Hurray! Time for PRS!! Steps to PRS bliss: Step 1: enter your student ID into the PRS clicker: Turn the clicker on (0/1 switch at left). When it says, Scanning classes, hit * twice to get to the Setup menu. Push the down arrow until you reach ID: blah blah blah. Hit enter (green arrow button) and then type in your student ID and hit enter again. Done. Your ID will be saved in the clicker for future classes, so you only need to do this once (unless you run out of batteries). Step 2: join the PRS session for this lecture: Turn the clicker off and then on again. When it starts scanning classes, just let it go it will find our class. Or, type 7 to jump to channel 7. When the clicker asks if you would like to join astro_trip, hit enter (green arrow) to join the session. Ready to go. You do need to do this every lecture. Astronomy 101: 9/16/2008 Announcements Pick up a golf ball at the front of the class or get one from Alex; you will need it for an in-class activity today. You will also need the question sheet from Alex. My office hours will be on Friday from 1-3 PM this week due to a conflict tomorrow The first real OWL homework is now available. Called Homework 1 Due 9/24 before midnight 1
Ch. 2: Discovering the Universe for Yourself! My Ch. 2 title: Stuff Gets in the Way. And It Doesn t Cooperate. The Earth? Gets in the way. The Moon? Gets in the way. The Sun? Oh, don t get me started... The Milky Way? Gets in the way. Isn t that special! Suppose we want to find some object such as the gravitational lens that we discussed in previous lectures: Where is it? How do we keep track of stuff so we can go back and study it carefully? 2
Can we use constellations to locate objects in the sky? PRS Question: Modern astronomers use constellations for which of the following purposes? 1. For specifying the locations of objects in the sky 2. For naming celestial objects Yes, occasionally (not very often 3. For studying the ways that stars interact and though) evolve 4. For determining when to plant crops and when to harvest For the most part, 5. All of the above constellations are not 6. None of the above used in most modern astronomy 3
Constellations are a very blunt tool. Constellations cannot be used to locate an object with sufficient precision for most astronomical objects of interest. A Constellation is just a region of the sky Most official constellation names come from antiquity. Some southern hemisphere constellations were named by European explorers in the 17 th & 18 th centuries. The patterns of stars have no physical significance! Stars that appear close together may lie at very different distances. Constellations were useful in prehistoric times to keep track of the year, e.g., when to plant or harvest the crops. Modern astronomers mainly use constellations as landmarks, but their usefulness is pretty limited. 4
The Horizon zenith The Local Sky the point directly above you horizon all points 90 from the zenith altitude the angle above the horizon nadir the opposite of the zenith, i.e., point directly beneath you meridian due north horizon zenith due south horizon (an object is highest on the sky when on the meridian) 5
To pinpoint an object in the local sky, we simply specify two angles: Problem: the local sky depends on time and place. altitude the angle above the horizon azimuth An angle between a defined point (say, due West) and the place on the horizon from which we measure the altitude. Sunset Consider the Sun: Noon Sunrise 6
One reason the local sky depends on time: diurnal motions due to the spin of the Earth Contrary to our perception, we are not sitting still. We are moving with the Earth. and not just in one direction Polaris (The North Star) The Earth rotates around it s axis once every day PRS question: If you travel north from the US into Canada, how will Polaris (the North Star) change? 1. It will be brighter. 2. It will be dimmer. 3.It will be higher in the sky. 4. It will be lower in the sky. 5. It will be exactly the same. 7
Polaris, the North Star Straight up at A Suppose that A is the North Pole. The Earth rotates around the North Pole. At this place, Polaris would be straight up. It would be as high in the sky as it can be. At B, Polaris would be just barely above the horizon. The angle between straight up and Polaris would be large. Straight up at position B This observer can only see objects above the horizon. If you want to study the entire sky, it is important to have telescopes in the Northern Hemisphere and the Southern Hemisphere. Observing stars in these directions would require the person to look through the Earth! 8
The Celestial Sphere The Sun rises in the east and sets in the west, and it takes 24 hours to complete one cycle of rising, setting, and rising again. Stars do the same -- rise in the east, move across the sky, and set in the west, once every 24 hours. This is daily or diurnal motion. The diurnal motion creates the illusion that the Sun and stars are on a vast sphere that is rotating around the Earth; this incorrect view was held by many ancient peoples. But, the North Star never rises or sets. And, some stars move but are always above the horizon. The Celestial Sphere -- a model for mapping the sky North & South celestial poles: the points in the sky directly above the Earth s North and South poles Celestial equator the extension of the Earth s equator onto the celestial sphere 9
The Celestial Sphere -- a model for mapping the sky The sky that you see depends on where you are located on the surface of the Earth. Stars and constellations that are in the direction of a celestial pole never set; these are called circumpolar objects. Daily, or Diurnal, Motion daily circles --- CCW looking north, CW looking south 10