Horizon Coordinate System (The Rotating Sky) (2)
docx
keyboard_arrow_up
School
Schoolcraft College *
*We aren’t endorsed by this school
Course
133
Subject
Astronomy
Date
Apr 3, 2024
Type
docx
Pages
5
Uploaded by PresidentExplorationCat81
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. 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
0.0
East
90.0
0.0
South
180.0
0.0
West
270.0
0.0
2. What is the value of the altitude coordinate at the horizon?
0.0°
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.
90.0°
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? These values are almost the same, on mine the declination of the star at the zenith is 44.3° and the latitude set is 43.9°, so I assume these numbers are actually supposed to be the same value.
b. The sun’s maximum declination is 23.5
∘
. Can the sun ever pass directly overhead in Michigan?
No, that declination is too low to pass directly overhead.
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.
Altitude: 45.0°
Azimuth: 0.0°
b. What do you notice about the altitude compared to the latitude of the observer?
They are nearly the same number.
7. Drag the star to various locations below the horizon. What do you notice about the altitude coordinate at all locations below the horizon? When placing stars below the horizon, the azimuth stays the same as above the horizon, but the altitude becomes 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 42
∘
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).
When looking north the stars are setting and moving eastwards. They travel towards the horizon and then start rising.
b. When facing east, how do the stars move from the perspective of the observer?
When facing east, the stars are rising at an angle, they are going up and right across the sky. c. When facing south, how do the stars move from the perspective of the observer?
When facing south, the stars are rising from the east and set as they travel towards the west. The stars are moving westward. d. When facing west, how do the stars move from the perspective of the observer?
When facing east the stars are setting at an angle, going down and right across the sky.
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
- Access to all documents
- Unlimited textbook solutions
- 24/7 expert homework help
e. Set the latitude to 0
∘
. Face east and advance time. It looks awesome! Check out the north pole as well (
90
∘
)
. (There isn’t a question that you need to answer here, but you can see how the sky looks different for people at different locations on the Earth.)
9. Head to https://stellarium-web.org
in your web browser. Turn off the atmosphere by clicking the “atmosphere” button if you can’t see any stars. Click the “Azimuthal Grid” button at the bottom of the screen to put the horizon coordinate system on the screen. Look in the simulated sky and choose any single star above the horizon. Notice the altitude and azimuth of the star in the pop-up window. Advance time so the star moves in the sky (you can hold down the arrows in the time box or use the slider-bar to advance time). What happens to the altitude and azimuth values of that star? As time advances, the altitude of the star decreases as the azimuth of the star increases. Both of these values are moving at a slow rate. Do the gridlines representing the horizon coordinate system move relative to the ground, or are they fixed to the ground? The gridlines are fixed to the ground, so as you change the direction you are facing, the gridlines stay fixed to the location they were previously in.
Do the objects in the sky move relative to the coordinate system? No they do not. Summary of Findings: In the horizon coordinate system, the azimuth is measured from north (0°) and increases toward the east (90°) with a maximum value of 360° when the observer has rotated back to north. The altitude coordinate is measured from the horizon (0°) and increases towards its maximum value, the zenith (90°).
The coordinate decreases towards its minimum value, the nadir (-90°). All stars with negative altitudes are located below the horizon. The horizon coordinates of an object in the sky depend entirely on the location of the observer. The coordinate system is used the same way, no matter where the observer is located on the Earth, so the altitude and azimuth of the cardinal directions (N, S, E, W), the horizon, the zenith, etc. are the same for any observer. The altitude and azimuth coordinates of any celestial object will change over time, because the horizon coordinate system is fixed relative to the Earth’s horizon (so the gridlines stay in the same place while
the celestial objects in the background move). At northern latitudes, stars that are further north stay in the sky longer (with any circumpolar stars that never set being located most to the north).