Celestial Sphere Lab
Introduction
The purpose of this activity is for you to gain a clearer understanding of the celestial sphere, the Sun, the Moon, and the Earth by making virtual observations of a simulated sky. You will be using a powerful computer program called “The Sky” which will chart and display the stars, planets, comets, Moon, Sun, galaxies, nebulae, etc. as they would appear from any spot on the Earth – north, south, east, or west, at any point in time – past, present, or future. This is a complicated and extensive program that has lots of menus and options – so please only make the changes to the program that are described below.
Start the program by double clicking “The Sky” icon. Go to the File menu and choose Open. Then choose the file called “Celestial Sphere Lab”. Once the file is opened you should see the sky and a curve at the bottom of the screen that represents the southern horizon. The opening view is from Knoxville at midnight on January 1, 2000 local time. Before making your observations you should familiarize yourself with the controls. If you make any changes be sure to return everything to the original settings that were loaded with the Celestial Sphere Lab file. If you get lost you can always revert to the original file. Complete the following checklist – marking off each item as you go.
q Under the Data menu choose Site Information. Use the tabs to check the current settings. As you can see this is where you may change the site and time and date for the simulation.
q Click on the push button with the smaller yellow tag. This is the Common Names button. This button toggles on and off labels of constellations, stars, planets, etc.
q Click on the push buttons marked with the arrows located on the tool bar. These buttons control the direction in which you are looking from your site of observation – you can look up, down, and around. (The arrow keys on the keyboard do the same thing.)
q Click on the push buttons marked with N, E, S, W, Z. These buttons will instantly change your point of view to the north, east, south, or west horizon. The Z will give you a view of the zenith. Finish here by returning to the south horizon. (These letters on the keyboard do the same thing.)
q Click on the push buttons marked with a magnifying glass and either + or –. These buttons will zoom you in or out without changing the direction in which you are viewing. Return the zoom to its original setting. (Pushing one of the horizon buttons also returns the zoom setting as well as directing your point of view.) (PgUp and PgDn do the same things.)
q Under the View menu choose Reference Lines. Experiment with these choices except the Telescope Limit Lines. Before continuing choose only the following reference lines: ecliptic, equatorial grid, local and opaque horizon.
q Note that there are four push buttons on the tool bar that can be used to toggle some of the reference lines. The equatorial grid consists of blue lines that are the celestial coordinates of declination and right ascension. The horizon grid consists of red lines that are the altazimuth coordinates for the observer’s location. There are also buttons that can toggle constellation lines and boundaries.
q Look at the “Status Bar” at the bottom of the screen. As you move the cursor around the screen you will notice that the values change. The values given are the coordinates of the cursor. These coordinates can be either equatorial coordinates of right ascension and declination or horizon coordinates of altitude and azimuth. Under the View menu choose Status Bar and try displaying different information. Note: in some cases you may have to uncheck a certain box before you can check another.
q Try the “tape player” push buttons. The amount of time in the box is used to skip ahead or back in time. You can either Step Forward or Backward or Go Forward or Backward. The Step Forward moves you ahead one. The Go Forward runs you forward like a movie. The amount of time in the box is the time increment used when stepping or going forward or backward in time.
q Find the push button with the blue sky and Sun on it. When this button is pushed the daytime sky is simulated and stars are not visible when the Sun is up. When this button is not pushed then the stars are always visible, even when the Sun is in the sky; this is how the sky would look if the Earth did not have an atmosphere.
q If you click on any star or other object in the window, a box will appear with information on that object. You can use this feature to identify what you are seeing and to determine celestial and/or altazimuth coordinates.
For this set of questions you will be observing simulations of the sky as it appears from Knoxville. Record all observations and answers on the given data sheet.
1. Set the time skip interval to 1 minute and push the “Go Forward” button to set the sky into motion. The status bar should show the time and date – if not, then make it so. Now use the arrow buttons or the N, E, S, W, Z buttons to “look around” at the celestial sphere in motion. Use your imagination – visualize a huge sphere rotating around the Earth. Also imagine you are on the spinning Earth simply looking “out there” almost like being on a merry-go-round. Observe the east horizon, west horizon and north horizon. Record the direction in which the stars rise and set. Record the direction (clockwise or counterclockwise) of the apparent rotation about Polaris.
2. Under the Data menu choose Site Information and make the date and time January 1, 2000, 12 hours (noon). Now you should see the Sun in the southern sky (press the S button and zoom out twice). Confirm the date and time in the status bar. Set the time skip interval to 1 day. This is a 24 hour day and is hence equal to the mean solar day. Now click the Go Forward button and watch. This is what you would see if you went outside and faced south every 24 hours. Describe the apparent motion of the Sun relative to your horizon over the course of the year. Describe the apparent motion of the Sun relative to the celestial sphere over the course of the year.
3. Click the Reset Date & Time button to go back to January 1, 2000, 12 hours. Now Go Forward in time until you see the Sun cross the celestial equator. By stopping the simulation and then stepping either forward or backward you should be able to find the date at which the Sun crosses the equator. You may want to Zoom in to get the best result. This will be the point on this year’s calendar where we have the vernal equinox. In a similar fashion you should be able to find the dates for the other equinox in 2000. Record these dates. Repeat the process for the year 2003 and record the results. You should notice that the dates are not exactly the same every year.
4. Under the Data menu choose Site Information and change the date to the year 7000. Try different views and time skips. Describe the ways in which the sky is similar to today’s sky. Describe the ways in which the sky is different from today’s sky.
