Lab Manual - Radford University

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CHAPTER A. LABORATORY EXPERIMENTS 3 Name: Section: Date: A.1 Celestial Coordinates I. Introduction How do you pinpoint the position of your house on the Earth? You can specify the street address or give a pair of coordinates. You can divide the surface of the Earth into grids in the east-west direction and the north-south direction. By measuring coordinates (i.e., distances or angles) from some reference points, you can determine the exact position of your house. For example, the City of <strong>Radford</strong> is located at a longitude of 80.6 ◦ west and a latitude of 37.1 ◦ north. In this case the reference points are the meridian through Greenwich, England, the reference point for longitude, and the equator, the reference point for latitude. In astronomy we are interested in specifying the positions of objects in the sky as seen by an observer on the Earth. It is accomplished by giving a pair of coordinates in a similar manner as determining locations on the Earth. It helps to picture the night sky as an immense glass sphere with the Earth (and the observer) at its center and all of the stars and planets projected on the sphere (see Fig. A.1). This sphere is known as the celestial sphere. ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ Observer ★ ★ Horizon Figure A.1: The observable half of the celestial sphere above the horizon. There are various ways to define coordinates on the celestial sphere. In this lab we are going to study two such systems: the alt-azimuth system and the equatorial system. II. Reference • The Cosmic Perspective, Supplement 1, pp. 94 – 104. III. Materials Used • planetarium • Starry Night Backyard
4 CHAPTER A. LABORATORY EXPERIMENTS IV. Activity The Alt-Azimuth System Let us define some terminology. Suppose the observer is located at the center of the celestial sphere in Fig. A.2. The point directly overhead on the celestial sphere is called the zenith, while the point directly opposite of the zenith is the nadir. The horizon is the circle extending around the celestial sphere and located exactly 90 ◦ from the zenith and the nadir. Local Celestial Meridian Zenith W S N Observer Horizon E Nadir Figure A.2: The Celestial sphere. The north point (N) is located on the horizon in the direction of geographic north as seen by the observer at the center. The east (E), south (S), and west (W) points are also located along the horizon at 90 ◦ intervals. The local celestial meridian is the imaginary circle on the celestial sphere that runs from the north point, through the zenith, to the south point and through the nadir back to the north point. Now let us consider a star on the celestial sphere (see Fig. A.3). The circular arc running from the zenith through the star to the horizon at H is a vertical circle. The azimuth of the star is the angle along the horizon from the north point eastward to H. This is basically the compass direction (SSW for example), but measured in degrees. The altitude of the star is the angle of the star above the horizon along the vertical circle. The altitude is a positive number if the star is above the horizon; it is negative if the star is below the horizon. Altitude combined with azimuth can specify the position of any object in the sky. Find the altitudes and azimuths of some reference points on the celestial sphere and complete the following table (Table A.1). If an item does not have a well-defined value or range of values, then it will be represented by an ×. The Celestial Sphere In part of this activity, you will use the planetarium software, Starry Night Backyard. This software can be used for many purposes, but its use in this lab will be to show you the sky as it appears from <strong>Radford</strong> or any other place on the Earth. In this way, it is very much like the planetarium.
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