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Ancient Astronomies Copyright © Mary Ann Sullivan. I have photographed (on site), scanned, and manipulated all the images on these pages. Please feel free to use them for personal or educational purposes. Since measuring time and determining seasons are important to all civilizations, we find that essentially all civilizations set up some kind of astronomical system. Some of these civilizations were in warm climates where the change of seasons was not so obvious, but they still needed to determine when the floods or the rains would come for agricultural effectiveness. Here are a few examples: Ancient Astronomies Egyptians based their agriculture on the flooding of the Nile. They noticed that the flooding occurred close to the heliacal rising of Sirius, the brightest star in the night sky. The pyramids have structures that are aligned with astronomical events. Ancient Astronomies Stonehenge is an ancient and strange site in Britain that has massive stones that appear to have a relation to various astronomical occurrences, such as summer and winter solstices. The Mayans had a calendar based on the motions of Venus. The Babylonians had a well developed astronomy that detailed the motions of the moon and planets. The North American Indians developed a Medicine Wheel that was aligned with the summer and winter solstices. Time Year: Cycle through the seasons. Month: Based on cycle of the moon – adjusted to make exactly 12 months in a year instead of the actual 12.4 cycles per year. Week: 7 is chosen since there are 7 astronomical objects besides the stars: Sunday, Moonday, Thorsday (Thor = Jupiter), Saturnday, in English can easily be identified. In Spanish you can easily identify Marsday (Martes = Tuesday), Mercuryday (Miercoles = Wednesday), and Venusday (Viernes = Friday). Time Day: Time from noon on one day to noon on the next. Hour: Day is broken into day and night, and the daytime is broken into 12 units, just like the year, and the night is broken into 12 units, also. Total is 24 hours in a day. Minute: A minute (pronounced my-nute’ meaning tiny) part of an hour – based on Egyptian idea of a minute (tiny) part = 1/60th . Second: A minute (tiny) part of a minute (tiny) part of an hour, or a second minute (tiny) part of an hour, or simply a second. The Classical Greeks Homer: 8th century BC; famous for the Illiad and the Odyssey : celestial objects are gods, flat earth, dome for sky. Thales: ~600 BC; Determined length of year, stars are not gods (since their motions are too orderly). Pythagoras: ~500 BC; geometry developed, idea of spherical objects. Earth is NOT flat! Plato: ~400 BC; pure form, ideals in the heavens, abstract, mathematical forms versus real objects. The Classical Greeks Aristotle: ~350 BC; placed emphasis on observations; postulated four elements: earth, air, fire and water, plus a fifth element, the quintessence, that filled the heavens. Earth not spinning since there is no great wind outside, so Earth must be stationary, so other things must be revolving around the earth (geocentric model). ** Alexander the Great lived during this time Eratosthenes: ~250 BC; used geometry and observations to determine the circumference of the spherical earth – accurate to about 1%. The Classical Greeks Ptolemy: ~150 AD, in Alexandria, Egypt. Put together a book, the Almagest. This offered a theory of the heavens that gave fairly good predictions. It was based on circular orbits of objects around the spherical earth (geocentric), but included circles on circles (epicycles) that accounted very well for the retrograde motion of the planets. Epicycles planet earth center Beginnings of Modern Theories ** Gutenberg Printing Press invented in 1450 ** Columbus discovers America in 1492 Copernicus (1473-1543) Proposed sun centered system (heliocentric) to more easily explain retrograde motion. (We’ll show this in lab with a computer simulation to see how this works.) Book published in 1543. Beginnings of Modern Theories Tycho Brahe (1546-1601) He collected great amounts of very precise data about the positions of the planets using instrumentation he developed. Johannes Kepler (1571-1630) He used Brahe’s data and mathematics to come up with three laws: 1. Planets follow elliptical instead of circular orbits. 2. Planets sweep through equal areas in equal times. 3. The square of the period is proportional to the cube of the semimajor axis of the ellipse. Beginnings of Modern Theories Galileo Galilei (1564-1642) contemporary of Kepler. He used the telescope to look at astronomical objects. He discovered craters on the Moon, moons going around Jupiter, rings around Saturn, and the fact that Venus has phases that are related to its orbit. The heliocentric model can explain all of these, whereas the geocentric model fails. Also, no need for Aristotle’s “quintessence” since what is up there looks like it is the same as what is down here. Modern Theories Issac Newton (1642-1727) Formulated Newton’s three laws of motion, and Newton’s law of gravity. He invented the calculus to work with these laws. Using these tools and the idea that what is up there obeys the same laws as what is down here, he explained Kepler’s three laws in terms of these more fundamental laws. He worked in optics as well, and invented the reflecting telescope (more about this in part 2 of the course). Newton’s Laws of Motion 1st Law of Motion: Objects tend to go in a straight line with constant speed. 2nd Law of Motion: Forces tend to change an object’s motion (speed it up, slow it down, or change its direction). The more mass an object has, the harder it is to change its motion: ΣF = ma . 3rd Law of Motion: For every action, there is an equal and opposite reaction. (You can’t push yourself. You have to push something else, and hope it pushes back.) Newton’s Law of Gravity Every object that has mass attracts every other object that has mass with a force that depends on the masses of both objects, and falls off with the distance between those objects. That force is very small unless one or both objects has a lot of mass (like the earth). F = G*m1*m2 / r2 . Planetary Motion All planets experience the gravitational force of the sun. They would all fall into the sun, except they are already moving sideways. They would normally continue their motion and move away from the sun, but the sun is pulling them in. To orbit the sun, they must move not too fast to escape, nor too slow so that they fall into the sun. The result is elliptical orbits. We’ll simulate this on a computer in one of the labs. Planetary Orbits The closer the planet is to the sun, the more force (from Newton’s Law of Gravity). In order not to fall in, the closer planets must move faster. The farther the planet is from the sun, the less force. In order not to escape from the sun, the farther planets must move slower. Not only do the inner planets have shorter orbits, they have to move faster around those orbits. Mathematically, this works out to explain Kepler’s 3rd law that relates the period of orbit to the planet’s distance from the sun. Modern Theories Albert Einstein (1879 – 1955) Theory of Relativity (1905 and following) predicted that different observers will measure different times, distances and masses based on the observers’ relative speeds. In particular, lifetimes and masses of particles differ if they are stationary compared to when they are moving. These effects are only noticeable as the speeds approach the speed of light (~670 million miles per hour). This theory implies that the measurement of time is not absolute, but rather relative. Modern Theories Quantum Theory (20th century) was developed by many different people in trying to make sense of measurements on the atomic scale. Basically, some things (for example, light and electrons) act like particles sometimes and act like waves at other times. This is not random – in certain well defined situations, they always act like particles. In other well defined situations, they always act like waves. Connecting Lives of Astronomers to Historical events 1066 Battle of Hastings (Norman invasion of England) 1450 Guttenburg printing press invented 1492 Columbus “discovers” America Copernicus 1473-1543 1588 Spanish Armada defeated Tycho Brahe 1546-1601 by Britain Johannes Kepler 1571-1630 1607 Jamestown established Galileo Galilei 1564-1642 (1st permanent English settlement) Issac Newton 1642-1727 1776 Declaration of Independence (America)