5. Determine the constellation and date of the vernal equinox in the year 7000.
6. Determine which bright star is fairly close to the celestial north pole and could be called the “north star” in the year 7000. Hint: point and click or use labels to find the name of the star.
7. Under the Data menu choose Site Information and change the time to January 1, 2000, 12 hours. Set the time skip interval to 24.83 hours. This interval is the approximate time for the moon to return to the same azimuth in the observer’s sky. Adjust the view so that the Moon is near the center of the screen and under the Orientation menu choose Zoom To and Wide Field. Click the Go Forward button to see the Moon’s changing appearance through the year. Compare and contrast to the results of the previous question regarding the Sun’s appearance.
In this section of the lab you will perform each of the following observations as simulated from different locations on Earth.. Set the site of observation to the given location and then make each observation in order to fill in the table.
q In the program you can specify a city or you can specify a latitude and longitude. This is done under the Data menu in Site Information. Just pick a city from the drop down list. In order to observe the sky from a foreign city you will need to go to the Site Information box and click on the Open button and select the file: Cities outside the USA.loc.
q Before proceeding you should reset the date to the year 2000.
q Under the View menu adjust the Status Bar so that it shows altitude and azimuth.
q At each location on Earth I invite you to try different views of the sky and different time skip intervals in order to get a real feel for the appearance of the celestial sphere from that site.
q You only need to determine coordinates in the table to the nearest one degree. There are many ways to obtain coordinates with the program but perhaps the simplest is just to click on the point in question and look through the info box for right ascension/declination and/or altitude/azimuth.
1. Stop the animation and turn to the north or south horizon so that you are viewing one of the celestial poles. Measure the altitude of the celestial pole.
2. Measure the maximum altitude of the celestial equator. This is the point on the celestial equator that intersects the observer’s meridian.
3. Turn to the opposite horizon from the celestial pole. Determine the declination at the horizon. This represents a limit on what part of the celestial sphere is visible from this site. In the table note whether this represents the minimum or maximum declination visible from this site.
4. Determine the declination at the zenith for observers at this site.
5. Determine the maximum and minimum altitude that the Sun will attain as it crosses the observer’s meridian over the course of one year. Hint: check the altitude of the Sun around noon on the dates of solstices and equinoxes.
In this section of the lab you will investigate the occurrence of eclipses. For these exercises you will remove the simulated horizon and atmosphere so that you can continuously observe the Moon, Sun, and stars.
q In the Data menu return the viewing location to Knoxville.
q Set the date to the year January 1, 2017, 0 hours.
q Under the Orientation menu choose Pole Up; this orients your view to the celestial coordinate system with the celestial north pole in an upward direction.
q Click on the Tracking Setup toolbar button (a little yellow “Saturn”). Select Earth’s Shadow and Lock On: Moon. You should see the Moon and the Sun nearby.
1. Step forward or backward in time until the Moon is nearest to the Sun.
2. Then set the time skip interval to 29.53 days (the Synodic month). Step forward and backward to observe the twelve times during the year 2017 that the Moon is near the Sun.
3. Find the two times during 2017 when the Moon is closest to the ecliptic and then change the time skip interval to 1 hour and step or skip forward or backward to search for an eclipse (Zoom in to see if the Moon actually covers part or all of the Sun). You should find that an eclipse is visible from Knoxville on one day in 2017. Use a slower time skip to observe this eclipse from beginning to end and determine which type it is.
4. Set the time skip interval to 1 day and search 14 days before and 14 days after for a lunar eclipse. You may need to Zoom out a bit to find the eclipse. But then Zoom in again and view from beginning to end and determine which type. Would this lunar eclipse be visible from Knoxville?
5. Under the Tools menu click on Eclipse Finder and check your answers. Try clicking on the Show Path of Totality option. Are there eclipses that occur during the year 2017 that are not visible from Knoxville?
The Sky from Knoxville
1.
Every night stars rise in the
__________ and set in the ___________.
Circumpolar stars revolve about polaris in a _____________________ direction.
2.
2000 2003 Date of Vernal Equinox Date of Autumnal
Equinox
Describe Sun’s motion relative to the
observer’s horizon over the course of one year:
Describe Sun’s motion relative to the celestial sphere over the course of one
year:
3.
4.
Year 7000
Similarities to today’s sky:
Differences from today’s sky:
5.
Year 7000
Constellation of Vernal Equinox:
Date of Vernal Equinox:
6.
Year 7000
Pole Star:
7.
Describe Moon’s motion relative to
the observer’s horizon over the course of one year:
Describe Moon’s motion relative to the celestial sphere over the course of one
year:
The Sky from Other Places
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Site Information |
Altitude of Pole |
Altitude of Equator |
Declination at zenith |
Limiting Declination |
Min/Max Alt. Of Sun |
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Location |
Longitude |
Latitude |
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Knoxville, Tennessee |
84° |
36° |
36° |
54° |
36° |
-54° (min) |
31°, 77° |
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Fairbanks, Alaska |
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North Pole |
N/A |
90.0°N |
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Quito, Ecuador |
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Sydney, Australia |
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Eclipses
A solar eclipse visible from Knoxville in
year 2017:
Date = _________________ Type = ___________________
The lunar eclipse that occurs
within about two weeks of the one above:
Date = _________________ Type = ___________________
Is this lunar eclipse visible from Knoxville? Explain.