Download ASTR 1010 – Spring 2016 – Study Notes Dr. Magnani

ASTR 1010 – Spring 2016 – Study Notes Dr. Magnani Classical Greek Astronomy The ancient Greeks were interested in studying the how and the why of natural phenomena and, in astronomy, their progress was unparalleled throughout Antiquity. These study notes should help you remember and understand some of their achievements. Like most astronomy-­‐minded civilizations (Chinese, Mayan, Babylonian, Egyptian), the Greeks were intrigued by the problem of the motions of the visible planets. The word planet in classical Greek means “wanderer” because the planets change their position with respect to the stars continuously. Recall that the stars rise in the east, reach their highest point on the Meridian and then set in west. So they move on a daily basis (diurnally). However, they keep their positions with respect to each other – at least over a human lifetime. In other words, if a constellation has three bright stars in a row (like Orion), then those three bright stars will remain in that pattern for many thousands of years (ASIDE: precession changes stellar positions with respect to the North Celestial Pole and the Celestial Equator. You can notice that change in about 500 years, but the patterns of the stars with respect to each other can be changed only by the motions of the stars themselves around the Galaxy. This takes from tens to hundreds of thousands of years to produce a change noticeable to the human eye.) So for all intents and purposes, we can take the pattern of stars (the constellations, if you will) as keeping the same shape over thousands of years. Precession shifts the whole celestial sphere, but the patterns of the stars remain the same (it’s just their location with respect to the North Celestial Sphere or the Celestial Equator that is affected by precession). The planets, however, move on the ecliptic in ways that change their orientation with respect to the stars on a daily or weekly basis. The inner ones (Mercury and Venus) are always found close to the Sun (as far as angular separation in the sky): Visible in the west right after sunset, or in the east right before sunrise. The three outer planets (Mars, Jupiter, and Saturn) move westward on the ecliptic for most of the year, but then they reverse their direction of motion with respect to the fixed background stars, move eastwards for several weeks, and then resume their westward travels. This backward-­‐moving behavior is known as “retrograde motion” and was well known by ancient astronomers. The million-­‐dollar question was how to explain it. Retrograde motion can be explained either with a heliocentric model of the Solar System (i.e., a Sun-­‐centered model), or with a geocentric model of the Solar System (an Earth-­‐centered model). The correct model is the heliocentric one and you can see how it explains the behavior of the outer planets in the image below: In a geocentric model, retrograde motion is more difficult to explain. The Greeks had to introduce epicycles – spheres or circles whose center orbits on the original sphere or circle: During the time when the planet is moving in the smaller, inner loop, it appears to be moving backwards with respect to the distant stars on the celestial sphere (as seen from the stationary Earth). The motion of the inner planets could also be described by both systems. Again, the heliocentric model explains the fact that inner planets are never very far from the Sun as far as angular distance is concerned: The geocentric model is more complicated (epicycles, again), but it can also do the trick: Basically, the orbits of Mercury and Venus were displaced epicycles which rotated on a fixed axis connecting the Earth and Sun. In the third century B.C., the great philosopher Aristotle argued that the geocentric system had to be correct. He did this by shooting down the heliocentric system. He argued that if the Earth moved around the Sun, then the positions of the fixed stars should change as the Earth went part of the way around its orbit. Since no observed shift in position is detected with the naked eye over many months, the Earth could not be moving around the Sun and the geocentric model was the correct one. Of course, if the fixed stars were very far away, then the angular shift between them would be too small to notice. Aristotle, rejected this objection to his argument by saying that it made no sense that a Creator would make a Universe where the distance between Earth and the stars on the celestial sphere was so much greater than the distance between the Earth and the other bodies in the Universe (Sun, Moon, and planets). Aristotle’s argument convinced most people at the time, but not Aristarchus of Samos, who argued for a heliocentric model based on his estimates of the relative sizes of Sun, Earth, and Moon. He determined that the Sun was the largest body and so should be at the center of the system. Aristarchus’ arguments convinced very few people and so the idea that the Universe was geocentric became entrenched. In the second century AD, the Greek-­‐Egyptian astronomer Ptolemy (a Roman citizen) refined the geocentric model in several sophisticated ways in an attempt to use it to calculate the positions of the planets in future times. Although his model with many epicycles and with other features such as the equant (an imaginary point some distance from the Earth around which the planets moved at a constant rate) did approximately predict were the planets would be over a period of several years, the longer term predictions were less and less accurate. It seemed through the Middle Ages that the problem of the motion of the planets could not be resolved by humans. Before we leave the story of Ancient Greek achievements in astronomy we should mention two more people: Eratosthenes and Hipparchus. Erastosthenes measured the size of the Earth (circumference and diameter). Make sure you understand his argument. We talked about it in class and it’s in your book. Hipparchus noticed the effects of precession; i.e., he realized that the positions of the constellations with respect to the North Celestial Sphere and Celestial Equator had slipped when he compared the sky he was looking at with Babylonian star maps from 500-­‐700 years earlier. Of course he didn’t understand why this slippage had occurred, but he did notice it.