Kepler
Kepler wrote five major astronomical works. Chronologically: Continue reading
Category Archives: Astronomy
From Kepler to Ptolemy 15
The Planetary Hypotheses
In the Planetary Hypotheses, Ptolemy lays out his cosmology: that is, the structure and arrangement of the universe. This work answers the question, did Ptolemy believe in the physical truth of the Almagest’s celestial geometry?—with an unambiguous Yes. Contrary to an opinion often expressed by earlier historians, he did not regard it just as a calculational scheme for predicting planetary positions.
From Kepler to Ptolemy 14
Cycle Counts
You may have heard that Ptolemaic systems grew to have 80 spheres or cycles, while the Copernican system had only 34. This is a myth.
From Kepler to Ptolemy 13
Latitudes
First thing to note about Ptolemy’s latitude theory: its decoupling from the longitude theory. For longitudes, one projects the orbits into the ecliptic plane. The actual speeds will differ from the projected speeds. However, the effect is small because the inclinations are fairly small (see the table below), and Ptolemy’s longitude computations ignore it. The latitude algorithms use the longitude as an input.
From Kepler to Ptolemy 12
Mercury
Mercury refused to cooperate with Ptolemy’s basic paradigm. You might guess that the fault lies with Mercury’s larger eccentricity, but studies show that bad data bears most of the blame. Mercury hugs the Sun, only appearing near the horizon close to sunrise or sunset, hardly ideal observation conditions.
From Kepler to Ptolemy 9
The Moon is the one solar body that does revolve around the Earth. It never displays retrogression. So you’d think Ptolemy wouldn’t “need no stinkin’ epicycles” for it. In fact, Ptolemy gave it a mechanism more complicated than that of any of the planets except Mercury! Here’s the model:
From Kepler to Ptolemy 7
The Full Ptolemy
We now start the second part of this series: an in-depth look at the Ptolemaic system.
