it again is pointing most away from the sun is called the tropical year. It is called the tropical year because the easiest way to measure its length is to go to the tropics and detect the day and hour and minute when the sun crosses over the equator. Modern astronomers prefer to call it axial precession.

LENGTH OF THE SIDEREAL YEAR
The sidereal year (also called the stellar year) is the time it takes the Earth to orbit the sun once with respect to the position of the stars. The sidereal year is about 20 minutes longer than the tropical year.
As a result, the constellations of astrology [when viewed on the same day each year] appear to slowly move across the sky, returning to their original positions about 26,000 years later. This 26,000 year cycle is called the precession of the equinoxes. The cause is that the number of times that the Earth travels around the sun in one 26.000 year cycle is one time less if your frame of reference is the stars instead of the Earth.
Astrologers divide the 26 000 year cycle into 12 ages

CALENDARS
The 1001 Inventions book falsely implies that Omar Khayyam’s year 1079 calendar was an improvement over the Julian calendar in preventing drift of the day for planting crops.
The calendar needed for knowing most precisely when to plant crops is the tropical year which we now know is 365.2422 days.
The Julian Calendar defined the year as averaging 365.2500 days. This caused a drift of (365.2500 – 365.2422) days, which is an annual drift of only 0.0078 days.
Omar Khayyam copied the Hindu sidereal calendar which we now know is 365.2564 days. The drift of Omar Khayyam’s year 1079 calendar was (365.2564 – 365.2422) days, which is an annual drift of 0.0142 days, which is almost double the drift of the Julian calendar.
The only change Omar Khayyam made to the Hindu calendar was to replace the Hindu constellations with images of animals, gods and goddesses of Roman and Arab astrology.

WHO FIRST DETECTED THAT THE SIDEREAL YEAR IS LONGER THAN THE TROPICAL YEAR?
The Hindus, Greeks, Romans and scholars of the Islamic Golden Age did not know that the Earth rotated, and did not know that the Earth traveled in an orbit around the sun. They believed that the apparent motion of heavenly bodies was them actually moving.
The angles involved in detecting and measuring the precession of the equinoxes are quite small. It amounts to 360 degrees of arc in about 26,000 years, which is about 1.4 degrees of arc per century. But 1.4 degrees of arc was roughly the amount of error in the measuring instruments.
Comparing results a hundred years apart would usually show that the sidereal year was longer than the tropical year, but if the data was bad it would sometimes show the tropical year as being longer. Early Greek scholars used the sidereal year to decide when to plant crops, but may not have known about the concept of a tropical year. The Greek astronomer Hipparchus (190 BC – 120 BC) probably was the first to claim that the sidereal year was longer than the tropical year, but some scholars say Hindu astronomers knew this centuries earlier.
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.
But some Roman Empire astronomers (and later some Islamic Golden Age astronomers) mistakenly thought there was a cycle of sometimes the sidereal year being longer and sometimes the tropical year being longer.
As Islamic Golden Age instruments became larger and more accurate, the value of the rate of the precession of the equinoxes was calculated more precisely.
Muslim websites make false claims that Muslim astronomers were the first to claim that the sidereal year is longer than the tropical year.

