CelestialSphere 2025-02-22 19:05:28 2025-02-23 04:40:09 Definition % this is the default PlanetMath preamble. as your knowledge % of TeX increases, you will probably want to edit this, but % it should be fine as is for beginners. % almost certainly you want these \usepackage{amssymb} \usepackage{amsmath} \usepackage{amsfonts} % used for TeXing text within eps files %\usepackage{psfrag} % need this for including graphics (\includegraphics) \usepackage{graphicx} % for neatly defining theorems and propositions %\usepackage{amsthm} % making logically defined graphics %\usepackage{xypic} % there are many more packages, add them here as you need them \usepackage{caption} % define commands here If you go on a camping trip or live far from city lights, your view of the sky on a clear night is pretty much identical to that seen by people all over the world before the invention of the telescope. Gazing up, you get the impression that the sky is a great hollow dome with you at the center (Figure 1), and all the stars are an equal distance from you on the surface of the dome. The top of that dome, the point directly above your head, is called the \textbf{zenith}, and where the dome meets Earth is called the horizon. From the sea or a flat prairie, it is easy to see the horizon as a circle around you, but from most places where people live today, the horizon is at least partially hidden by mountains, \PMlinkescapetext{trees}, buildings, or smog. \\ \begin{center} \includegraphics[scale=0.8]{celestial_sphere_2_2.jpg} \\ \begin{center} \caption{Figure 1: }{The Sky around Us.} \end{center} \end{center} \PMlinkescapetext{If you lie back in an open field and observe the night sky for hours, as ancient shepherds and travelers regularly did, you will see stars rising on the eastern horizon (just as the Sun and Moon do), moving across the dome of the sky in the course of the night, and setting on the western horizon. Watching the sky turn like this night after night, you might eventually get the idea that the dome of the sky is really part of a great sphere that is turning around you, bringing different stars into view as it turns. The early Greeks regarded the sky as just such a \textbf{celestial sphere} (Figure 2). Some thought of it as an actual sphere of transparent crystalline material, with the stars embedded in it like tiny jewels.} \\ \begin{center} \includegraphics[scale=0.7]{celestial_sphere_2_3.jpg} \\ \caption{Figure 2: }{ \textbf{Circles on the Celestial Sphere.} Here we show the (imaginary) celestial sphere around Earth, on which objects are fixed, and which rotates around Earth on an axis. In reality, it is Earth that turns around this axis, creating the illusion that the sky revolves around us. Note that Earth in this picture has been tilted so that your location is at the top and the North Pole is where the N is. The apparent motion of celestial objects in the sky around the pole is shown by the circular arrow.} \end{center} \PMlinkescapetext{There is even a special theater, called a \textit{planetarium}, in which we project a simulation of the stars and planets onto a white dome.} \\ This article is a derivative work of the creative commons share alike with attribution in [1]. \begin{thebibliography}{9} [1] Fraknoi, Andrew, David Morrison, and Sidney Wolff. The Sky Above. In Astronomy 2e. Houston, Texas : OpenStax, 2022. \PMlinkexternal{The Sky Above}{https://openstax.org/books/astronomy-2e/pages/2-1-the-sky-above} \\