AST 101, Week 1 Notes
AST 101, Week 1 Notes 101
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This 4 page Class Notes was uploaded by Bethany Marsfelder on Monday September 19, 2016. The Class Notes belongs to 101 at Syracuse University taught by Professor Walter Freeman in Fall 2016. Since its upload, it has received 281 views. For similar materials see Our Corner of the Universe in Astronomy at Syracuse University.
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Date Created: 09/19/16
August 30, 2016 AST 101 – Professor Freeman Lecture: Introduction to Astronomy (What’s out there, why we learn it, and why it’s awesome) Textbook Pages: 2-16 Contact Information: o walterfreeman.github.io/ast101 o Dr. Walter Freeman (email@example.com or firstname.lastname@example.org ) Course Outline: four units o Naked-eye astronomy What can we see from Earth? What changes do we see in the sky? How are they explained by Earth’s motion? o Astromechanics How does scientific thought work? How do we know planets orbit the Sun? Planetary motion – what do they look like and what causes them to move this way? o The science of light/Light and Electromagnetism What is light? Where does it come from? What does it do? How do we use light to study the sky? What has this taught us about the Sun? o Humanity and the cosmos What are the past and present of spaceflight? What might its future be in our lifetimes and beyond? Where else in the Universe might we find life, and what might it look like? Units of measurement o 1 kilometer (Earth-size things) o 1 AU = 150 million km = about 9 light-minutes (planets) o 1 light-year = 60,000 AU = 9 trillion km Planet Solar System Galaxy Galaxy Cluster Supercluster Universe Basic Astronomical Objects: o Star – A large, glowing ball of gas that generates heat and light through nuclear fusion in its core. Example: the Sun o Planet – A moderately large object that orbits a star and shines primarily by reflecting light from its star. According to a definition adopted in 2006, an object can be considered a planet only if it 1. Orbits a star, 2. Is large enough for its own gravity to make it round, and 3. Has cleared most of the other objects from its orbital path. An object that meets the first two but not the third, like Pluto, is designated a dwarf planet. o Moon – An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object. o Asteroid – A relatively small and rocky object that orbits a star. o Comet – A relatively small and ice-rich object that orbits a star. o Small solar system body – An asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet. Collections of Astronomical Objects: o Solar system – The Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system (solar means “of the Sun”), it is often applied to other star systems as well. o Star system – A star (sometimes more than one star) and any planets and other materials that orbit it. o Galaxy - A great island of stars in space, containing millions, billions, or even trillions of stars, all held together by gravity and orbiting a common center. o Cluster/group of galaxies – A collection of galaxies bound together by gravity. Small collections of galaxies are generally called groups, while larger collections are called clusters. o Supercluster – A gigantic region of space in which many groups and clusters of galaxies are packed more closely together than elsewhere in the universe. o Universe/Cosmos – The sum total of all matter and energy – that is, all galaxies and everything between them. o Observable universe – The portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe. Astronomical Distance Units: o Astronomical unit (AU) – The average distance between Earth and the Sun, which is about 150 million kilometers. More technically, 1 AU is the length of the semimajor axis of Earth’s orbit o Light year – The distance that light can travel in 1 year, which is about 10 trillion kilometers (more precisely, 9.46 trillion km) Terms Relating to Motion: o Rotation – The spinning of an object around its axis. o Orbit/Revolution – The orbital motion of one object around another due to gravity. o Expansion (of the universe) – The increase in the average distance between galaxies as time progresses. September 1, 2016 AST 101 – Professor Freeman Lecture: The celestial sphere Textbook Pages: 25-30 Lecture Tutorial: 1-4 (Motion, Part I) “I know that I am mortal by nature and ephemeral, but when I trace at my pleasure the windings to and fro of the heavenly bodies, I no longer touch earth with my feet. I stand in the presence of Zeus himself and take my fill of ambrosia.” – Claudius Ptolemy Focus Questions: o What does the night sky look like? o What are we able to see, and why does the night sky move the way it does? o How have we affected the night sky? o How does the night sky move each night? The Stars to Us o The view of the stars is “the cultural heritage of humanity” o Astronomy is the first science and the first shared science o The one thing all of Earth shares The light from the sun scatters off atmosphere – this is why stars “go away” Sky seems to “spin” about Polaris, the pole star o Why? Ptolemy theorized that it must be because all the stars are attached to a sphere very far away and rotates around the earth once a day But, the Earth rotates and the stars don’t move Plus, the stars are far apart, and we don’t have far reaching depth perception, so we can’t really comprehend this Key Ideas The stars are very far away compared to the Earth’s motion Concept of parallax: it doesn’t matter the stars are different distances away since those distances are so huge compared to our motion. Ptolemy’s celestial sphere’s predictions will be badly wrong for the motion of the Sun and the planets It doesn’t matter if Earth rotates or if the celestial sphere rotates: relative motion controls what we see. Summary o We can treat the stars as all rotating together, on an invisible sphere far away o We can expect this to get the stars “right” and the planets and Sun “wrong” o The axis of rotation is the same as the Earth’s, and it rotates once per day o Only half of the sphere is visible, because the Earth is in the way o Horizon: a plane lying along the Earth at our location o Zenith: the point directly overhead o Celestial pole: the point about which the stars appear to rotate Additional o Constellations A region of the sky with well-defined borders: the familiar patterns of stars merely help us locate these constellations. o Celestial Poles North Celestial Pole – the point directly over Earth’s North Pole South Celestial Pole – the point directly over Earth’s South Pole Celestial Equator – a projection of Earth’s equator into space, makes a complete circle around the celestial sphere Ecliptic – the path the Sun follows as it appears to circle around the celestial sphere once each year. It crosses the celestial equator at a 23.5-degree angle, because that is the tilt of Earth’s axis. o The Milky Way A band of light we call the Milky Way circles all the way around the celestial sphere, passing through more than a dozen constellations Relationship to the Milky Way Galaxy: it traces our galaxy’s disk of stars- the galactic plane- as it appears from our location within the galaxy. o The Local Sky The sky as seen from wherever you happen to be standing and appears to take the appearance of a hemisphere or dome. The dome shape arises from the fact that we see only half of the celestial sphere at any particular moment from any particular location, while the ground blocks the other half from view. o Pinpointing the Direction of a Star Direction/Azimuth – is degrees clockwise from due north Altitude – above the horizon o Circumpolar A star that remains perpetually above the horizon, circling counterclockwise around the north celestial pole each day. o Variation with Latitude Latitude measures north-south position on Earth Longitude measures east-west position. Latitude affects the constellations we see because it affects the locations of the horizon and zenith relative to the celestial sphere.
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