Lab 2 Exloring the Night sky

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CHARLESTON SOUTHERN UNIVERSITY PHYSICS 115 ASTRONOMY-STARS AND GALAXIES Student Name: Jewel Ash Date: October 29,2023 Student ID: 204386 LAB 2: EXPLORING THE NIGHT SKY: APPARENT MOTIONS OBJECTIVE: The main goal of this lab is for you to gain an understanding of how the sky changes during the night and over the course of a year. We will see how the stars move on a daily to yearly basis. We will explore the altitude-azimuth and RA-declination coordinate systems and the advantages and disadvantages of each. Introduction As a first orientation to the sky and the motions of celestial objects, we will be using a computer planetarium -STELLARIUM. Such programs are very powerful and can display the positions and trajectories of celestial objects observed from any place and time on Earth, as well as from other planets. They also allow one to step forward and backward in time and make it easy to view how the sky changes over periods of a few minutes to hundreds or thousands of years. Coordinate Systems To pinpoint any location on Earth, you must specify two coordinates (i.e., latitude and longitude). Similarly, two coordinates are required to locate an object on the celestial sphere. There are actually two different coordinate systems commonly used by astronomers. We will examine both of these systems. The Local Coordinate System The local (a.k.a. altitude-azimuth) coordinate system is based on two perpendicular coordinates: the azimuth angle, or the angle along the horizon from North, and the altitude angle, which is the angle above the horizon. An azimuth of 0° corresponds to due North. East has an azimuth of 90°, South has an azimuth of 180°, and so forth, just like a compass.

The altitude can range from an angle of 0° (along the horizon) to 90° (right above your head, your zenith point). Using these two coordinates, you can locate any object above the horizon. The local coordinate system is easy, but it has its flaws As objects appear to move, their alt-az coordinates change. Furthermore, these coordinates are related to your position on Earth. So while they’re useful for finding objects when you’re standing outside, they’re not useful for collaborating with your astronomy friends who live in different cities. Fortunately, there’s another coordinate system that astronomers employ. The Celestial (a.k.a. Equatorial) Coordinate System For the Equatorial coordinate system, imagine that Earth’s latitude and longitude lines have been projected onto the celestial sphere. This coordinate system is measured in units of Right Ascension (RA) and declination (decl). RA corresponds to lines of longitude; declination corresponds to lines of latitude. This coordinate system is based on the Celestial Equator and the Celestial Poles, and thus doesn’t depend on your location on Earth. In a sense, this coordinate system is attached to the stars. Each star has a unique Right Ascension and a unique declination. These coordinates can be used to determine when a star will be above your horizon, no matter where you are on Earth. To understand the Celestial coordinate system, take our picture of the Earth at the center of the Celestial Spherical shell. Extend the Earth’s equator to

the stars and call it the Celestial Equator. Extend the Earth’s polar axis to the stars and call the points of intersection the North and South Celestial Poles. As the Earth orbits the Sun over the course of a year, the Sun appears to live in different constellations (i.e., the zodiac) on the Celestial Sphere. We’ll add in the path that the Sun takes: This figure provides a lot of insight into this coordinate system. Notice its features: the Sun’s path intersects the Celestial Equator on the equinoxes. The sun is farthest away from the Celestial Equator on the solstices. Now, we’ll introduce some nomenclature: add lines of celestial latitude and call it declination. Add lines of celestial longitude and call it Right Ascension.

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Notice how these coordinates are measured. Declination is measured in units of degrees. A degree can be broken up into arcminutes (′), which can be broken up into arcseconds (′′). There are 60 arcseconds in an arcminute and 60 arcminutes in a degree. Declination ranges from −90° at the South Celestial Pole to +90° at the North Celestial Pole. A declination of 0° corresponds to the Celestial Equator. Right Ascension is measured in units of hours, not degrees, and runs around the Equator. As a convention, an RA of 0 hours corresponds to the point in the sky where the Sun crosses the Celestial Equator at the Vernal Equinox. RA increases going East. An RA of 12 hours corresponds to the point in the sky where the Sun crosses the Celestial Equator at the Autumnal Equinox. An RA of 24 hours is the same as an RA of 0 hours. The following figure shows the entire Celestial Sphere filled in with lines of RA and declination:

Measuring an angle in units of time may seem counter-intuitive at first, but there’s a reason for it that we will learn shortly. We can always convert back to units of degrees: From the above figures, we can see that the entire Celestial Equator occupies 24 hours of Right Ascension. You should also know that a circle occupies 360°. Therefore: ;00 24 hr = 360°, thus 1 hr = 15° This simple conversion tells us an important piece of information: in 1 hour of time, the stars appear to move 15° because of the Earth’s rotation. It’s essential to notice that the coordinate system is now fixed with respect to the ‘fixed’ stars. In this system, a star’s RA and declination do not change. What does change is the rotation of the coordinate system with respect to the local observer (you). In other words, as the Earth turns, you see the ‘fixed’ stars move. The point directly above your head is called the zenith. The imaginary line connecting the poles, which passes through the zenith, is called the meridian. What changes throughout the night, in a very regular way, is the value of the RA that corresponds to your local meridian. This is why RA is measured in units of time! If your meridian currently has an RA value of 5 hours, then a star with a RA of 6 hours will lie on your meridian in one hour. Procedure Locating Stars Open the Stellarium program on your computer. Ensure that the location is set to North Charleston SC. You should now see the stars as they appear in the night sky. 1. Click on the ‘Cardinal Points’, ‘Planets Labels’, ‘Equatorial Grid’ button on the bottom control menu if the icons are not already highlighted. Advance the time to approximately 10 pm (22:00) tonight using the ’Increase time speed’ button. You can move around the sky by clicking and dragging the sky or using the left, right, up and down keys. Move so that you are pointed toward due north. 2. At the North Celestial Pole (90 o declination where all the right ascension lines meet on the Equatorial grid) you should see a bright star which is Polaris. Click on the star and information about the star will be displayed in the upper left of the screen. Fill in the appropriate data below. Use the ’RA/DE (of date)’ for the Celestial coordinate system data.

