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67 votes

What is Ptolemy holding in this picture?

This device was invented by a Jewish Rabbi, Levi Ben Gershon. It was used to measure the angular distance between two stars or, in general, any pair of celestial bodies. Ptolemy lived 1000 years ...
Riccardo.Alestra's user avatar
61 votes
Accepted

What is Ptolemy holding in this picture?

It is called "Jacob's staff". It was an old astronomical tool used for trigonometric purposes.
Euler_Salter's user avatar
22 votes
Accepted

How did Ptolemy know that days were unequal lengths?

You are right: at the time of Ptolemy they could not measure the length of a day directly. Actually Ptolemy never discusses any clocks in his book, he probably used some crude devices record the ...
Alexandre Eremenko's user avatar
19 votes

What is Ptolemy holding in this picture?

And this tool has been known under many other latin names than baculus Jacob (or Jacob's staff): radius astronomicus (astronomic ray), crux geometrica (geometrical cross), revelatorem secretorum (...
Laurent Duval's user avatar
8 votes

Where did Ptolemy compare the Earth to the distance of fixed stars?

Ptolemy's argument can be simply explained in plain English. If the distance to stars was comparable to the size of the Earth, the stars would experience a diurnal parallax, that is the visible ...
Alexandre Eremenko's user avatar
6 votes

Has anyone explored Ptolemy's epicycles as an early form of Fourier analysis?

As @Andrei Kopylov noticed, epicycle theory is not the theory of Fourier series of a periodic function. Still this is called (generalized) Fourier analysis. Such functions are called almost periodic ...
Alexandre Eremenko's user avatar
5 votes

How did Ptolemy calculate the distance to the Moon?

Ptolemy knew about the Moon's parallax (he explains it in section 11, Ch. V of Mathematical Syntaxis). To measure it he invented the "parallactic instrument" described in section 12. Section 13 is ...
Alexandre Eremenko's user avatar
5 votes

Was Fourier inspired by Ptolemy?

Fourier was not inspired by Ptolemy or by mechanics. His work where he developed Fourier series and invented Fourier transform was about heat. He never mentions Ptolemy (or any astronomer). The ...
Alexandre Eremenko's user avatar
5 votes

On Ptolemy climes

Originally, the Greek word κλῐ́μᾰ means “slope, incline, inclination,” and has nothing to do with the modern meaning of the word (long-term weather). Ptolemy defined the seven climata by the length of ...
Pierre Paquette's user avatar
5 votes

Did Ptolemy and other Greek scientists actually measure the distance to the Sun?

No, they did not. Several methods were proposed but they do not give "correct" distances. Of course, all depends on the exact meaning of the word "measure" and "correct". But their estimates were ...
Alexandre Eremenko's user avatar
4 votes
Accepted

Did the Ptolemaic system have rotating center of the deferent for Venus?

There were multiple versions of the Ptolemaic system circulating in the middle ages, including those where Venus did have a rotating eccentric, the center of the deferent, e.g. in Regiomontanus's ...
Conifold's user avatar
  • 77.6k
3 votes
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Earth's and Sun's rotation in the Ptolemaic world

In Ptolemy system, there is a sphere of fixed stars which rotates about the Earth with constant speed 1 rotation per 24 hours. This rotation is about the axis which passes through the North and South ...
Alexandre Eremenko's user avatar
3 votes
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What was the Ptolemaic system used for?

One specific application was the eclipse prediction. For example, Columbus used a predicted Lunar eclipse at least once, trying to determine his longitude. He was not very successful. Ref. S. E. ...
Alexandre Eremenko's user avatar
3 votes

Has anyone explored Ptolemy's epicycles as an early form of Fourier analysis?

This is very popular myth, but it is not true: Ptolemy's epicycles are not Fourier analysis! Fourier series can indeed approximate an arbitrary periodic function. And you can approximate an arbitrary ...
Alexei Kopylov's user avatar
2 votes

Has anyone explored Ptolemy's epicycles as an early form of Fourier analysis?

Yes. That was done by Giovanni Schiaparelli in a book called Scritti sulla storia dell'astronomia antica, published in 1926 and reprinted in 1997.
José Carlos Santos's user avatar
2 votes
Accepted

Where did Ptolemy compare the Earth to the distance of fixed stars?

Almagest, Book 1, chap. 5 contains what you are looking for.
sand1's user avatar
  • 2,422
2 votes
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The Rejection of Solid Orbs by Geo-Heliocentric (Tychonic) model of the solar system

I can explain the principle (question 1). In Ptolemy's theory (in Almagest), the sizes of planetary orbits are not specified. They can be arbitrary, since scaling of any single orbit does not change ...
Alexandre Eremenko's user avatar
1 vote

The Sun's direction of rotation in the geocentric model

Sun (and everything else in the sky) participate in two motions. Sky as a whole (the sphere of fixed stars) rotates clockwise, 1 rotation per 24 hours. BTW for this reason we call this direction ...
Alexandre Eremenko's user avatar
1 vote

On the Ptolemaic theory of the planet's latitude

tl;dr: No. According to Ptolemy's model, the external planets will not get their maximal latitude exactly at the opposition. However, his model makes the maximum latitude to occur closer to the ...
d_e's user avatar
  • 261

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