TILT OF THE EARTH’S AXIS
All early astronomers who measured the tilt of the Earth’s axis (also called the obliquity of the ecliptic) thought that it was the sky that was tilting.
Ptolemy’s 2nd century measurement in the Greek-speaking Roman colony of Alexandria Egypt had a 2% error.
Nine centuries later, Al-Biruni’s 11th century 1% error measurement was twice as accurate.
[Ptolemy wrote of 23.86 degrees at a time when the tilt was actually 23.42 degrees. Al-Biruni measured 23.58 degrees at a time when the tilt was actually 23.31 degrees.]
Over that 9 century period, the actual change of the tilt was (23.42 – 23,31) degrees, which is 0.11 degrees in 9 centuries, which is 0.012 degrees per century.
The amount of error in early measurements of the tilt was about 0.500 degrees, but later measurements had an error of only about 0.100 degrees. [This improvement may have been due either to better instruments or to luck.] Thus none of the astronomers were able to detect the 0.012 degrees per century change in the tilt of the Earth’s axis.
http://adsabs.harvard.edu/full/2002JHA….33…15J
http://en.wikipedia.org/wiki/File:Obliquity_of_the_ecliptic_laskar.PNG
https://books.google.com/books?id=Y61qolT7m98C&pg=PA130&lpg=PA130&dq=biruni+23&source=bl&ots=6fJnZpXd3P&sig=S3yHXNPBeq1Gn_LFCvvIyuFczxo&hl=en&sa=X&ved=0ahUKEwjo2ZSwtvXMAhUGyGMKHSjBDhAQ6AEILTAD#v=onepage&q=biruni%2023&f=false

FORECASTING EVENTS
In the 10th century Al-Qabisi (Alcabitius or Alcabitius) wrote about judicial astrology, which is the forecasting of future events by examining the positions of the planets.

JUDGEMENT OF THE STARS
Around the year 1000, Haly Abenragel wrote the astrology book “The complete book on the judgment of the stars”.

ASTROLOGY CALENDAR
A team led by poet Umar al-Khayyām (Omar khayyam) created the Jalali astrological signs calendar by copying the Surya Siddhanta calendar of the Hindus, but changing the constellations.
[Muslim constellations are based on Arab and Roman myths.
For example, stars representing Na’sh and  his daughters are forced to circle al-Jaday (the North Star) to prevent the daughters of Na’sh from killing al-Jaday, whom they accused of murdering Na’sh. This forms a portion of al-Dubb al-Akbar (the greater bear), copied from the Roman constellation Ursa Major. The Romans believed their goddess Juno created Ursa Major when she learned that her husband Jupiter had gotten Callisto pregnant, so Juno transformed Callisto into a bear in the sky.]

PLANETS ARE RATIONAL ENTITIES
Al-Kindi (Alkindus) (801-873), living in Baghdad wrote that the planets are rational entities, whose circular motion is in obedience to and worship of Allah.
wrote that the influence of heavenly bodies caused people in different lands to look and behave differently.
Al.Kindi developed a mathematical scale to quantify the strength of a drug, and developed a system, based on the phases of the moon, that predicted the most critical days of a patient’s illness.

LUNAR MANSIONS
In the 9th century, al-Farghani wrote about the astrology concept of lunar mansions, which divided the sky into 28 divisions. Observing which lunar mansion was visible at sunrise might have been used by Muslims to calculate the day on which to plant crops. [The Muslim calendar could not be used for this, because some years had 12 months and some years had 13 months.]

MORE ACCURATE PREDICTIONS
The 13th century Persian scholar. (Tusi) Nasir al-Din al-Tusi (1201 – 1274) , born a Shia Twelver, was captured by a Mongol army. Al-Tusi made his most important contributions after Hulagu Khan built him in 1259 the Rasad Khaneh observatory in what is now Azerbaijan , so al-Tusi could make accurate astrology predictions. (Hulagu Khan was probably a Nizari Shia Sevener, the same sect as Aga Khan is head of today.)
Al-Tusi wrote Shia Sevener religious writings, but Shia Twelvers claim al-Tusi remained a Twelver and was only pretending to be a Sevener.

HANAFI MUSLIMS OPPOSED ASTROLOGY
In 1580 the chief Hanafi Sunni sharia law advisor to the Sultan ordered that the observatory near Istanbul must be destroyed, because a comet observed there was blamed for a disease epidemic.

ISLAMIC YEAR AH CALENDAR

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.
Image by painters of Sultan Murad III, via Wikimedia Commons.
image credit https://en.wikipedia.org/wiki/File:Celestial_map,_signs_of_the_Zodiac_and_lunar_mansions..JPG

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