Re: VMs: How did Ptolemy know about time zones

["Dan Gibson/CanBooks" <dangibson@xxxxxxxxxxxx>]

I believe that Ptolemy wrote around 130 A.D. By that time Arab merchants 
(Nabataeans) had over two hundred years experience sailing from India to the 
Red Sea. Some had sailed to China. Arab merchants (Nabataeans) had also 
established a thriving colony near Rome itself. These merchants used the 
stars for navigation. Thus they were in an excellent position to observe 
events taking place from China to Europe. They would surely notice and 
discuss how events in the stars took place at different times of the day in 
different locations.

The Arabs used several ways to determine latitude. Before the invention of 
the compass, watch, and the sextant, the Arab mariner's main guide was 
latitude. To obtain their latitude, Arabs measured the altitude above the 
horizon to a known star, and then deduced from this the altitude of the Pole 
Star, (since the Pole Star was the one star that did not move in the sky). 
In some cases ancient navigators measured directly the altitude of the Pole 
Star. This was the simplest method, and was known as the science of qiyas. 
The easiest method was to use the width of a finger. When held at arm's 
length, the width of four fingers was considered to measure 4 isba'. In a 
360 degree circle there were 224 isba'. It was considered that a day's 
sailing due north would raise the Pole Star 1 isba' from the horizon. For 
those traveling on land, the isba' was further divided into 8 zam. Thus land 
distances were often measured in zams.

A more accurate, but still simple instrument was known as a kamal. This was 
a small parallelogram of horn or wood measuring about one by two inches with 
a string inserted in the center. On the string were nine knots at measured 
intervals. The end of the string was held in the teeth. The lower edge of 
the horn was placed on the horizon while the horn was moved along the string 
until the upper edge touched the required star. The knot at which the horn 
covered the exact distance signified a certain number of isba' of altitude 
of the star. The altitude of the Pole Star could then be deduced from the 
rahmani.

An alternative way of using a kamal was to move the knots through the teeth 
until the piece of horn or wood covered the required star altitude.
Vasco da Gama's pilot from Malindi used a kamal, and the Portuguese adopted 
it and eventually modified the spacing of the knots to measure degrees. 
Sometimes Arab and Indian seamen added extra knots marking the latitudes of 
particular ports of call, or they simply used a kamal on which all the knots 
indicated particular ports of call.

Another very simple navigation method that was used by many early dhow 
captains was simply the position of the sun or North Star above the boat. By 
standing on various locations on the boat, they could place the sun or North 
Star above, right, left or behind the dhow. As long as they kept the stars 
at a correct position above the rigging they were assured that they would 
arrive at their destination.

Thus merchants traveling at sea would notice that events in the stars 
observed in China would have taken place at a different time of day than 
they did in India, Arabia, and Europe. Ptolemy, who was ever collecting 
knowledge about science and geography.

His book, The Almagest, is divided into 13 books, each of which deals with 
certain astronomical concepts pertaining to stars and to objects in the 
solar system. In essence, it is a compilation of Greek astronomy, and the 
major source of knowledge about the work of Hipparchus, most probably the 
greatest astronomer of antiquity. As already mentioned here, Ptolemy 
compiled a star catalog of 1,022 stars.
Ptolemy also wrote books about mathematics and geography. As a geographer, 
Ptolemy's reputation rests mainly on his Geographike hyphegesis (Guide to 
Geography), which was divided into eight books; it included information on 
how to construct maps and lists of places in Europe, Africa, and Asia 
tabulated according to latitude (and longitude I believe, although I do not 
know how he would have calculated these. Pamela, do you know if he 
calculated longatude?

There are, I understand, many errors in the Guide. For example, the Equator 
was placed too far north, and the value used for the circumference of the 
Earth was nearly 30 percent less than a more accurate value that had already 
been determined before, as well as some contradictions between the text and 
maps. The Guide is an important work from a historical point of view 
because, like the Almagest, it exerted a great influence on later 
generations. Christopher Columbus, for example, used it to strengthen his 
belief that Asia could be reached by traveling westward because Ptolemy had 
indicated that Asia extended much farther east than it actually does. 
Ptolomey suggested that the Indian Ocean was bounded by a southern 
continent. (Perhaps reports from Arab merchants of Australia, Antarctica and 
South America caused him to think this. There are a number of research 
projects currently going on to find and record all of the old Arabic 
inscriptions in South, Central and North America.) It wasn't until the 
return voyage from the Southern Hemisphere by Capt. James Cook that the 
belief in a southern continent was disproved.

I have looked at the star charts in the VMS, but since so much was known by 
100 BC it is very hard to date the VMS by what is shown. If only we could 
read the labels under the charts! 


______________________________________________________________________
To unsubscribe, send mail to majordomo@xxxxxxxxxxx with a body saying:
unsubscribe vms-list