Ancient Astronomy Introduction • The Golden age of early astronomy began in Ancient Greece around 600 B.C. The Greeks were able to explain natural phenomena from their observations of the world. Applying geometry and mathematics, the Greeks were able to measure distances (from Earth) and size of the moon with good accuracy by just looking at them from afar. • The Greeks, because of their close affinity to mysticism , have held the idea of a Geocentric view of the Universe. Aristotle (384-322 B.C) • concluded a spherical Earth because of the curved shadow it casts on the moon during eclipses. Eratosthenes (276-194 B.C.) • was able to measure the circumference of the Earth using principles of geometry Aristarchus (312-230 BC) • the first Greek to speculate about a Heliocentric universe through his geometric calculations of the size and the distances of the moon and the sun. However, because of the strong Aristotlean idea, the idea remained that the Earth was the center of the universe. Ptolemy (A.D 141) • was another ancient Greek philosopher. He noticed that some of his observations did not fit with Aristotle’s ideas. He suggested the idea of epicycles to explain the way some of the planets moved. • He still put Earth at the center of the Solar System. Modern Astronomy Introduction • For quite some time, the Aristotlean view of the universe persisted mainly through the works of Ptolemy and Greek philosophers. • For modern astronomy to succeed, it took numerous scientific breakthroughs to disprove religiously-rooted notions of the universe. We will discuss the works of five notable scientists involved in the transition of astronomy to a science that we know today. Nicolaus Copernicus • Very few scientific advancements were made for centuries after Ptolemy's works. It was then from a Polish astronomer, Nicolaus Copernicus (1473-1543) that the idea of a spherical earth was revived. • He said that Earth is just like any other planet and that the movement of the heavenly bodies around it can be attributed to a rotation. • Copernicus sticked to spherical planets and perfect circular orbits making his predictions on the locations of the planets full with errors. Tycho Brahe • Born three years following Copernicus' death, Tycho Brahe (1546-1601) became interested in astronomy and had persuaded King Frederick II to establish an observatory in Copenhagen. • From this observatory, he began to observe the locations of heavenly body with such precision that his works became a legacy to astronomy. • Brahe believed that if the Earth indeed revolves around the sun, the apparent position of the stars should shift accordingly. This apparent shift is called the stellar parallax. and is now used in measuring the distances of nearby star. Stellar Parallax • Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects. Johannes Kepler • Johannes Kepler (1571-1630) was an assistant of Brahe after he left his observatory in Copenhagen. Years after Brahe's death, Kepler derived the three basic laws of planetary motion. He was also the first to speculate about the irregularity of Mar's orbit and its variation in the orbital speed. Kepler's 3 Laws of Planetary Motion 1. Law of Ellipses • The path of a planet around the sun is not perfectly circular but an ellipse. Having an ellipse would mean that the orbit has two foci (focus) and one being the sun. Kepler's 3 Laws of Planetary Motion 2. Law of Equal Areas • The line joining the Sun and planet sweeps out equal areas in equal times, so the planet moves faster when it is nearer the Sun. Thus, a planet executes elliptical motion with constantly changing angular speed as it moves about its orbit. • The point of nearest approach of the planet to the Sun is termed perihelion; the point of greatest separation is termed aphelion. Hence, by Kepler's second law, the planet moves fastest when it is near perihelion and slowest when it is near aphelion. Kepler's 3 Laws of Planetary Motion 3. Law of Harmonies • Kepler's Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit. Thus, we find that Mercury, the innermost planet, takes only 88 days to orbit the Sun but the outermost planet (Pluto) requires 248 years to do the same. Galileo Galilei (1564-1642) • Supported Copernican Heliocentric theory. With the help of a telescope he constructed on his own, Galileo was able to view heavenly bodies no one was ever able to do in the past. From here, he made discoveries that shook the foundations of ancient astronomy. Galileo’s Discoveries • Jupiter's moon Galileo’s Discoveries • Planets are just like the Earth and not just points of light like stars • The phases of planet Venus as seen from Earth Galileo’s Discoveries • The moon surface is not smooth as previously thought (glass sphere). Instead, craters and mountain-like features were observed Galileo’s Discoveries • The sun has darker spots (sun spots) • Sunspots are areas that appear dark on the surface of the Sun. They appear dark because they are cooler than other parts of the Sun's surface. The temperature of a sunspot is still very hot though— around 6,500 degrees Fahrenheit! Sir Isaac Newton • It was on the same year of Galileo Galiei's death that a mathematician and physicist by the name of Sir Isaac Newton was born. At the early age of 23, Newton was able to theorize the existence of a force that held the moon in orbit around the Earth. Law of Universal Gravitation • All objects attract each other with a force of gravitational attraction. Gravity is universal. This force of gravitational attraction is directly dependent upon the masses of both objects and inversely proportional to the square of the distance that separates their centers. Newton's conclusion about the magnitude of gravitational forces is summarized symbolically as G= 6.67 𝑥 10−11 𝑁𝑚2 𝑘𝑔2
