92023stars

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School

Eastern Michigan University *

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Course

105

Subject

Astronomy

Date

Dec 6, 2023

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pdf

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Uploaded by diontaelt

Horizon Coordinate System This activity will teach us how to use the observer-centered Horizon Coordinate System . Unlike the Equatorial Coordinate System , which allows observers to measure the same coordinates for any celestial object from any location on the Earth, the coordinates of every celestial object measured in this coordinate system will have coordinates that change with time and also change with location on the Earth. While this coordinate system is not very useful for communicating the location of celestial objects to other astronomers around the world, it is useful for communicating with people you are observing with and attaining a better understanding of how the sky moves from your perspective on the Earth. The Horizon Coordinate System has two coordinates, azimuth and altitude . The azimuthal coordinate tells the observer the direction that they need to face to locate an object in the sky. Once facing the correct direction, the altitude coordinate tells the observer how high to look above the theoretical horizon. Open the ‘Rotating Sky Explorer’ simulation ( https://astro.unl.edu/naap/motion2/animations/ce_hc.html ). Make sure the only checked boxes in the ‘Appearance Settings’ panel (bottom of simulation window) are ‘show labels’, ‘show celestial equator’, and ‘show underside of horizon diagram’. The top-left panel shows the celestial sphere, the location where all celestial objects are projected in the equatorial coordinate system. In this window we will be able to adjust the declination and right ascension of stars to see how an observer at a specific location on the Earth would perceive the motion of that star as the Earth rotates. The top-right panel shows how the observer will perceive the celestial sphere in the horizon coordinate system. In this window we will be able to adjust the altitude and azimuth of stars. Both windows can be manipulated by clicking and dragging on the sphere. Set the latitude of the observer to 42°N, 84°W. These are approximately the GPS coordinates of EMU. 1

1. Hold the shift key and click on the sphere in the top-right window to add a star to the northern point directly along the horizon (the star should be centered on the northern point right on the line where the ground meets the sky). Record the altitude and azimuth values (you can estimate what the values should be if you are having difficulty putting the star exactly in the right location). Drag the star to the eastern, southern, and western point directly along the horizon and record the altitude and azimuth values. * All numbers in the table should be integers. Cardinal Direction Azimuth (°) Altitude (°) North 0 0 East 90 0 South 180 0 West 270 0 2. What is the value of the altitude coordinate at the horizon? - 0 degrees 3. Which direction from north must an observer rotate for the azimuth they are observing to increase, east or west? - east 4. Drag the star to the zenith. Note that at the zenith, all azimuthal coordinates yield the same location (you do not have to face a specific N/S/E/W direction in order to look “up”). What is the maximum value of the altitude? *It may be difficult to get the exact value, but it should be a whole number. - 90 degrees 5. Compare the declination of a star at the zenith to the latitude of the observer. Objects at this declination always pass directly overhead. a. What do you notice about these values? - the azimuth stays exactly the same while the altitude decreases towards the horizon b. The sun’s maximum declination is 23.5. Can the sun ever pass directly overhead in 2

Michigan? - yes! 6. Drag the star to the north celestial pole (NCP). a. What are the horizon coordinates of the NCP? *It might be difficult to get the exact value, but it should be a whole number. - If you are looking north, the azimuth would be 0 degrees and the altitude would be 90 degrees b. What do you notice about the altitude compared to the latitude of the observer? the altitude of the star is much higher than the latitude of the observer. To have the altitude be eye level with the observer the number would have to basically be cut in half 7. Drag the star to various locations below the horizon. What do you notice about the altitude coordinate at all locations below the horizon? - they are negative 8. In the star controls window, click the ‘add star randomly’ button 15 – 20 times. Make sure the ‘long star trails’ option is selected. Set the latitude to N. Advance time to create star trails and keep track of the motion of the stars through the sky. * You will need to look through the sphere to see the observer’s perspective. a. When facing north, how do the stars move from the perspective of the observer? *When you are writing your descriptions, you can focus on details like whether stars are rising or setting, whether they rise/set at an angle, or if they are traveling across the sky in a certain direction/pattern (instead of rising/setting). - the stars are not rising or setting, the travel in circles around the NCP and stay at the same altitude. They move from north to east going right. b. When facing east, how do the stars move from the perspective of the observer? - When facing east, the stars are setting and going from east to west like going overhead the observer. If not, then they are traveling along the horizon from east to south c. When facing south, how do the stars move from the perspective of the observer? - When facing south the stars are rising in the east and setting in the west. They are going 3

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