Ptolemy and longitude?

Question

1.In Ptolemy’s time, why was latitude so easy to find and longitude so difficult?

Answer 1

I am sorry I don't agree with Mlk256.

Since the beginning of time, men in the northern hemisphere had noticed that one star, the north star, was nearly not moving in the sky.

By measuring the altitude of that star above the horizon, travelers and seafarers were able to find a line of common elevation, the latitude.

A simple piece of wood where the latitude of known places was marked, was enough for them to navigate, following that latitude until reaching that place.

Finding the longitude is, in fact, finding a hour angle from an arbitrary place of reference; today being Greenwich, UK. That requires an accurate timepiece. Alas that was not possible before the end of the 18th century and John Harrison's invention of the marine chronometer.

This is why Christopher Columbus thought India was closer sailing westward from Spain. At the time, no one had a way to measure east-west distances.

Answer 2

You can find latitude easily by knowing the length of the day (or by measuring the angle of stars relative to the horizon), but to find longitude, you have to know the locations of both the magnetic and true north pole and then calculate the angle made between the observer and the poles and plug that into some calculus equation, but since the Greeks in Ptolemy's time didn't have compasses, they didn't know about magnetic north, and so didn't have a system of longitude.

Answer 3

Latitude was measured from the equator, as it is today, but Ptolemy preferred in book 8 to express it as the length of the longest day rather than degrees of arc (the length of the midsummer day increases from 12h to 24h as one goes from the equator to the polar circle). In books 2 through 7, he used degrees and put the meridian of 0 longitude at the most western land he knew, the "Blessed Islands", probably the Cape Verde islands (not the Canary Islands, as long accepted) as suggested by the location of the six dots labelled the "FORTUNATA" islands near the left extreme of the blue sea of Ptolemy's map here reproduced.

AM