Ptolemy’s Eccentrical And Epicyclical Hypotheses
Beauty of Ptolemy’s Hypotheses
In his theory published in his Almagest circa 150 CE, Ptolemy had a base on the earlier written works by the Greek scholars who had it that the earth was the stationery center of the universe. The theory of epicycle which literally means circle in Greek was a geometric model that Ptolemy used to clarify the speed and direction variations of the apparent moon, sun and all other planets motion. He drew the theory from Apollonius of Perga in the 3rd Century BC but was later formalized by Ptolemy in his work Almagest in the 2nd Century AD. This theory was very influential at that time in explaining retrograde motion of the only five planets that were known by the world then. It even went further to explain changes in the evident planet distances from the earth.
Eccentric that literally means out of the centre is normally used to refer to any difference from the normal. Astronomically it refers to a circle round which a body revolves but its centre is displaced from the visible centre of movement. The ellipses which is planets are known to revolve around the sun could not be differentiated from the circles in the ancient astronomy. The disparate angular motion was observed due to ellipticity though (Goldstein, 1972). This theory has it that the centre of the epicycle of every planet moved evenly in a circle which is the centre displaced from that of the earth by an amount twice the eccentricity of the actual ellipse as it is understood now.
These two hypotheses are very beautiful considering how they helped shape modern day astronomy since those days of ancient astronomy. The Epicycle theory now known as Ptolemaic after the renowned Greek astronomer Ptolemy was although developed long before he came into existence by fellow Greek astrologers such as Hipparchus of Rhodes and Apollonius of Perga. Close to five centuries prior to the arrival of Ptolemy in astronomy. Ptolemy tries to point out that the beings in the sky move around, not because they follow the laws of nature but because they are free of spontaneity that is so common in the world.
After Hipparchus observed summer solstice in the year 135 BC, he found the time of equinox more accurate, Ptolemy then later on gave a extensive explanation of Hipparchus works. He quotes equinox timing by Hipparchus on 24th March 146 BC, a time that differs by 5 whole hours from the citing made on Alexandria’s equatorial ring on the same day. It’s interesting because Hipparchus may have visited Alexandria but he did not observe the equinoxes there, probably he was on Rhodes that is almost on the same Geographical longitude. The fault with an equatorial ring is that atmospheric refraction lifts the solar enough to make a change above the horizon therefore when it’s a northern observer making the observation, its declination is too high which goes on to change the observed time when the sun is crosses the equator. This is usually if the person observing is inexperienced enough to trust that it’s very near dawn or dusk.
This Ptolemy’s hypotheses of the sun’s motion which was close to that of Hipparchus observes that for Ptolemy, the sun moves with even speed along a circle that is a bit eccentric with regard to the earth and also inclined at an angle of 23 degrees which was with regards to the plane of the equator. The apsidal line through earth and the center of the sun’s eccenter is constant considering the equinoctial points. Ecliptic on the other hand, which is the projection of sun’s eccentric circle on the celestial sphere, is a great intersection of the celestial equator in the two equinoctial points.
But with the current studies and inventions, we all know that Ptolemy was wrong, this is because the sun is found to be the centre of the universe and not the earth, which is just a planet like any other. Contrary to what Ptolemy taught, the heavenly bodies are not divine, they are just matter moving in space regarding the fixed laws of nature. Matter is matter, whether earthly or heavenly. Though without a close study of Ptolemy, one would not understand the origin of astronomy as we know it today, that is why students are urged to read Ptolemy’s works but not necessarily believe them.
Ptolemy starts by observing the world and taking measurements of position and time for the sun, star and the planet, he therefore constructs tables on the basis of these findings, these serve as a means by which an astronomer can plot the course of a planet. The hypothesis must have a predictive power, not only for the present but also for the future. He then weds the facts to the adjusted hypothesis of the regular motion. It is therefore not easy to understand Ptolemy just by taking the doctrines from the book form his assumptions and conclusions only. One has to spend time and study carefully his confusing demonstrations.
Ptolemy urges us in almagest that mathematical astronomy is good for our souls. In studying the motions of the heavenly beings, we tend to become like the according to Ptolemy; ordely good and regular. We hence cultivate the correct perspective on the human being in relation to divine being. Studying Ptolemy is hence to come to realization that knowledge of the visible world should not be peculiar to dignity. Ptolemy might have been wrong actually speaking, but his hypotheses have important fact; that when mortals like us observe the beings in the Sky, we have no choice but to revere.
He is therefore responsible to what has shaped modern-day astronomy because his hypotheses generated conflicts that resulted in Ptolemaic science. It is that it’s not the science that has revolved but the form of it.