Observing the Sky

Name: Introduction: This lab is designed to provide a hands-on learning experience in observational astronomy. The lab handout just provides a general description of the exercise. Learning Goals: Students will learn to ● use Stellarium ● explore methods of finding objects in the night sky ● understand celestial coordinates and time Learning tools: Introduction to celestial coordinate systems, Stellarium software, Stellarium “User Guide”. Stellarium is free software that you can download from www.stellarium.org Astronomical terms and definitions: An arrangement of reference lines or curves can be used to identify the location of points in space. In two dimensions, the most common system is the Cartesian (after René Descartes) system. Points are designated by their distance along a horizontal ( ) and vertical ( ) axis from a reference point, the origin, designated (0, 0). This arrangement does not work too well on a curved surface like the surface of the Earth. Before starting this lab, you need to become familiar with three coordinate systems used in astronomy as well as some basic definitions. The horizon or skyline is the apparent line that separates the earth from the sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon . The astronomical horizon is what you’d see as a visible horizon if you were on a perfectly level ground with no obstructions. The astronomical horizon, thus, is a boundary of an imaginary horizontal circle whose plane makes a 90∞ angle with the line pointing straight overhead at the point we call zenith . Opposite zenith, directly under your feet, is a point we call the nadir . Sunset: Sunset is defined as the instant at which trailing edge of the Sun’s disk disappears below the horizon. Sunrise: Sunrise is defined as the instant at which the leading edge of the Sun appears above the horizon. Ecliptic: The ecliptic is the apparent path of the Sun across the sky. It depends on where you are on Earth, on your latitude, to be exact, as well as on the time of the year. Geographic coordinate system: (see figure 2) To uniquely describe the position of an object on Earth we use geographic coordinate system. Since the surface of the Earth is two-dimensional (roughly a surface of a sphere), two coordinates are needed. Those are: Latitude : Latitude specifies the north – south position of a point on the Earth’s surface. It is an angular measurement expressed in degrees. Latitude ranges from 0° at the equator to 90° (North or South) at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Longitude : Longitude specifies the east – west position of a point on the Earth 's surface. It also is an angular measurement expressed in degrees . Meridians (lines running from pole to pole) connect points with the same longitude. By convention, one of these, the prime meridian , which passes through the Royal Observatory, Greenwich , England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the prime meridian, ranging from 0° at the prime meridian to +180° eastward and −180° westward. Horizontal coordinate system: (see figure 4) To uniquely describe an object’s position in your local sky one uses the horizontal coordinate system, also known as the altitude-azimuth system. Since your local sky looks like a hemispherical dome, two coordinates are needed. Those are: Altitude : Altitude is the angle between the horizon and an object in the sky. It is measured in degrees. Altitude of an object on the (astronomical) horizon is 0∞ (when an object is rising or setting), 45∞ when it’s halfway up the sky, and 90∞ when an object is directly overhead (at the zenith). In figure 4, a star has an altitude of 60∞ measured up from 0∞ at the horizon (i.e. it is "60∞ high").

Note that 0∞altitude is not necessarily the same as your observed horizon, because the ground may not be flat, and buildings or trees might be in your sight line. It is important to measure altitude from the astronomical horizon, not the observed one. Azimuth: Azimuth is measured from due north in the plane of the horizon. It’s an angle with a value between 0 and 360 degrees. Like the markings on a compass: 0∞ is due North, 90∞ is due East, 180∞ is due South, and 270∞ due West (see figure 5). If only approximate directions are required, the azimuthal direction can be specified with just cardinal directions, for example "this star is in the NNE". Like the local sky, altitude and azimuth of a celestial object depend on the location of the observer on Earth. Celestial coordinate system: (see figures 6 and 7) To describe positions of objects in the sky without regard to the observer’s location on Earth, we use celestial coordinate system. Before we introduce this third coordinate system, let’s look into its roots. The sky we see overhead is the half of the sphere that appears as a dome. The other half of the sphere is hidden below the horizon. These two hemispheres form a celestial sphere: an imaginary sphere with the Earth at its center. At any one time, an observer on the Earth’s surface can only see half of the celestial sphere since the other half lies below the horizon. Because the stars are so far away, from the perspective of an observer on Earth, all stars seem to be equally far away, as if affixed to the inside of the celestial sphere of large but unknown radius. To us the celestial sphere is just a practical tool that allows us to locate objects in the sky but to our ancestors who thought the Earth was at the center of the Universe, it was a real thing. They viewed the Earth as centrally located and unmoving and thought that the celestial sphere rotated around it resulting in celestial objects rising and setting. The extension of the Earth's rotation axis to the sky defines the north celestial pole and south celestial pole , while the extension of the Earth's equatorial plane defines the celestial equator (see figure 7). The line on the celestial sphere joining the observer’s zenith with the north and south celestial poles is the celestial meridian (see figure 8). The plane of the ecliptic intersects with the celestial equator at an angle of 23.5∞(see figure 9). Watch the video #1 linked here to better visualize the celestial sphere and ecliptic orientation: Earth’s geographic coordinate system of longitude (meridians) and latitude (parallels) can be projected on the celestial sphere, giving rise to the celestial coordinates: right ascension and declination. The points where the ecliptic crosses the equatorial plane of the celestial sphere are called equinoxes . On those dates there are 12 hours each of daylight and dark. At an equinox, the Sun is at one of the two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. These points of intersection are called equinoctial points . The most northern excursion of the sun is called the summer solstice and will have the longest amount of daylight. The winter solstice opposite it is the shortest period of daylight. As a result, the northern and southern hemispheres are equally illuminated Declination: Declination (dec) is defined as an angle that describes an object’s position north or south of the celestial equator. It is similar to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees. Points north of the celestial equator have positive declinations, while those to the south have negative declinations. ● An object on the celestial equator has a declination of 0°. ● Celestial north pole has a declination of +90°. ● Celestial south pole has a declination of −90°. Right ascension : The Right Ascension (RA) is measured counterclockwise along the celestial equator from the intersection point of the celestial equator and the ecliptic at the vernal equinox. It is similar to longitude, but it is measured in hours. There are (360 degrees / 24 hours) = 15 degrees in one hour of right ascension. ● Vernal equinox point has RA = 00 h 00 m 00 s and declination = 0∞ ● Autumnal equinox point has RA = 12 h 00 m 00 s and declination = 0∞. A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, or mythology. Different cultures and countries adopted their own constellations.

