The History of Astronomy Theories, People, and Discoveries of the Past
Early man recorded very little history. Left some clues in the form of petrographs. Stone drawings that show eclipses, comets, supernovae. The petrograph to the right represents a supernova. Found in a pueblo in southwest America. Prehistory
Prehistory Early man was frightened by the sky. Believed that the heavens held power over earthly existence. Psychology of the Unknown Astrology born as an attempt to understand, predict, and influence events.
History First written records were astronomical observations. Early Chinese, Central American (Mayans), and Northern Europe. Stonehenge in England marks solstices.
1600 B.C. Babylonians First to write stuff down. This early culture recorded the positions of the planets, the times of eclipses, and other celestial phenomena.
~500 B.C. Hellenistic Culture Greeks acquired the records of the Babylonians. Conquest of Alexander the Great. Greeks applied data to construct a cosmological framework. Basically means this is the way heaven is arranged.
~500 B.C. The Greeks used the star charts for more than just practical uses. Navigation on the seas. Began to develop new ideas about the universe. Devised new experiments to try to prove their theories.
~ 480 B.C. Thales of Miletus Proposed that the universe was rational and could be understood by humans. Used the data of the Babylonians to predict the occurrence of eclipses.
400 B.C. Plato Made what seems like a minor contribution. But his statement dominated astronomical thinking for centuries. Heaven is perfect and a circle is the perfect form.
330 B.C. Heraclides Proposed the first model of the solar system. Geocentric Earth at the center of the universe.
330 B.C. Orbits were perfect circles (Plato). Was the first round in the debate over geocentric vs. heliocentric solar system.
270 B.C. Round 2 Aristarchus of Samos Made several important contributions. He was the first person to propose a heliocentric model of the solar system. Mathematically proved the Sun is farther away from the Earth than the Moon.
270 B.C. This model put the Sun at the center (helios is Greek for Sun, thus heliocentric model). Still used circles for the orbits (Plato).
270 B.C. Used trigonometry to prove that the Sun is farther away from the Earth than the Moon. Idea was correct, just made mathematical errors.
270 B.C. Parallax Problems with Aristarchus model: 1. If the Earth is moving, why couldn t people feel it? 2. No parallax seen in the stars. a. As Earth moves in orbit, the apparent position of nearby stars move in relation to stars farther away. 3. Geocentric = Egocentric a. More natural. b. Highlights importance of man!
100 B.C. Hipparchus Produced the first star catalog showing positions and magnitude (brightness). Recorded names of constellations.
200 A.D. Ptolemy Librarian in the ancient city of Alexandria. Resurrected Heraclides geocentric model. Used the library resources containing centuries of data to formulate a complete description of the solar system.
200 A.D. Ptolemy explained and predicted the apparent motions of celestial bodies (planets). He used perfect circular orbits for the planets, drawing on Plato s idea that all things in heaven were perfect. Major problem was retrograde motion.
Retrograde Motion Retrograde motion is movement in the opposite direction from the expected motion. Some planets would move backwards during their motion across the nighttime sky over the year. This became very difficult to explain in a geocentric model because the orbits were supposed to be perfect circles around the Earth. Epicycle Motion
Ptolemaic Model Ptolemy developed a very complex system using circles to explain the retrograde motion of the planets. He used circles on circles as seen below. The Sun and Moon did not need them because they did not exhibit retrograde motion.
Ptolemaic Model The main circle around the Earth was called a deferent. The smaller circle on which the planet traveled around the deferent was called an epicycle. To explain all the known motions, Ptolemy was forced to use 28 circles. Epicycle Motion
As the planet moves around the epicycle, it is also traveling along the deferent. This causes the retrograde motion of the planets (points 3 to 5 in the diagram to the right). This is a way to diagram how Ptolemy s theory would work. Ptolemaic Model
200 A.D. Ptolemy realized that the planets were a lot closer to Earth than the stars, but he still believed in the existence of celestial spheres. Stars and planets were attached to these spheres. He also realized there might be other objects outside the sphere of fixed stars.
200 A.D. So, Ptolemy proposed there were other spheres outside those visible. Ended with the Primum Mobile (God), which provided the motions of the other spheres. This was the first mathematical attempt to explain nature.
~ 410 A.D. Alexandria burns and is destroyed along with most of the records in the library. Loss of most knowledge acquired by the Greeks and Romans. Roman culture collapses leading to the Dark Ages. Roman Catholic Church absorbs Aristotle s scientific method and Ptolemy s model of the universe. Ancient Lighthouse at Alexandria
~ 1500 s The Renaissance People returned to scientific values where new ideas were more important than religious beliefs. Art and science began to flourish. Da Vinci, Copernicus, Raphael, and others served the church, but they began to rebel (secretly).
~ 1500 s Nicholas Copernicus Reinvented the heliocentric model. More than just an attempt to solve retrograde motion. It had both social and political consequences by challenging the power of the Catholic Church. Psalm 93 Thou hast fixed the Earth immovable and firm.
