Using a Planisphere

 | Basic Celestial Phenomena | Constellations |


Where can I get a planisphere?


Practice using the planisphere (one of the three main astronomical tools for planetarium and skywatch labs at OBU --the other two are the star-clock and the protractor quadrant). This page contains the lesson plan for a planetarium lab. In fact, it may not make much sense until after you have attended the first planetarium lab in which we will do what is indicated here. Yet many students may get more out of the planetarium lab by previewing or reviewing this page.

  1. Set the planetarium sky so that Scorpius and Sagittarius are visible due south. With a little foresight, this can be done during the last diurnal rotation of Cowboy Astronomer.
  2. Using the planetarium star projector, introduce the constellation Scorpius.
    • Fish-hook to Polynesians.
    • What is it known as in the western tradition? A scorpion, creeping low along the horizon, its stinger the tip of the fishhook.
    • Antares, as red as its mythological rival Mars, is the heart of the Scorpion.
    • Its claws were chopped off by the Romans to make the tiny constellation Libra, or Balance. Not only are they equally bright (dim, but equally so); in the days of the Roman empire, the sun was in Scorpius on the day of equinox, when the hours of daylight and nighttime were equally balanced.
  3. Using the star projector, introduce the constellation Sagittarius.
    • Archer. Chiron the Centaur.
    • Teapot.
    • Teaspoon.
    • Galactic center.
    • Star clusters rise like steam from the teapot.
  4. General instructions.
    1. Examine your planisphere, front and back. Note that constellations are labelled in ALL CAPS with a large font; bright stars or asterisms are labelled in lower case with a smaller font.
    2. Find the north, south, east and west horizons on the planisphere. First find north (designated by an arrow), then the other cardinal directions ("never eat slimy worms").
    3. Position yourself so that you face south. Star charts are usually designed for facing due south, optimized for about half-way up from the horizon.
    4. Read road maps looking down, but read star charts looking up. Hold the planisphere above your head so that the north arrow points north.
    5. Find Sagittarius and Scorpius on the planisphere, turning the inside dial as needed. Note that the planisphere is a guide to the sky, not a replica of the sky's actual appearance. Expect distortion in the arrangement of stars and constellations that are close to the horizon. Check with your neighbor to see if your results agree.
  5. Practice aligning planispheres.
    1. With the planetarium starfield on, and using your red flashlight as needed, turn the planisphere dial so that the position of Sagittarius and Scorpius are aligned on the planisphere's south horizon corresponding to their position on the dome. Check with your neighbor to see if your results agree.
    2. Without changing the relative position of the two dials, find the current date and read the corresponding standard time.
    3. From April to October add 1 hour to obtain Daylight Savings Time.
    4. Compare with your neighbor to see if your times agree.
  6. Predict the appearance of the sky at a specified time.
    1. With the room lights on, set your planisphere to show tonight's sky at 10:00 pm our time (DST if necessary). Remember that DST is one hour later than the standard time the planisphere indicates.
    2. Note carefully the positions of Sagittarius and Scorpius.
    3. With room lights off and the planetarium stars on, the star projector will simulate the passing of the evening hours. Call "HALT!" as soon as the sky reaches the position you expect to find at 10:00. Don't be shy; try to be the first one.
    4. Go outside tonight, listen to the perfect silence of the stars, and practice aligning your planisphere with the night sky.
  7. Look for any visible planets.
    1. The distortion of the star patterns on the planisphere for constellations near the horizon, such as Scorpius and Sagittarius, is not the only complication. Sometimes there are extra bright stars in the real sky not shown on the planisphere! What might these extra bright stars be, that wander from constellation to constellation across the sky?
    2. (Display the ecliptic and celestial equator.) The celestial equator, shown as a solid line, is a projection of the earth's equator out into space. The stars lying on the celestial equator would pass directly overhead an observer located on the earth's equator. The ecliptic, shown as a dotted line, is the path travelled by the sun throughout a year. By the end of the fall semester, the sun will enter Scorpius and then Sagittarius. At that time these constellations will lie in the daytime sky, invisible unless there is a solar eclipse.
    3. The planets generally follow the path of the sun, never straying too far above or below the ecliptic. Constellations that include the ecliptic are called zodiac constellations, and often contain planets. If you see a bright star in a zodiac constellation that doesn't belong, you've found a planet!
    4. Note some bright stars that give a general indication of the path of the ecliptic around the sky. For this part of the sky, you can use lambda-Sagittarii, the top of the teapot, or Antares, the heart of the Scorpion.

    Next: Using the Starclock

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