Astronomy 101 Lab 3-1

Astronomy 101 Lab 3 Stellarium is a free, open-source planetarium program available for use online or to download. The program can be downloaded at: https://stellarium.org/ Or used on the web at: https://stellarium-web.org The advantages of using a computer-generated sky are numerous. First, and most obviously, it’s never too cloudy to observe the sky on the screen! Another powerful advantage is time. We can examine the sky on any day, at any time: past, present, or future. We can watch the stars and planets move quickly across the night sky in a way that would not be possible outdoors, or even with a professional telescope. We can change our location and see the sky from parts of the world we have never visited...or from other worlds entirely. Objectives ๏ Become familiar with the Stellarium program and its user interface ๏ Define and distinguish between systems of astronomical coordinates ๏ Examine the position of the sun in the sky over the course of a year Procedure 1. Launch the Stellarium program. When the program has loaded, your screen should show you located in a field of grass, facing north. You have multiple tools that you can use, located at the top and bottom of your screen. On the bottom left, click on the location label and make sure it is set to view from your current location. On the bottom right, click on the date and time and make sure it is set for today’s date and the current time. Play with the other screen options in the bottom center. What does each of them do? 1

Screen Option Icon What it Does Constellations – shows you all the constellations Constellations Art – shows you the art of the constellations Atmosphere – shows you all the stars Landscape – gives you the option to have landscape off or on Azimuthal Grid – gives you a grid that has all the degrees that you need Equatorial Grid - Deep sky objects – shows you all the objects in the sky Night mode – Enters night mode Full screen – gives you a full screen Astronomical Coordinate Systems There are several systems that can be used to locate objects in the sky. We will distinguish here between two coordinate systems: azimuth/altitude (Az/Alt) and right ascension/ declination (RA/DE) . Either one of these systems is analogous to using x- and y-coordinates on a piece of graph paper. They are two-dimensional systems, which means that they can tell you where to look in the sky to find an object, but not how far away that object is from the Earth. 2. Turn and face north: Toggle on the Azimuthal grid , look up a bit, and examine the coordinate system. Azimuth is the angle measured in a horizontal circle, around your horizon. Due N = 0°. Head around the circle, and E = 90°, S = 180°, W = 270° and back to N = 360°. Altitude is the angle measured from the horizon (0°) to the zenith (90°, or directly overhead). Thus, any point in the sky that you can see can be specified by telling you which direction to face (Az), and how high up to look (Alt). This is the type of coordinates to use when specifying locations of objects in your Observation Log. 2

8. What does that mean? You have just demonstrated the difference between the coordinate systems! Notice that Az/Alt depends entirely on where you are located, and RA/DE does not. Relating this back to question 7, if you were an astronomer here in Vancouver who needed to tell a colleague at Kitt Peak (in Arizona) about a sky object, how would you choose to communicate its location? I would find out which object I was finding, then tell the person to look up 90° Sunrise and Sunset We all know that the sun rises in the east and sets in the west, right? Let’s examine the position and time of the rising and setting sun over the course of the year and see how true that is. Toggle back to the azimuthal grid for this exercise. 9. Find the rising sun: Set your date for 09/21/20 and find the rising sun. Guess at the time, then refine your time until the sun has an altitude of 0°. Adjust the time in seconds to get as close as you can, but because you cannot adjust time in smaller than 1 second increments, understand that you will probably not be able to obtain an altitude of precisely 0°0’0”. Get as close as you can and record the time and both azimuth and altitude coordinates in the table below. (Note, the coordinates of the sun may not automatically update as you change the time. You might have to unselect the sun and then select it again after adjusting the time.) 10. Find the setting sun: For the same date, advance the time to sunset. The setting sun will again have an alt = 0°. Record the time and Az/Alt coordinates in the table below. 11. Change the date and repeat: Record the sunrise and sunset times and coordinates again on 12/21/20, 03/21/21, and 06/21/21 in the table below. Date Sunrise time Az. Alt. Sunset time Az. Alt. 9/21/20 06:59 AM 088° 45’ 02.8” -00° 06’ 10.1” 19:10 PM 271° 22’ 37.8” -00° 26’ 14.8” 12/21/20 07:49 AM 123° 44’ 02.2” -00° 09’ 52.0” 16:31 PM 236° 35’ 09.8” -00° 22’ 55.2” 3/21/20 7:11 AM 088° 25’ 17.” -00° 09’ 24.1” 19:26 PM 272° 03’ 42.3” -00° 18’ 53.7” 6/21/20 05:22 AM 054° 23’ 05.7” -00° 09’ 26.5” 21:05 PM 305° 56’ 23.9” -00° 22’ 28.0” 12. Does the sun rise due east (Az = 90°) every day? Set due west (Az = 270°)? On which day does the sun rise farthest to the north (smallest azimuth, or north of east)? On which day does it rise furthest south (greatest azimuth, or south of east)? The sun rises farther East during the middle of the day. 4

13. Why look at the sun on those days in particular? Why not four other random days chosen three months apart? Note the significance of each of these particular dates. Each of these times have different Az. And Alt. as well as different sunrise and sunset times. 5