~ 1500 s Copernicus model also questioned the authority of the most revered wise men of the ancient world (Ptolemy, Plato, Heraclides). He forced a change in humanity s view of the world and our importance in it. Yet, even his model had problems.
~ 1500 s Copernicus still used circular orbits. As a result, he still was forced to use epicycles and deferents. His model was even more complicated than Ptolemy s. Would have failed our modern criteria that models be as simple as possible (Occam s Razor).
Tycho Brahe 1 st true observer. Built the Danish Observatory. 1580 Measured the positions of planets and the stars to a very high degree of accuracy for his time. He also used trigonometry to measure the distance to the Sun.
1600 s Johannes Kepler A student of Tycho Brahe with access to all of Brahe s data. Used this data to formulate the Laws of Planetary Motion. He was the first to realize that orbits were elliptical. Theory fit the data, not the other way around.
1600 s Solved one problem of heliocentric model. Eliminated the need for epicycles and deferents.
1600 s This highly accurate system determining all the motions of the planets marked the beginning of the clockwork universe concept.
First Law The orbits of celestial objects (planets) are ellipses. The Sun is not in the center, but at one of the two foci of the ellipse. No matter where you are on the ellipse, the sum of the distances from the foci to the object is constant. This ellipse is exaggerated, they are very close to being circular.
Second Law The planets sweep out equal areas in equal times. If you look at the shaded areas to the right, those two areas are equal to each other. Which means the planet moves slightly faster in its closest pass to the Sun, slower farther away. Kepler's Laws with Animation
The square of a planet s orbital period is proportional to the cube of its average distance from the Sun. The period is time of revolution. The radius can be miles, kilometers, or astronomical units. Third Law
Examples of 3 rd Law Planet Period A.U. T² R³ Mercury 0.24 0.39 0.06 0.06 Venus 0.62 0.72 0.39 0.37 Earth 1.00 1.00 1.00 1.00 Mars 1.88 1.52 3.53 3.51 Jupiter 11.9 5.20 142 141 Saturn 29.5 9.54 870 868 Period = Earth Years T² = Period Squared R³ = Average Radius of Orbit (A.U.) Cubed
You Do The Math The A.U. of a planet is 2.53. About how long will it take the planet to orbit the Sun? 2.53³ = 16.19 The square root of 16.19 = 4.02 Earth Years
1620 s Galileo Finished off the idea of a geocentric universe. Used a telescope to make the following discoveries: 1. Sunspots 2. Mountains on the Moon 3. Milky Way with Lots of Stars 4. Venus had Phases Like Our Moon 5. Jupiter s Four Main Moons
1620 s Most damaging discovery was the four moons of Jupiter. Those moons were orbiting Jupiter instead of the Earth. This did not fit the idea of a geocentric model. At this point, the Catholic Church was forced to give in and accept a heliocentric model. Began the decline of religion.
1680 s Isaac Newton One of the most intelligent men in history. Two main applications to Astronomy. 1. Law of Universal Gravitation 2. Laws of Motion
Law of Universal Gravitation Simply put every object in the universe has a gravitational attraction for every other object. An atom in your fingernail has a gravitational pull on the Moon!
Law of Universal Gravitation Newton developed a formula to go with the Law. F = G M 1 M 2 / r² F = force of attraction; G = gravitational constant; M 1 = mass of 1 st body; M 2 = mass of 2 nd body; r² = the distance between the two bodies squared.
Law of Universal Gravitation The gravitational constant is a very small number. It is for this reason you can t feel a gravitational pull between you and another person. Mass has a role as well, you may pull on the Moon, but your mass is so small there is no apparent change in its orbit.
Law of Universal Gravitation Lets look at an mathematical example. It will not be precise, because the gravitational constant is a lot smaller than the number we will use for this example. What is the attractive force (F) of two bodies in space with the following information: G = 0.00006 M 1 = 500 kg; M 2 = 3000 kg R = 10 m
Law of Universal Gravitation Remember the formula: F = G M 1 M 2 / r² 500 kg x 3000 kg = 1,500,000 kg 1,500,000 kg x 0.00006 = 90 kg (10m)² = 100m² 90 kg 100m² = 0.9 kg/m² This is a very small amount of force between two huge objects that are very close. Imagine if we used the real G or the objects were a lot farther apart! Try it at a distance of 100 m apart!
Law of Universal Gravitation Remember the formula: F = G M 1 M 2 / r² 500 kg x 3000 kg = 1,500,000 kg 1,500,000 kg x 0.00006 = 90 kg (100m)² = 10,000m² 90 kg 10,000m² = 0.009 kg/m² The distance increased by a factor of 10, the force decreased by a factor of 100! Try to imagine the force between the two objects at astronomical distances! It is very tiny, but still there.
Law of Universal Gravitation This law also predicts that, in general, the orbit of an object can be any of four conic sections as well as a straight line. Conic sections are slices thru a cone at different angles.
Law of Universal Gravitation Scientists have discovered some comets that have parabolic or hyperbolic orbits. In these cases, they do not return to the Sun after they pass by it.
Laws of Motion We have looked at the Laws of Motion in Motion Unit. Will not go over them again. Necessary for spaceflight.