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Chapter 2

Chapter Two

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Wilfrid Laurier University
Victor Aurora

AS101 Chapter 2: Patterns and Cycles Week Two January 13 th RECAP • There are different objects at different scales in the visible Universe, but mostly invisible • Objects in the Universe are separated by astronomical differences • Our place in the Cosmos is our observatory of the Universe • What does this photo show? o Circle and a spiral pattern with different faded colours Learning Goals • Describe Earth’s rotation How the Earth is Moving • Two basic motions • • 1. Revolves around (“orbits”) the Sun (very close to being a circle) • 2. Rotation on its (tilted) axis o Which motion is responsible for rising and setting of the Sun? Its rotation • Assume for now that the Sun rises from the Eastern half of the sky and sets towards the Western half of the sky • To a moving observer, stationary objects appear to move in the opposite direction as the observers motion • Which way does the Earth rotate • Quiz question: Counterclockwise is where the Earth rotates as viewed from above the North Pole Stimulating the Sky • Do the stars rise and set like the Sun? o Sort of, some rise and set, some don’t • Are the stars up in the daytime? o Yes the daylight hides the stars The Celestial Sphere • A conceptual model of the sky o An imaginary sphere of very large radius surrounding the Earth • Assumptions o Objects in the sky appear to be attached to the rotating celestial sphere • Allows us to specify the direction to an object in the sky and how it moves over a single night • From the inside, you can see exactly one-half of the entire sky at any given time, from your horizon up to your zenith (directly overhead) • The North and South Celestial Poles are directly above the Earth’s North and South poles • Your Meridian is the line passing through the North point on your horizon, through your Zenith and to the South point on your horizon • Some stars never appear to set – in the Northern Hemisphere, they are located near the North Celestial Pole (NCP) o These are called circumpolar stars because they appear to circle the pole, and never set o The North Celestial Pole is near the star Polaris • Other stars also make circles around one of the celestial poles, but part of their daily path is below the horizon • Stars reach their highest points when they cross the meridian (also true for the Sun) Terrestrial coordinates • We specify locations on Earth using latitude and longitude • Longitude: in degrees East of West of the Prime Meridian • Latitude: in degrees North or South of the Equator • On the Earth, intersecting lines of longitude and latitude form a gird • Imagine this grid Celestial coordinates • We specify positions in the sky using Right Ascension (RA) and Declination (Dec) • Right Ascension: in hours East of the Vernal Equinox o From 0h to 24h • Declination: in degrees North (+) or South (-) the Celestial Equator o From +90 degrees to -90 degrees Astronomy Brain Training • Recall: we can only see one-half of the sky at one time • The half that we see at a given time of night changes depending on o The position of the Earth along its annual orbit o The latitude of the observer Summary • Objects in the night-sky rotate due to the Earth’s rotation axis • The celestial sphere is a model that helps us map out the relative positions and the nightly motions of these objects • What we see in the night-sky depends on our latitude • Readings: 2.1, 2.6, 2.7, 2.8 th January 15 Recap • Earth’s rotation on its axis causes objects in the night sky to rotate as well • Can specify the positions of these objects and visualize their motion using the celestial sphere model • The stars that we can see, and how they move over the course of a night, both change with latitude Learning Goals • Find our way around the night sky • Measure angular size and the brightness of stars • How the earth revolution changes over the course of a year Precession: a slow wobble • The Earth is not perfect, has a bit of a bulge in the middle • The Sun and Moon are unevenly pulling on the slight (43 km) bulge around the Earths equator o This causes a very slow but regular wobble of the Earths axis • Changes the NCP o This very slowly changes the position of the NCP, and all the other coordinates  One full “wobble” takes 26 000 years o Catalogues of stellar coordinates are slightly adjusted every 50 years Chapter 2.1: The Stars • All around the world, ancient cultures celebrated heroes, gods, and mythical beasts, by naming groups of stars called constellations • Constellation: one of the stellar patterns identified by name, usually of mythological gods, people, animals or objects. Also, the region of the sky containing that star pattern • Originated in ancient civilizations of Mesopotomia, Babylon, Egypt, and Greece • In recent centuries, astronomers have added 40 modern constellations to fill gaps o In 1928, the AU established a total of 88 official constellations with clearly defined permenant boundaries that together cover the entire sky  Thus a constellation is now a section of the sky not only a group, and any star within the region belongs only to that one constellation • Asterisms: a named grouping of stars that is not one of the recognized constellations o The Big Dipper is an example from the constellation Ursa Major The Names of Stars • Names are in Latin or Greek and derived from ancient Arabic • Another way to identify stars is to assign Greek letters to the bright stars in a constellation in approximate order of brightness o Alpha (brightest) o Beta and etc. The Brightest of Stars • Magnitude scale: The astronomical brightness scale, the larger the number, the fainter the star • Ancient astronomers divided the stars into six brightness groups o Brightest – first magnitude stars o And so on to the sixth magnitude stars which is the faintest visible to the human eye • The Greek Astronomer Hipparchus is believed to have compiled the first star catalogue, and there is evidence he used the magnitude system in that catalogue • Apparent visual magnitude: A measure of the brightness of a star as seen by human eyes on Earth (brightness is quite subjective) • Flux: A measure of the flow of energy out of a surface. Usually applied to light • Limitations of the apparent visual magnitude system have motivated astronomers to supplement it in various ways o 1. Some stars are so bright the magnitude scale must extend into negative numbers, as demonstrated in Figure 2.2 o 2. With a telescope you can find stars much fainter than the limit for your unaided eye. Thus the magnitude system has also been extended to numbers larger than sixth magnitude to include faint stars o 3. Apparent magnitude tells only how bright the star is as seen from Earth but doesn’t tell anything about a stars true power Chapter 2.6: Stellar Coordinates • Today, astronomers recognize 88 constellations • The sky is divided into parcels that each contain the constellation and other stars or objects in the same part of the sky • The stars in a constellation are not at the same distance from the Earth, just in the same direction o All the patterns made by the stars will change as the Sun (and other stars) slowly orbit the center of the Milky Way • In the sky, latitude is called Declination o The angular distance of an object on the celestial sphere measured north (+) or (-) from the celestial equator • Longitude is called right ascension o The angular east-west distance of an object on the celestial sphere measured from the vernal equinox; measured in hours, minutes, and seconds rather than angular degrees • The constellations are arbitrary patterns of stars invented, mostly by ancient cultures January 20 th RECAP • Objects in the sky move in circles around the celestial poles due to the rotation in axis • What we see in the sky depends on our latitude • Earths revolution around the Sun causes different constellations to be visible at night through the year Chapter 2.7: Timekeeping Timekeeping by Day • Our local meridian is the imaginary line ending at the north and south celestial poles that cuts through our zenith • How can we define what a “day” should be? Two ideas: o Solar Day: The average time between successive crossings of the Sun on the local meridian (24 hours) o Sidereal Day: The time between successive crossings of any star on the
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