RanaAditya_Lab3

M ONTGOMERY C OLLEGE – R OCKVILLE A STRONOMY 101 ASTR101 Laboratory 3 – Mapping the Night Sky 1 Name: Aditya Rana Ever tried to give someone directions to a friend's house without knowing the exact address? It’s hard! Just as buildings have addresses to help us find them, astronomers need ways to locate things in the sky. Ideally, there should be a precise way to map the sky, just as we have a map of the surface of the Earth. To locate any point on the surface of the Earth, we specify the point’s LATITUDE and LONGITUDE , two angles that measure a spot's position relative to the center of the Earth. For example, Rockville (at least the center of it) is located at 39º North , and 77º West . Similarly, we need a way to tell other astronomers where to look for things in the sky. We can’t just say “Look to the left of that bright red star and then go up a little.” In this assignment, we’ll study the two ways of locating objects in the sky using angles – one called Altitude & Azimuth (Alt/Az) , and the other called Right Ascension & Declination (RA/Dec) - and their relative advantages & disadvantages. PART A Start Stellarium . Make sure your location is Rockville . If it is not, open the Location window, enter Rockville in the search box and then click on Rockville, United States , and then close the Location Window. Turn off the Atmosphere and Fog (if they're on), by pressing the A and F keys (sky should be dark). Put the horizon at the center of the screen by dragging it until it's at the center. It doesn't matter which part of the horizon – N, S, E or W, you choose. Pick a star that is as close to the horizon as possible. Click on it. Information on your star will appear in the upper left-hand corner of the screen. Use red font for your answers. What is the common name of your star? Procyon In the information (upper left-hand corner), look for the line that starts Az/Alt . Az/Alt stands for Azimuth/Altitude , and the numbers on this line list the angles 1 Last edit Spring 2022. 1

ASTR101 L ABORATORY 3 that are the star's Azimuth and Altitude, as discussed in class. The first number listed is the Azimuth, and the second number, after the slash, is the Altitude. What are the Azimuth and Altitude of your star? ROUND OFF ALL YOUR AZIMUTH AND ALTITUDE READINGS TO THE NEAREST DEGREE – IN OTHER WORDS, IGNORE EVERYTHING AFTER THE ° SYMBOL! Azimuth : +272 Altitude : +4 Try a few other stars along the horizon. Click on them. Look at their altitudes. Compare the answers to what you saw for your first star. Do you notice a pattern? What is the approximate value of altitude (in degrees) for any star on the horizon? +3-4 Now find a star that is straight above one of the stars you just chose. In other words, look a little higher in the sky. Click on the new star. Note its altitude and azimuth, and compare them to the values for the stars on the horizon. One of the numbers will have changed, and the other will be (approximately) unchanged. As you move the cursor straight up from the horizon (in other words, as you look up from the horizon, toward the zenith), which angle changes, Altitude or Azimuth? The Altitude Changes Zoom out and re-center the sky until you are looking at the entire sky at once. Click as close as you can to the zenith (the point at the center of the screen). Remember, the zenith is the point directly over your head in the sky. What is the value of altitude (in degrees) at the zenith? 90 Altitude is an angle that tells you where a star is above the horizon. Picture two rays coming from your head – one pointing at the horizon directly below the star, and the other pointing at the star you're looking at. The angle between those rays is the altitude of a star. Click the cursor on a star that is below the horizon – in other words, click on the ground. Notice the value of the altitude here. How can you tell by looking at a star’s Because the value of the altitude is in 2

ASTR101 L ABORATORY 3 Comparing the answers to the previous two questions, what might you guess is the relationship between an observer’s latitude and the altitude of Polaris above his or her horizon? The altitude of Polaris is always same to the observer’s latitude. Stellarium can superimpose a grid on the sky that allows you to easily measure Altitude and Azimuth. To show this grid, press the “ Z ” button on your keyboard (or go to the toolbar at the bottom of the screen and click on Azimuthal grid) . The lines in the grid that run parallel to the horizon are lines of altitude, measuring from the horizon to the zenith, and the lines that are perpendicular to the horizon, and which all meet at the zenith like the spokes of a wheel, are lines of azimuth, measuring along the horizon, from North. The grid lines are labeled, in degrees where they hit the edge of the screen. Set the time for 8 PM tonight by changing the time in the Date/Time window. Label the brightest stars by opening the View window and putting a check next to Stars (in the Sky sub-menu). Also, move the slider next to Labels and Markers to about half way from the left. Close the View window. Now search for the star named Dubhe. You should see that it is located in the bowl of the Big Dipper in the constellation Ursa Major . Don't forget, if you want to see the constellation outlines and names on the screen, press C and then V . What is Dubhe’s Altitude and Azimuth at 8 PM tonight? ( Remember, if you click on a star, its data will be shown onscreen.) Azimuth : 333 Altitude : 22 Change your location to Pueblo, CO ( Lat=38º N, Lon=104º W ) in the Location window by entering Pueblo in the search box and then scrolling down until you find Pueblo, United States of America in the resulting list and clicking on it. Stellarium will put you at 6pm local time (18:00), the same as 8pm in Rockville. Before answering the following, change the time to 8 PM in Pueblo (20:00)! Find Dubhe's altitude and azimuth in Pueblo. Azimuth : 334 Altitude: 20 Are Dubhe's altitude and azimuth similar in Pueblo and Rockville at 8PM Yes 4

