Scholars Online Astronomy - Chapter 2: 1-4 Coordinate Systems
Reading: Astronomy, Chapter 2: Knowing the Heavens, sections 1-4
Study Notes: notes on your assigned reading from the text
- Section 1: There is evidence that ancient cultures all over the world studied the skies, named constellations, and recorded astronomical events such as conjunctions and eclipses. In Europe and the Middle East, we have monuments such as Stonehenge, Karnak, the Pyramids, and the ziggurats that were aligned to lunar and solar positions at key points of the year. In the Americas, similar events were captured in cave paintings.
- Section 2: Constellations of the northern hemisphere were identified in prehistoric times; the major constellations were those along the zodiac, Orion, and Ursa Major (the Great Bear or Big Dipper). Constellations in the southern hemisphere are modern, dating from the age of European exploration. In the nineteenth century, astronomers agreed to specific boundaries for the constellations to allow better communication of the location of observed phenomena.
- Section 3: Three primary motions dictate how we see object in the sky move. The combination of the earth's spinning, moving platform along with the actual motion of the objects themselves produces a complex pattern over time.
- the earth rotates on its axis once every 24 hours. As a result, sun, moon, planets, and stars appear to rise in the east and move westward across the sky.
- the earth revolves around the sun over the course of a year. As a result, even if they were not moving, the sun appears to move eastward against the background sky, or (to put it another way) the stars appear to move westward, rising a bit earlier each morning ahead of the sun.
- the moon and planets are themselves revolving around their common centers of gravity, so they appear to move eastward against the background of the sky (except when in retrograde), rising later each day.
- Section 4 [and Box 2-1]: Celestial Sphere To locate stars in the night sky, we imagine that they are all at the same distance on a great celestial sphere, rotating around the axis of the earth, and divided by the plane of the earth's equator. We use two coordinate systems to identify locations. Each is like the earth's latitude-longitude system: we identify "poles", an axis through the poles, a plane that intersects the axis at right angles, and the circle caused by the intersection of the plane with our imaginary sphere. We fix a starting point on this circle, and the we can then measure around the circle (on earth, this is longitude), and up or down toward the poles (on earth, latitude).
- The alt-azimuth system, based on the observer's local view.
- Poles: Zenith (point directly overhead), nadir (point directly below -- opposite side of the earth and sky).
- Plane: Horizon
- Measure along the circle: Azimuth in degrees, starting at the north point and going to the east.
- Measure above/below plane: Altitude in degrees, starting at the horizon. There are no stars visible with negative altitude; they would be below the horizon.
- The celestial-coordinate system
- Poles: North Celestial Pole (directly above Earth's North Pole); South Celestial Pole
- Plane: Celestial Equator (extrapolated outward from Earth's equator)
- Measure along the circle: Right ascension in hours and minutes of arc, starting at the vernal equinox and counting
- Measure above/below plane: Declination in degrees, starting at the celestial equator and measuring north (positive) or south (negative)
Key Formulae to Know
Be sure that you feel comfortable with the discussion of degrees on a sphere. A complete circle (all the way around the equator) is 360°, so a half-circle (from north pole to south pole) is 180° and a quarter-circle (from the equator to the pole, for example) is 90°. We use degrees on the celestial sphere to track positions of stars and planets.
Read the following weblecture before chat: Coordinate Systems
Planetarium program: Startup your chosen program. Advance the time so that the sky is dark and position your view so that you are looking due south at the horizon at 9pm for the night on which you do this, or for a night within the next week when you plan to do some observing..
- Add the Celestial grid, or celestial equator, declination, and right ascension markings to your display.
- Notice the direction of the lines. The vertical lines mark right ascension, the horizontal lines mark declination north and south of the celestial equator. Note the "20h" type marks and the "15°" and "-15°"marks.
- Using your field of view controls (often on a table, this just means dragging the display), adjust your view so that you are looking due east at a level horizon. What direction do the lines go now?
- Now move so that you are facing north. Where do the lines of right ascension converge?
- Facing due north, note the time. Advance the time by 1 sidereal day. If you don't have sideral day controls, advance by one day. What happens to the display? What time is it now? How much time has passed?
- If you were able to advance the time by a sidereal day, now advance the time by one civil day (24 hours). What happened to the display? What time is it now?
- Change orientation until you are looking at the east horizon. What line of right ascension intersects the horizon closest to the east point? Advance the time by one hour. What happened to the line of right ascension?
- Face the western horizon and repeat your observations of the right ascension lines. Face south and advance the clock one hour.
- Face north and let the clock run at one hour increments through a day and night. What happens to the sky?
- Add the Ecliptic path to your display.
- Set the date and time to back to "now", then roll back the sky until you can see the sun above the western horizon for whatever "today" is (about 6pm). What are the approximate right ascension and declination of the sun?
- Set the time increment to 1 day. Move forward a day at a time until the sun is at 0° declination and 12hr right ascension. What is the date? What is this particular occasion called? Is the sun moving "northward" or "southward" in the celestial coordinate system?
- Orient your display so you are looking south south and back the sun up using hours and minutes until you reach civil noon. Note the height of the sun above the horizon (the exact height will depend on your observing location). Is the sun at its highest point for the day (move forward and back one hour to check). Why is there a difference between noon and the sun's highest point in the sky?
- Set the time increment to 1 day and start the sky moving continuously. Note when the sun is lowest above the south horizon: what is the date? Estimate in celestial degrees how far it is above the horizon. Note when the sun is highest at noon: what is the date and how high (in degrees) is the sun? What is its RA and dec in both places?
- Reset back to "now". What is the RA and dec of your zenith?
- Use the appropriate controls to turn on the Constellation figures and their labels. What constellation is at your zenith now? What constellation is on the east horizon? What constellations lie along the ecliptic?
- There is a bright star near the 40° north declension line between 18h and 19h right ascension. Find it and use the appropriate controls to access information about the star. What star is it?
Online Planetarium (alternative to Starry Night): Visit the Neave online planetarium. This is another online planetarium site where you can explore the night sky.
- Determine your own latitude and longitude (if you didn't already do that as part of last week's homework!). Another good option for figuring this out is Infoplease Latitude and Longitude of US and Canadian Cities, where you can enter your city information in the upper right of the display screen. For example, Seattle is at 47°37' north and 122°20' longitude west.
- Use your mouse to move around the sky. Clicking starts and stops the motion of the sky; you can control the direction by moving your mouse across up down, or left, right.
- Click to stop the motion of the sky. Move the mouse over a star. You should see the name of the star, its constellation and magnitude in the upper left corner.
- Manipulate the time minute by minute. What direction do the constellations move? (left? right? east? west?)
- Change the times for 9pm, local time. What stars are above your south horizon?
- Click below the horizon. Line drawings for the constellations should appear.
You now have two online planetaria programs to "play" with. From time to time in this course we will use these to discover the current sky and plan observing sessions. You may use whichever will provide the answers for questions and whichever you feel most comfortable using. Neither is the "right" program — they are each good for different things. You may also use any of the iPhone planetarium programs available if you have the appropriate equipment.
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- There is no chapter quiz yet -- we haven't finished the chapter.
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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