Lab2+Celestial+Sphere+I 2

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1403

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Astronomy

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Oct 30, 2023

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PHYS 1403L Lab 2 Name:______________________ PHYS 1403 Lab 2: THE CELESTIAL SPHERE I Instructions OBJECTIVES This two-part exercise teaches you some important facts about the celestial sphere and the meaning of the terms right ascension, declination, altitude and azimuth. By exploring the Stellarium software, you will see what these terms mean. You will learn how to identify the location of various stars and planets. Additionally, you will also learn about the causes for the different seasons and how your location on earth determines the seasonal changes. EQUIPMENT “ Stellarium ” software INTRODUCTION The stars are stationary in the sky, at least by the time scale of human lives. The apparent movement of the stars across the sky that we see every night is due to the rotation of the earth on its axis. Because of this, the stars, the Sun and the Moon all appear to travel across the sky in circles, when actually we are the ones that are moving while we stand on the Earth’s surface. However, it becomes obvious that the simplest conclusion for the ancients to reach was that the stars were moving and that the Earth was standing still. Some early cultures believed the stars were fixed on a giant sphere in the sky to keep them from falling to the Earth and this sphere was rotating about the Earth. This giant sphere was called the celestial sphere. Today, we still use the idea of the celestial sphere to locate objects in the sky with great precision. We do this with a coordinate system that is nearly identical to the geographic coordinates (latitude and longitude) we are familiar with. In order to use a coordinate system, we must have reference points, places to start counting from. On our celestial sphere, our reference marks are: 1) The Celestial Equator . This is a line in the sky directly above the Earth’s equator. If you were standing on the Earth’s equator, the celestial equator would make an imaginary line in the sky connecting the eastern and western points of the horizon and passing through a point in the sky directly above you. This point directly above you is called the zenith. 2) The North and South Celestial Poles . These are points in the sky directly above the Earth’s north and south spin poles, respectively. If you were standing on one of the Earth’s poles, the corresponding celestial pole would be at the zenith. 3) The Vernal Equinox . The vernal equinox is the point in the sky where the Sun

PHYS 1403 – Lab 2 is located when it is directly above the Earth’s equator. Since this occurs twice a year, once in the spring and once in the fall, we must specify which of the two points we are referring to. The vernal equinox is the point when the Sun is moving from the southern hemisphere into the northern hemisphere along its annual path on the celestial sphere. This apparent path of the Sun is known as the ecliptic . Equipped with these reference points we can explore the coordinate system. Refer to Figure 1. First comes declination (DEC), which is measured in degrees. Declination gives us the number of degrees that an object is located above or below the celestial equator. Declination is zero for objects located on the celestial equator and gets larger in magnitude the farther away from the celestial equator is located. The maximum declination is 90 o , for objects located at a celestial pole. If it is in the southern hemisphere declination is measured in negative degrees, while it is positive for objects in the northern hemisphere. So, -90 o DEC means it is at the south celestial pole. Figure 1 Declination only provides us with a circle that is either north or south of the celestial equator. It doesn’t tell us where on that circle the object is located (with the exceptions of the celestial poles, which are points, not circles). In order for us to locate an object, we need to know its location not only to the north and south, but to the east and west, as well. This location is provided with right ascension (RA), which is measured in hours, minutes and seconds. Declination had a convenient starting point, the celestial equator, but where do we start right ascension? The answer is the vernal equinox, that point where the Sun appears to be directly over the equator while moving from the south to north. This point is zero hours RA. There are twenty-four hours of RA, each of which is broken down into 60 minutes. Each minute is then broken down into 60

PHYS 1403 – Lab 2 seconds, yielding 3600 seconds in each hour of RA. Because there are 360 o around a circle, each hour of RA encompasses 15 o of the sky. RA increases to the east from the vernal equinox, until it comes around to 24 h RA, or 0 h RA again. This point use to be in the constellation Pisces, but is now entering the constellation Aquarius, due to a slow movement of the Earth’s axis called precession. It is important to understand that these coordinates are located in the sky. Therefore, with the exception of the celestial poles, they are not located over any one particular point on the face of the Earth. Instead, the Earth rotates beneath them and these coordinates rotate with the stars. Thus the declination and right ascension of an object in the sky as measured by one observer in one country, will be the same as measured by another observer in another country. At first the night sky appears to be constant, but it isn't. There are small changes that are taking place every hour. Stars are seen to rise above the horizon and set just like the Sun and the Moon. From a local perspective, it is useful to know how high a star is, above the horizon. This number is called the star’s altitude . A few hours later, the position of the star will be noticeably different, and its altitude will also be different. While it is very easy to measure altitude, it is not as useful as declination for professional astronomers, because the altitude of a star will change depending on the observer’s location, day and time of observation. To be able to measure the altitude of a star, you must first look in the proper direction! This means you must turn to face the star. The angle through which you turn, starting from north, is called azimuth . If you are facing due north, the azimuth angle is 0  if you face east, the azimuth is 90  . Facing south gives an azimuth of 180  and facing west is 270  . Like altitude, the azimuth depends on the observer’s location, date and time.

