Polaris Star Clock

| Assembly Instructions | Telling Time | Basic Celestial Phenomena | Constellations |

Courtesy of the OBU Planetarium
Designed by Ben Randell, Science Education major, class of '98

Assembly Instructions

  1. Print and cut out the outer dial, which indicates dates of the year.
    Click here to view a printable Outer Dial.
  2. Print and cut out the inner dial, which displays a circumpolar star chart. Be sure to cut out its little notch also (in order to reveal the time of night written on the outer dial).
    Click here to view a printable Inner Dial.
  3. Fasten the two dials together at their centers. The center of the outer dial is clearly marked, but note that the center of the inner dial (where the grid lines converge) does not exactly coincide with the location of Polaris, the North Star. Polaris is displaced about a degree away from the center.

    Possible fasteners include a staple, a piece of thread, a paper brad or rivet, or a thumbtack; any arrangement that allows the dials to rotate freely around their centers without tearing.

    • If you use a staple or piece of thread, secure each end by taping it down on the front and back sides.
    • If you use a thumbtack, an eraser (e.g., a small cylindrical one removed from a pencil) makes a good fastener on the back side.
  4. A larger laminated card stock, laser-printed version of the dials is available from the Planetarium for $1. Or laminate the printed versions yourself for increased durability and performance, which will cost about 30¢ in the Media Center (located in the lower floor of the Mabee Learning Center).
  5. Try out your star clock! You may need to cut the notch in your small circle slightly deeper if the a.m./p.m. indicators under the clock face hours do not appear within the notch at every point around the circle.

Telling Time with the Star Clock

  1. Go outdoors at night with a red flashlight and face north. Find Polaris (the North Star) in Ursa Minor (the Little Bear); the Big Dipper (in Ursa Major, the Big Bear), and Cassiopeia, which are illustrated on the inner dial.
  2. Find the Date: Locate today's date on the outer dial and hold it at the top with your thumb.
  3. Position the Clock:
    • Hold the star clock up at arms length, keeping today's date at the top.
    • Close one eye, and gaze above the star clock directly at Polaris.
    • Tilt the clock face so that it is perpendicular to your line of sight; that is, it should be tilted and not vertical.
    • Keeping your gaze steady, raise the star clock until your line of sight to Polaris would line up with, or pass through, the center of star clock. Of course, the center of the star clock is not transparent, so getting a consistent result may take a little practice!

      (Hint: Point the index finger of the hand not holding the star clock horizontally toward Polaris. Then when you raise the star clock up vertically you can use your index finger to show you when to stop.)
  4. Align the Star Chart: Without moving the orientation of the outer dial or the clock as a whole, rotate the inner dial until the star chart on its face corresponds with the location of the stars in the sky. Look especially for the location of the Big Dipper and Cassiopeia, but any of the constellations identified on the star clock will work.
  5. Read Time: Read the approximate Central Standard time off the scale on the outer dial revealed by the notched area of the inner dial.
    • The star clock is accurate to within a quarter of an hour.
    • The star clock is calibrated for the Central time zone. If you use it in California or Florida, you'll have to convert to your own time zone.
    • The star clock displays Standard time. Add one hour for Daylight Savings Time, if needed, between April and October.
  6. Can't get it to work? Come to the Friday night shows of the OBU Planetarium. Weather permitting, a telescope is set up outside for viewing after each show. The students at the telescopes will be more than happy to assist you in using your star clock.
  7. Optional: The star clock is a convenient way to convert Sidereal Time to local time.

Previous: Using the Planisphere

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Welcome to the Basic Celestial Phenomena web site provided by the OBU Planetarium.

The logo of the OBU Planetarium is not based on a medieval woodcut...

To provide explanations of basic observational astronomy to students, teachers, families, and visitors to the planetarium these pages have been written by the Planetarium Director, Kerry Magruder; the Unified Studies Natural Science Coordinator Mike Keas; and the students who work in the planetarium. These web pages may be printed, copied, and distributed for educational use by any non-profit educational group so long as they are not edited or altered in any way, nor distributed for profit, nor repackaged or incorporated into any other medium or product, and so long as full credit is given to the OBU Planetarium.

This Basic Celestial Phenomena page and many others from the same web site may be accessed from the OBU Planetarium home page: http://www.okbu.edu/academics/natsci/planet/index.htm.