1. What is the name of the brightest star in the constellation of Cygnus? Stars are represented by small circles/dots of various sizes. The larger the circle, the brighter the star. Star Name Azimuth Altitude Right Ascension Declination Deneb +40 degrees +12 degrees 20h41m23.92s +45 degrees 1. What is the name of the brightest star in the constellation of Lyra? Star Name Azimuth Altitude Right Ascension Declination Vega +60 degrees +27 degrees 18h37m38.91s +38 degrees 1. Complete the table below. Star Name Constellation Name Right Ascension Declination Arcturus Bootes 14h16m37.53s +19 degrees Capella Auriga 5h18m14.97s +46 degrees Regulus Leo 10h9m29.47s +11 degrees Pollux Gemini 7h46m36.8s +27 degrees Procyon Canis Minor 7h40m24.14s +5 degrees Spica Virgo 13h26m18.25s -11 degrees Part C: Sunset and Sunrise 1. Set the location to Fairfax, Virginia set today’s date and the time at noon. What is the Sun's altitude at noon? Record the altitude : +67 degrees Is it the highest the Sun gets to be in the sky in this day? No Should it be? Why or why not? On May 26, the sun will reach its highest at 1 PM as the altitude is 72 degrees. Therefore, the position of the sun depends on the location of a point on Earth, time of day and the time of year. 1. What direction would you face in order to see the Sun at noon? SE Is it due south? No Should it be? Why or why not? It should be since it is solar noon where the sun crosses the celestial meridian and the sun is due south in the Northern Hemisphere however it is not reaching its highest point in the sky until 1 PM where the sun is due south. Record the azimuth: +136 degrees 1. Fast forward the time to find how many hours after 12 PM sunset occurs. (Give your answer to the nearest ½ hour). Today Sun sets: 8 hours after noon. 1. What is the Sun's altitude now (i.e. at sunset)? +20 degrees What direction would you face to watch sunset today? West Is it due west? Yes Should it be? Why or why not? The Earth rotates from west to east, so if the sun is going down, it is essentially west from us. Record the azimuth : +281 degrees

1. Does the Sun rise due east every day? Set due west? Sun rises due to ENE and sets W 2. On which day does the Sun rise farthest to the north? June 21 3. On which day does the Sun rise farthest to the south? December 21 4. In the table below list two stars that are just rising in the east at the time of sunset today. Star Name Constellation Right Ascension Declination Azimuth Altitude Rasalhague Ophiuchus 17h35m54.75s +12 degrees +85 degrees +14 degrees Alphecca Corona Borealis 15h35m34.86s +26 degrees +90 degrees +45 degrees 1. Now change the time to 3 hours past sunset and record the altitudes and azimuths of the stars you listed above in the table below. Star Name Constellation Right Ascension Declination Azimuth Altitude Rasalhague Ophiuchus 17h35m54.76s +12 degrees +119 degrees +48 degrees Click or tap here to enter text. Corona Borealis 15h35m34.86s +26 degrees +155 degrees +76 degrees Which coordinates changed over the course of three hours? Azimuth and Altitude Which coordinates remained the same? Right ascension and declination How did these stars move over the course of 3 hours? Went SE Submission details: Submit into this lab’s drobox on Blackboard: MS Word report (this document with your entries) only, Image credit: Pearson Education