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PHYS 1403 – Lab 2 PHYS 1403 Lab 2: THE CELESTIAL SPHERE I Worksheet Enter you answers to each question in the data tables and highlighted areas below. When completed, you may leave. PRELAB QUESTIONS 1. What is the unit of measure of Right Ascension? of Declination? 2. How much does the declination of the Sun vary over one year? 3. If you find two stars with the same Right Ascension, are they necessarily close together in the sky? Why or why not? EXERCISE In this exercise you will be using Stellarium in order to familiarize yourself with the celestial sphere. Once you have installed the program, find Stellarium in the “Start” menu at the bottom of your computer desktop. After the program has started, you will need to set your home location. If you move your cursor to the lower part of the left side of the screen, a vertical menu of icons should appear. Click the top icon c alled “Location window”. Choose “Plano, United States” from the list of cities given, and then click the “Use as Default” box in the bottom left of the window. Now that you have set your location (and closed the Location window), you will see a scene representative of the sky conditions outside. If it is still daylight outside, you will see daylight on your computer screen; and if it is night outside, you will see night on your computer screen. Since we are interested in things other than the sun in this lab, we need to get rid of the daylight. Move your cursor to the left side of your screen to bring up the vertical menu again, and click the “Sky and viewing options window” icon. In the “Sky” tab within this window, un- check the box that says “Show atmosphere”. Daylight should fade and stars should now be visible (although the sun may still be displayed in the sky). You can move around by simply clicking on the screen and moving the mouse. We now want to set up our celestial coordinate system (we will be using the Equatorial coordinate system). Open the Sky and viewing options window again, and click on the “Markings” tab. Now, check the boxes labeled “Equatorial Grid”, “Equator

PHYS 1403 – Lab 2 Line”, and “Ecliptic Line” for the current date, not J2000. This will display the Celestial Equator (in blue) and the Ecliptic (in Red) on the night sky, as well as lines of Right Ascension and Declination (these lines are displayed in blue). Also, please be aware that the labels for the Right Ascension and Declination lines are along the edge of the screen and move around a lot as you move the screen, so in some cases you may need to look around very carefully to find the label for a grid line, or even click and drag the sky around a bit. Hours of Right Ascension are labeled at the top of the screen, and degrees of Declination are labeled at the right side of the screen. In this lab, we want to be able to view the whole sky, not just what is above the horizon. Therefore, we need to remove the ground from the screen. To do this, open the Sky and viewing options window again, and click on the “Landscape” tab. Now, un -check the boxes labeled “Show ground” and “Show fog”. This should remove the ground from the screen. Close the Sky and viewing options window, and now you are ready to start the exercise. READ AND DO THIS IF THE PROGRAM CRASHES WHILE YOU'RE WORKING: Unfortunately, because it is such a complex program, Stellarium might occasionally crash (depending on the speed of your computer) as you are trying to work on the lab. Don't panic! When Windows asks to send an error report, click "Don't Send." Then click on the Stellarium icon again to restart the program. Follow the steps listed above, and continue working on the lab. SOME OF THE INSTRUCTIONS MAY BE CONFUSING AT FIRST. TAKE YOUR TIME AND READ THEM SEVERAL TIMES TO MAKE SURE YOU UNDERSTAND. IF YOU GET CONFUSED, ASK FOR HELP (EITHER BY EMAILING ME OR POSTING ON THE DISCUSSION BOARD). BE PATIENT AND DON’T GET FRUSTRATED. 1. Using the lines of Right Ascension and Declination displayed on the celestial grid, find the bright stars at the following coordinates (you can find the name of a star by just clicking on the star): Right Ascension Declination Star Name 04 h 36 m +16 o 31’ 06 h 45 m -16 o 43’ 13 h 25 m -11 o 10’ 16 h 29 m -26 o 26’ 18 h 37 m +38 o 47’ 19 h 51 m +8 o 52’ 20 h 41 m +45 o 17’

PHYS 1403 – Lab 2 2. Now let’s look at the constellations. To display the constellations, open the Sky and viewing options window again, and click on the “Markings” tab,.Under “Constellations”, check the boxes labeled “Show labels” and “Show boundaries”. This will display t he constellation names (in blue) and boundaries (dotted red lines) on the night sky. Close the Sky and viewing options window, and locate the constellations that the following points are within: Right Ascension Declination Constellation 01 h 00 m +40 o 00’ 05 h 30 m 00 o 00’ 07 h 00 m -22 o 00’ 08 h 00 m -55 o 00’ 10 h 50 m +17 o 00’ 11 h 30 m +55 o 00’ 15 h 30 m -15 o 00’ 3. Below is a list of 20 deep sky objects from the Messier catalog (the “M” is for Messier). We will use Stellarium to gather information on this group of objects. First, open the “Find” tab on the left -side menu bar. To find an object, move your cursor to the left side of your screen to bring up the vertical menu again, and click the “Search window” icon. The first object is M1. Type this object name in the box in the Search window and press “Enter”. Stellarium should now move to MI. Information on this ob ject is found in the text box in the upper right corner of the screen (where you found the star names for part 1). Record proper name of the object as well as its type (nebula, galaxy, cluster, or “cluster with nebulosity”) below. You can also see what the object looks like in greater detail by using the scroll wheel on your mouse to “zoom in”. Object Proper Name Type M1 M8 M16 M20 M27 M31 M42 M44 M57 M104

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