Polaris Right Ascension Declination Azimuth Altitude 3h02m12.50s -+89 o 21’32.1- -+0 o 40’05.1----- +32 o 29’43.6” 56 o 15’29.5” 3. The two ‘pointer stars’ of the Big Dipper are known as Merak(β UMa) and Dubhe(α UMa). Find the Big dipper and record their information below in the table below. Star Name Right ascension Declination Azimuth Altitude Merak (β- Uma) 11h03m13.3 8s +56 o 15’29.5” +330 o 58’25.3 ” +12 o 03’22.4” Dubhe (α- Uma) 11h05m08.1 1s +61 o 37’38.2” +334 o 54’31.3 ” +15 o 49’12.5” Phecda ( γ Uma) 11h55m02.3 1s +53 o 34’01.1” +324 o 03’38.5 ” +16 o 13’23.2” Megrez ( δ Uma) 12h16m33.3 5s +56 o 54’19.7” +325 o 22’37.2 ” +20 o 34’00.1” Alioth ( e Uma) 12h55m02.5 3s +55 o 50’10.1” +321 o 50’20.3 ” +24 o 53’53.6” 4. Find three other bright stars and record their data in the table above 5. Now Set the time to 4 am and date to 4/14/2020. Locate the following Planets on your local sky and identify which constellation each object is located: For each object, complete the table below: Object Constellatio n Distance from sun (AU) Right Ascension Declination Jupiter Sagittarius 5.189 AU 19h51m21.41s -21 o 04’03.5” Mars Capricornus 1.459 AU 20H51M33.10S -18 o 55’06.6” Saturn Capricornus 10.023 AU 20h14m31.13s -19 o 55’43.6” 6. Stellarium allows you to correct this by getting closer views of objects in the sky. After selecting the planet with the mouse, you may zoom in close to the planet by using the shortcut

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"/". To zoom out, you may use the shortcut "\". You may also use the Page Up and Page Down keys (Fn+Up on a Mac). After magnifying the planet, list the size of the field of view (FOV) and describe the planet/ object as you see it. You should also see several of the planet's moons. If not, press "\" once. Name these moons and describe their layout relative to the planet where applicable. Write a brief description of your observation for each object below: 1. Jupiter Jupiter appears to be positioned between two moons. With a RA of 19h51m26.42s, (JII Europa) is the moon that is farthest to the right, and (JI) IO is the moon that is closest to Jupiter, with a RA of 19h15m23.15s. This moon appears to be more yellow than the other moon. FOV: 0.0429 o 2. Mars There appears to be a moon with the name (MI) above Mars and a shadow covering part of the planet's right side. The width of Phobos with respect to the sun is 1.459 AU. FOV: 0.00768 o 3. Saturn There appear to be several moons around Saturn. Tethys (SIII) and Janus (SX) are on the right. Dione (SIV), Enceladus (SII), and Epimetheus (SXI) are positioned on the left side. Below it are Calypso (SXIV), Mimas (SI), and Pandora (SXVII). FOV: 0.0183 o Questions 1. Describe how the stars move during the night in North Charleston. The earth's rotation and mid-altitude will cause the stars to appear to be moving from east to west. The Earth rotates from west to east, which causes the stars to appear to move. The stars that were once hidden by

the eastern horizon ascend to greater heights in the sky as the Earth rotates, and eventually set in the western horizon. 2. What do you think the motion of the stars would look like at the North Pole? Where Would Polaris be located? Because there won't be as much artificial light, the stars will appear to rotate counterclockwise and will be brighter. The stars that may appear to be the center of rotation are called Polaris, and they will be situated almost exactly above the North Pole. 3. If you were on the equator, what would be the altitude of Polaris? What would be Declination of Polaris? Polaris would be visible near the northern horizon if I were on the Equator at a latitude of 0 degrees. Why? Because I live near the equator, the celestial North Pole would be directly above the North Pole of Earth's axis, and the North Pole would be at the horizon facing north. 4. How long will it take a star to return to its initial position? It takes around 23 hours and 56 minutes for a start that is not close to a celestial pole to return to its starting position. Stars that are close to celestial poles seem to revolve in extremely tiny circles around the pole. It would take a full day for these stars to move back to their starting positions.

Lab 2 Exloring the Night sky

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