ASTR101 L ABORATORY 3 in each location? They should be similar! Explain why. Because the change in latitude was insignificant to affect the viewers observation of dubhe Change your location to Fairbanks, AL ( Lat=64º N, Lon=147º W ) in the Location window by entering Fairbanks in the search box and then scrolling down until you find Fairbanks, United States of America in the resulting list and clicking on it. Before answering the following, change the time to 8 PM in Fairbanks (20:00)! Find Dubhe's altitude and azimuth in Fairbanks. Azimuth : 318 Altitude: 47 Are Dubhe's altitude and azimuth similar in Fairbanks and Rockville at 8PM? NO They should be different! Explain why your results for Pueblo and Fairbanks are different. Because of the location change the latitude value is significantly different which affects the observers view Change your location back to Rockville , and the date back to today's date, at 9 PM . Back in ROCKVILLE, what is Dubhe’s altitude and azimuth at 9 PM ? Azimuth : 339 Altitude : 17 Do a star's altitude and azimuth stay constant as the night goes on? (Compare the Alt/Az for Dubhe at 9 PM with the Alt/Az you found at 8 PM in Rockville.) It does not What is Polaris' altitude and azimuth at 8 PM ? Azimuth : 0 Altitude : 38 What about at 9 PM ? Azimuth : 0 Altitude : 38 You should find that the Alt/Az of Polaris does NOT change. Explain why Dubhe behaves differently from Polaris. The reason why there is no difference in Polaris’s values is because it is very close to the celestial pole, where as the other stars are not and the movement of the earth affects their values. 5

ASTR101 L ABORATORY 3 The Right Ascension and Declination of any star's position are listed in the information that appears when you click on that star. Click on any star. The information about that star appears in the upper left-hand corner of the screen. Look for the line that reads RA/DE (J2000) . The two sets of numbers separated by a slash are the star's Right Ascension and Declination. Let's look at Declination first. What units is declination measured in? degrees Press the “.” (the period) key on your keyboard. This will highlight the Celestial Equator in brighter blue. Pick a star lying right along the Celestial Equator. Click on it. Look at the star's Declination. Do this for a few more stars along the Celestial Equator. (If you can’t see it because it’s the same color, toggle the equatorial grid on and off with the E key.) What is the Declination anywhere on the Celestial Equator? 0 Make sure you're looking at the whole sky by zooming out and centering the sky on the screen. Click on various stars and watch the information that appears for them. Notice that the Right Ascension (RA) and Declination (Dec) change in various places in the sky. Try to find the place where Declination reaches its maximum value. What is the maximum value of declination? 90 What is the name of the bright star almost exactly at the point where declination reaches its maximum value? Polaris What angle does declination measure? The angle from the celestial equator Now let’s look at the Right Ascension lines. Again, these are the lines that circle the celestial sphere but run through the north and south celestial poles. Unlike Declination, which is measured in degrees (and fractions of a degree called arc-minutes and arc-seconds), Right Ascension is measured in a different unit. Click on a few stars and look at their Right Ascension values. 7

ASTR101 L ABORATORY 3 What are the units RA is measured in? (Hint: look at the letters that appear in the RA scale.) In time Click on various stars along the Celestial Equator (the brighter blue line) and note their Right Ascension. Do you see a pattern? Keep clicking on stars and try to find the maximum value of RA. You may have to let some time pass to click all the way along the entire Celestial Equator. What is the maximum value of RA (before it starts over again at zero)? 23h 59m What is the exact RA of Polaris? (Click on Polaris to see information.) 2h33m09.64s Center the whole sky on the screen, and click on a star as close as possible to the zenith to estimate the RA and Dec coordinates of the zenith itself. Don't forget to write the units for each measurement below! Zenith RA: 19h Zenith Declination: 41 Now change the time to 1 hour later. Again, click on a star as close as possible to the zenith to estimate the new RA and Dec coordinates of the zenith. DO NOT CLICK ON THE SAME STAR . Click in the center of the circle. Your zenith is always the center of the circle. IF YOU ARE GETTING THE SAME RA EACH TIME, YOU’RE DOING IT WRONG . Zenith RA: 20h Zenith Declination: 41 What would you guess the RA and Dec coordinates of the zenith would be six more hours later? Zenith RA: 2h Zenith Declination: 41 The answers to the previous questions should help you understand why RA is measured in hours (and minutes and seconds). Each hour that passes causes one hour of Right Ascension to pass by any point in the sky. 8

ASTR101 L ABORATORY 3 In the space below, write a brief conclusion explaining what the advantages and disadvantages are of using Altitude & Azimuth or Right Ascension & Declination, and what you learned about them in this exercise. The benefits of using the altitude and azimuth is that although it might give the accurate position for the current time the values change quite frequently. That's why the use of right ascension and declination is better because the values are fixed on the Equatorial map, and they do not change. 10